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CallNo. 5*51 T^V AccewionNo.
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Fundamentals
of physical
geography
McGRAW-HILL SERIES IN GEOGRAPHY
JOHN C. WEAVER, Consulting Editor
BENNETT Soil Conservation
CRESSY Asia's Lands and Peoples
CRESSY Land of the 500 Million: A Geography of China
FINCH, TREWARTHA, ROBINSON, AND HAMMOND Elements of Geography: Physical and
Cultural
FINCH, TREWARTHA, ROBINSON, AND HAMMOND Physical Elements of Geography (a
republication of Part I of the above)
POUNDS Europe and the Mediterranean
RAISZ General Cartography
TREWARTHA An Introduction to Climate
TREWARTHA, ROBINSON, AND HAMMOND Fundamentals of Physical Geography
WHITBECK AND FINCH Economic Geography
VERNOR C. FINCH was Consulting Editor of this series from its inception in 1934 to 1951.
GLENN T. TREWARTHA
Professor of Geography, University of Wisconsin
ARTHUR H. ROBINSON
Professor of Geography, University of Wisconsin
EDWIN H. HAMMOND
Associate Professor of Geography, University of Wisconsin
NEW YORK TORONTO LONDON
Fundamentals
of physical
geography
McGRAW-HILL BOOK COMPANY, INC. 1961
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
Copyright 1961 by the McGraw-Hill Rook Company, Inc.
Portions of the material in this book have been taken from
Physical Elements of Geography by Vernor C. Finch, Glenn
T. Trewartha, Arthur H. Robinson, and Edwin //. Hammond,
copyright 1957 by the McGraw-Hill Book Company, Inc..
copyright 1936, 1942, 1949, by Vernor C. Finch and Glenn T.
Trewartha. Printed in the United States of America. All rights
reserved. This book, or parts thereof, may not be reproduced in
any form without permission of the publishers.
Library of Congress Catalog Card Number: 61-8661
65180
CREDITS FOR CHAPTER-OPENING PHOTOGRAPHS
Chap. 1, Bradford Washburn. Chap. 2, Royal Canadian
Air Force. Chap. 3, Spence Air Photos. Chap. 4, Soil
Conservation Service, USD A. Chap. 5, Spence Air
Photos. Chap. 6, Kirk H. Stone. Chaps. 7, 8, and 9,
Standard Oil Co. (N.J.). Chap. 10, Westinghouse
Photo. Chap. 11, Hamilton Rice Expedition of 1924-
1925. Chap. 12, George A. Grant, National Park
Service. Chap. 13, Standard Oil Co. (NJ.). Chap. 14,
Royal Canadian Air Force. Chap. 15, F. P. Shepard.
Chap. 16, Royal Canadian Air Force. Chap. 17, U.S.
Forest Service. Chap. 18, H. Suito. Chap. 19, Oliver
Iron Mining Company.
Introduction
The surface of the earth is a complicated
combination of a great many things. Some
of these are the natural features, such as the
land, air, vegetation, soil, and water. Man
lives within this complex natural environ-
ment, and, according to his interests and
his knowledge, he takes it into account in
planning and carrying out his activities.
If a person travels over the earth, he will
observe that the various physical features
change from place to place: it is warmer
here; it rains more often there; the land sur-
face is relatively smooth in one place, moun-
tainous in another; and so on. To appreciate
and understand the character of the differ-
ences and similarities of the earth's surface
from place to place, he must turn to the
study of physical geography, the body of
knowledge that deals with the description
and interpretation of the physical features
of the surface zone of the earth.
In general, the description and interpreta-
tion of the myriad interrelationships among
the physical elements of the earth is properly
called earth science. Earth science includes
many fields of systematic investigation. Physi-
cal geography is the segment which studies
the elements that man finds significant in
his use of the earth; in this respect, physical
geography is the study of man's natural en-
vironment. In particular it concentrates upon
the manner in which the environment differs
from place to place and upon the reasons for
the differences.
Man studying the earth has been likened
to a curious ant upon a patterned rug. Be-
cause of the ant's diminutive size and limited
range of vision it cannot easily comprehend
vii
Vlll
the broad arrangements of the different
colors. To develop a clear description and
interpretation of the general pattern, the ant
would need to employ several scientific
techniques, such as careful observation, data
reduction, the development of systematic
classifications, and mapping. The physical
earth is like a patterned rug, albeit a rela-
tively large and complex one. But rather
than being simply a uniform surface on
which the only thing that changes from
place to place is the color of the fibers, the
earth's surface zone is made up of many
different elements, ranging from air tempera-
tures to the flatness of the land, each with
its own more or less complex pattern.
Through the scientific study of physical
geography the student will become aware
that there are both striking similarities and
fundamental differences in the physical en-
vironment from place to place. He will learn
of the great and important variations in the
surface forms of the land, from the broad
patterns of the continents to the smaller
irregularities that complicate the local scene.
He will come to appreciate the general char-
acter and movements of the great mass of
water that not only floods the great depres-
sions of the earth but also exists on and
beneath the surface of the land. He will
become acquainted with the nature and
behavior of the atmospheric film which en-
velops the earth and acts as a transporting
and distributing agent for life-giving energy
and water. He will find also that the mate-
rials and forms of the earth's solid crust and
th^ behavior of the gaseous envelope are all
interrelated, and that they, together with
organic life, combine to produce yet other
patterns such as those of soils and natural
vegetation. In fact he will find that the pat-
Introduction
terns of the several physical elements are all
interrelated and that their spatial relation-
ships are at once simple and complex.
To study scientifically the elements of the
physical environment, one must consider
the physical processes involved in their
interaction in place; this is necessary back-
ground for an understanding of the place to
place variation of each physical element.
One of the purposes of this book is, how-
ever, to focus attention, as much as is pos-
sible in a survey treatment, on the areal
distributions and functional interrelation-
ships of the physical elements over the
earth's surface. In this it strives to emphasize
the basic locational aspects of these matters.
Thus the broad earth patterns of variation
and their interrelationships are stressed, with
less emphasis placed upon the mechanics of
process independent of place. It is hoped
that this will enable the student to obtain
in a direct manner that appreciation of the
earth as a physical environment without
which he cannot be considered properly
informed as a tenant.
Although this briefer book has been
organized and written afresh, much of its
content is based upon materials in the more
comprehensive Physical Elements of Geogra-
phy by Finch, Trewartha, Robinson, and
Hammond. The selection of materials has
been made to fit a one-semester, one-quarter,
or two-quarter introductory college survey
course in the fundamentals of physical geog-
raphy. In every case the degree of general-
ization has been kept at a high level, with
the focus on general world patterns and their
interrelationships. In some sections of this
book a completely different approach has
been taken from that in the earlier book,
and recent materials have been included.
Introduction ix
A number of new illustrations have been
prepared and procured, but many are taken
from the larger book.
The student and the instructor will note
that there are no chapter outlines or review
questions. It is the authors' opinion, and
they feel it to be the judgment of many
instructors, that to include such materials is
to subvert an important part of the learning
process. The good student finds them use-
less, and the mediocre student is likely to
grasp them as straws without going through
the essential learning process of formulating
them for himself. For the student who wishes
to range further, brief bibliographies are
appended at appropriate places.
The authors acknowledge a debt to both
their colleagues and their former students.
At the University of Wisconsin most of the
physical science departments, among them
the Department of Geography, offer one-
semester survey courses as well as year-
length courses. Each of the authors has
taught the survey course in physical geogra-
phy and by contact with the students has
become familiar with the capabilities of
those who take such a course with little or
no background in the subject. Colleagues in
the Department of Geography and in other
departments of the earth sciences have been
helpful in many ways.
Glenn T. Trewartha
Arthur H. Robinson
Edwin H. Hammond
Contents
INTRODUCTION, vii
CHAPTER i The earth: basic facts and mapping 1
CHAPTER 2 The varieties of surf ace form 28
CHAPTER 3 How surface form develops 42
CHAPTER 4 Plains 72
CHAPTER 5 Surfaces rougher than plains 98
CHAPTER 6 The margins of the land 120
CHAPTER 7 Introduction to climate; air temperature and solar
energy 134
CHAPTER 8 The circulation of the atmosphere: winds and pressure 1 55
CHAPTER 9 Precipitation 171
xi
xii Contents
CHAPTER 10 Atmospheric disturbances; air masses and fronts 188
CHAPTER 11 Classification of climates and their distribution; the
tropical humid climates 209>
CHAPTER 12 The dry climates 223
CHAPTER 13 Humid mesothermal climates 235
CHAPTER 14 Humid microthermal, polar, and highland climates 253
CHAPTER 15 Water and the seas 277
CHAPTER 16 The waters of the land 292
CHAPTER i? Natural vegetation 317
CHAPTER is Soils 334
CHAPTER 19 Mineral resources 359
APPENDIX A selected list of United States topographic quad-
rangles 391
1 Average annual precipitation
2 Climates of the earth (front end paper)
3 Terrain of the earth
4 Lithic regions
5 Natural vegetation
6 Distribution of soils
7 Distribution of population
8 Agricultural types and regions
9 Surface temperature regions of the continents
INDEX, 395
Fundamentals
of physical
geography
CHAPTER 1
The earth:
basic facts
and mapping
SIZE AND FORM OF THE EARTH
The earth is almost a true sphere, with a
radius of nearly 4,000 miles and a surface
area of about 197 million square miles. The
earth rotates steadily, and for some time the
surface has maintained its position relative to
the axis of rotation. The opposite, or anti-
podal, points on the surface that lie in the
axis of rotation are called the earth poles; an
imaginary line encircling the surface midway
between the poles is called the equator.
The greatest departure of the earth from
sphericity is a flattening in polar areas and a
correspondent bulging in equatorial regions.
The polar radius is about 13.5 miles shorter
than an equatorial radius. Yet given the size
of the earth, this spheroidal deformation is
small: it would amount to less than Vio in.
on a ball 5 ft in diameter. None of the other
departures from sphericity is even this great,
for the highest mountain projects only about
5.5 miles above the general level of the sea,
and the greatest ocean depth is less than
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
7 miles below sea level. The almost perfect
smoothness of the earth's surface relative to
its size may be illustrated this way: on a true
reduction of it with a diameter of 1 ft, the size
of an ordinary desk globe, one could hardly
feel any roughness. On such a globe the peak
of Mount Everest, approximately 29,000 ft
above sea level, would be less than Woo in.
above the general surface of the globe.
The exterior of the earth nonetheless con-
sists of markedly different substances. Most
of the solid surface is covered with water
that has an average depth of more than 2
miles. Only a little more than 29 per cent of
the solid material protrudes above the water
surface, and this but slightly, since the aver-
age elevation of the land surface above the
water is only about % mile (Fig. 1.1). Over-
lying both the solid and liquid surfaces is a
mixture of gases, the atmosphere. The distri-
bution of the land and water surfaces is not
symmetrical, especially in terms of an equa-
torial division. A separation of the earth into
hemispheres by the equator finds twice as
much land in the Northern Hemisphere as in
the Southern. The total area of the exposed
solid surface is about 57 million square miles,
equal to about nineteen times the area of the
United States. Upon this surface the entire
human population of the earth resides and
endeavors to secure a living. Because of its
physical geography, i.e., its natural variation
from place to place, there are large parts of
the land area which, for one reason or another,
are ill suited to intensive human occupation
or use.
The polar flattening and the equatorial
bulging of the earth indicate it is plastic
since this is the sort of deformation that would
occur in any nonrigid sphere as a consc-
quence of gravitational force and the _centrif-
ugat force resulting; from its^rolajtion. The
outer zone of this plastic ball, the surface
crust, apparently consists of adjacent segments
that vary slightly in the average density of
the rock types of which they are composed.
The continental segments are thought to be
made of rocks that together are slightly less
FIG. 1.1 Cumulative graph showing the relative amount of land and
water surface on the earth and the average elevation and depth of the solid
surface in relation to sea level. Note that most of the earth's surface is
water and that most of the land lies beneath the sea.
DEPARTURE FROM SEA LEVEL
IN THOUSANDS OF FEET
.Average elevation of land (approximately 0.5 mile)
SEA LEVEL
veraae depth of ocean
approximately 2.5 miles
50 100 150
ARE.A OF EARTH IN MILLIONS OF SQUARE MILES
FIG. 1.2 The inferred distribution of densities
of the solid materials of the earth's crustal zone.
dense (average density approximately 2.7)
than those that form the crustal sections
underlying the ocean basins (average density
approximately 3.0). It is because of this dis-
crepancy that the continents stand slightly
higher, it is believed. The various crustal
segments or blocks are thought to float, so to
speak, on the subsurface in balance, a state
called isostasy (Fig 1.2). Yet the relative ver-
tical position of the materials that form the
segments is transitory since the density rela-
tionships are constantly being disturbed by
such events as the removal of material from
the continents by streams and its deposition
in the oceans. The variances that result ap-
pear to be regularly counteracted by the slow
movement of the more plastic material
thought to exist beneath the earth's crust.
This, together with still other forces, results
in the bending, breaking, and warping of the
materials of the crust. These in turn cause
the minor surface irregularities of lesser mag-
nitude, such as the mountain masses and
depression areas on the continents and within
the ocean basins. The detailed unevenness of
a surface made up of mountains, hills, and
plains is of critical local and regional signifi-
cance to diminutive man.
Yet it must be emphasized that the most
important fact of the earth's form is its almost
perfect sphericity. A large proportion of the
fundamental processes that together cause
natural variations in the surface from one
place to another are direct or indirect con-
sequences of this simple fact of spherical
form. For example, the variation in the receipt
The earth: basic facts and mapping 3
of energy from the sun and all its conse-
quences such as air temperature, precipita-
tion, and the circulation of the atmosphere
and the oceans, to name but a few of the
elements of physical geography are directly
attributable to the earth's being a rotating
spherical body. In order, then, to understand
many important things about the earth, a
person must be thoroughly familiar with the
geometric properties of the sphere, the circle,
and the arc.
Any intersection of a plane and a sphere
results in a circle that contains 360 degrees. If
an intersecting plane includes the center of
the sphere, the circle is termed a great circle;
it is the largest that can occur on a sphere
and divides it into hemispheres. (The equator
is the great circle the plane of which is per-
pendicular to the earth's axis of rotation.)
There can be an infinite number of great
circles and they have the significant property
that they all bisect one another (Fig. 1.3).
F I G . 1 . 3 A great circle lies in a plane that
passes through the center of a sphere;
consequently a great circle divides a sphere into
hemispheres. A sphere can have any number of
great circles arranged in an infinite number of
ways, and they will all bisect one another. These
concepts apply to the earth since it is a sphere.
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
The course along the arc of a great circle is
the most direct route from one place to an-
other on the curved surface and is thus anal-
ogous to the straight line on a plane as the
shortest distance between two points. One
degree of arc distance along a great circle on
the earth is about 69 statute miles or 60
nautical miles. Any other circle on the earth
is a small circle, and arc distances along small
circles are shorter in miles.
These simple geometric consequences of
the earth's shape and size will repeatedly be
found to underlie the complex facts of physi-
cal geography that appear in the balance of
the book.
EARTH MOVEMENTS
This rotating sphere, the earth, is also a
planetary satellite of the sun, around which
the earth revolves in an orbit some 93 million
miles from it.
The earth rotates at an almost constant
speed, and the time required for the surface
to rotate once in relation to the sun is desig-
nated as 1 day. During each rotation most of
the surface successively turns toward and
away from the sun. The greater part thus
experiences a period of light and energy
receipt and a period of darkness and energy
loss. Most places, therefore, pass twice
through the boundary between light and dark,
the circle of illumination, which is a great
circle, once at dawn and again at sunset.
The eastward rotation of the earth deter-
mines the direction in which the sun, moon,
FIG. 1.4 Relation of the inclination and parallelism of the earth's axis
to the periods of the year. The observer of this perspective drawing is far
outside the earth's orbit and slightly above the plane of the ecliptic.
Compare with Fig. 1.8, which shows views of the earth in the plane of the
ecliptic at the time of the solstices.
and stars appear to move across the sky and
affects other earth phenomena of far-reaching
consequence, such as the general circulation
of the atmosphere and oceans, which will be
studied later.
The earth revolves around the sun in a
slightly elliptical orbit. In the course of its
revolution, it moves faster when it is closer
to the sun. The average time required for the
earth to complete one circuit of its orbit is
designated as a year. During this period the
earth rotates in relation to the sun approxi-
mately 365 V4 times, thus determining the
number of days in the year. Primarily because
of slight variations in the speed of the earth
along its orbit, the time interval between
successive complete rotations relative to the
sun is not constant. The average rotational
day is arbitrarily divided into 24 hr of con-
stant duration.
All points in the earth's orbit lie in a plane,
which includes the sun, called the plane of
the ecliptic. The axis of the earth's rotation is
inclined about 66% from this plane (or
23^6 from perpendicular to it). This angular
inclination is nearly constant, and moreover
the axis at any time during its orbit is parallel
to the position that it occupies at any other
time (Fig. 1.4). This is called the parallelism
of the axis.
The inclination of the^ earth's axis and its
parjdlelism, together withjhe earth^s,srTapc,
its "rotation, and its revolution about the sun,
causesevcral earth phenomena that are^of
vital importanccjo^hysical geography. Some
otthese are (a) thedistribution over the earth
of the rccciptofsplar^nergy, (b) the^changing
of the seasons, (c) thechanging lengths j)fday
and mgh^juxd (d) the generajinajinej^ in
which the atmosphere and oceans circulate.
These matters and other related ones will be
discussed more fully in their connection with
climate.
The earth: basic facts and mapping 5
LOCATION ON THE EARTH
Coordinate system The earth's form
and its movements are also significant as the
bases upon which man has developed the
system he uses to determine and describe
position and relative location on the earth's
surface. On an ordinary sphere there is
neither beginning nor end, no natural point
or line of reference from which to begin to
measure the relative positions of other points.
If it were not for its systematic motions and
planetary relations, the earth also would have
no natural point or line from which to meas-
ure distance or direction. But the fact of rota-
tion establishes the geographic poles of the
earth, and these serve as reference points for
the coordinate system by means of which
directions and locations are determined.
The system is similar to the familiar rectan-
gular-coordinate system on ordinary graph
paper, modified to fit the spherical earth. An
infinite number of small circles is conceived
parallel to the equator, as illustrated in Fig.
1.5. All, including the equator, are called
parallels, and the earth directions east and
west are determined by their orientation on
the surface. Since each of the small circles is
parallel to the equator, every point on a given
parallel will be the same distance from the
equator, the same distance from the North
Pole, and the same distance from the South
Pole. The distance of a point from the
equator or one of the poles is called latitude
in the earth's coordinate system and is ex-
pressed by identifying the parallel on which
the point is located.
Of course a statement of latitude is not
enough to locate a point since a parallel is a
circle, and the point could be anywhere on
it. Position east or west on a parallel, called
longitude, may be determined by reference to
a different system of circles arranged per-
FIG. 1.5 Arrangement of the parallels in the
earth's coordinate system. The parallels establish
the directions east and west and provide a method
for designating positions north and south. Only a
few of the infinite number possible are shown here.
pendicular to the parallels. In a plane, or
flat, coordinate system these lines are also
parallel to one another, but on the spherical
earth they must converge and intersect at the
poles; they are, however, equally spaced on
any parallel. These great circles are called
meridians, and the earth directions north and
south are determined by their orientation on
the surface, as shown in Fig. 1.6. In practice
each meridional great circle is halved at the
poles to form opposite pairs of semicircular
meridians extending from pole to pole.
Latitude In numbering the earth's co-
ordinate system, the great circle formed by
each pair of meridians is divided into quad-
rants, the points of division being the poles
and the two intersections with the equator.
Each meridional quadrant is divided into 90
of latitude, and the numbering of the latitude
proceeds from the equator (0 Lat) to each
pole. Location along a meridian is established
by noting the intersection of it by a partic-
ular parallel. Thus latitude is reckoned from
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
the equator northward to the North Pole on
any meridian and, in the same way, from the
equator to the South Pole. Consequently the
terms low, middle, and high latitudes refer to
the values of the numbering system; for ex-
ample, the lower latitudes are those areas
near the equator.
The lengths of the degrees of latitude are
not quite identical along a meridian. On a
sphere each arc unit of a great circle has the
same length, of course, but the earth is not
quite a true sphere. The latitude of a point
is determined by observing, at the point in
question, the angular difference between the
horizon and some celestial body, such as
Polaris (the North Star) or the sun (Fig. 1.7).
A degree of latitude is, therefore, the distance
north or south a person must move along a
meridian in order to observe 1 change in
this angle. Because of the flattening in the
polar regions, a traveler must go farther along
a meridian there to obtain a change of 1.
FIG. 1.6 Adding the meridian system to the
parallel system provides for an infinite number of
coordinate intersections specifying the location of
any and all points on the earth. The meridians
themselves establish the directions north and south
and provide a method for designating positions
east and west.
Polaris
Polaris
Polaris
FIG. 1.7 The relation between (1) the curvature
of the earth's surface (in this case north-south),
(2) the angle above the horizon of Polaris, and
(3) the latitude.
The first degree of latitude from the equator
covers a distance of 68.7 miles, while the
first degree from either pole is 69.4 miles
long. Each degree of latitude is divided into
60 minutes ( ' ), each minute into 60 seconds
( " ). One minute of latitude is very nearly 1
nautical mile, or about 1.15 statute miles,
and one second of latitude is about 101 ft.
The length of the meter, the standard of dis-
tance measurement in the metric system, is in
theory one ten-millionth of the meridional
quadrant, the distance from the equator to
the pole.
Commonly only a few of the infinite num-
ber of meridian and parallel circles are shown
on maps, such as those of the multiples of 5
or 10. Often, however, four particular par-
allels in addition to the equator are indicated
because they have special significance. These
are the parallels of approximately 23^6 N
and S Lat and of 66% N and S Lat. They
are important because the sun appears at dif-
The earth: basic facts and mapping 7
ferent angular elevations above the horizon in
different regions. The parallels of 23% N
and S are called the Tropics of Cancer and
Capricorn, respectively. They mark the limits
of the zone near the equator within which
the sun ever appears directly overhead. The
parallels of 66% N and S are called the
Arctic and Antarctic Circles, respectively.
They mark the limits of the polar area in each
hemisphere within which the sun ever ap-
pears above the horizon continuously for 24
hr or more, or, at the same time in the
opposite hemisphere, remains below the hori-
zon for 24 hr or more (Fig. 1.8).
Longitude Longitude is reckoned east or
west along the parallels, but there is no par-
ticular meridian marked by nature (as the
equator is for specifying latitude), from which
a system of specifying longitude may be
started. All meridians are exactly alike, and
any one of them could be designated as the
zero meridian (0Long). In fact, for several
centuries each important country numbered
a meridian that lay within its own borders as
0Long. So much confusion resulted that,
in the year 1884, the meridian passing
through Greenwich Observatory at London,
England, was chosen by international agree-
ment as 0Long. It is called the prime
meridian. It intersects the equator in the Gulf
of Guinea at a point which has the distinc-
tion of having 000'00"Long and O'OO'OO"
Lat. This point is, then, the point of origin
of the earth's coordinate system. The degrees
of longitude in each parallel are numbered
east and west to 180, the meridian opposite
the prime meridian; together the two merid-
ians (0 and 180) make a great circle.
The circumference of each parallel is less
than that of a great circle in the ratio that is
given graphically by the lengths in Fig. 1.9.
Since each parallel, whatever its circumfer-
ence, is divided into 360, it follows that the
8
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
FIG. 1.8 The angular relationship between the direction of the sun's
earth-striking energy and points on the earth's surface changes during the
year, and thus so does the sun's apparent position in the heavens. For
example, on December 21 a person on the Tropic of Capricorn would see
the sun directly overhead (90 above the horizon) at noon; on the same day
a person on the Arctic Circle would see the sun on the horizon; i.e., the
rays would be tangent (at elevation) to the earth's surface. The segments
of the sun's surface shown here to scale represent only about one two-
hundredth of its circumference; consequently for practical purposes they
have almost no curvature.
higher the latitude, the less will be the dis-
tance represented by 1 of longitude. The
length of 1 of longitude along the equator,
a great circle, is 69.17 miles, which is about
the same length as an average degree of lati-
tude. At 30 N or S Lat the length of a de-
gree of longitude is 59.96 miles, at 60 it is
34.67 miles, at 80 it is 12.05 miles, and at
the poles it is, of course, nil.
Determining Latitude and Longitude Any
point on a perpendicular-coordinate system
may be designated by establishing its ordinate
and abscissa values; consequently, any point
on the earth's surface may be located pre-
cisely by determining its latitude and longi-
tude, i.e., that it lies at the intersection of a
certain parallel and a certain meridian. Thus
if a person were to say that the dome of the
National Capitol at Washington was located
at 3853'23"N Lat and 77 00'33"W Long,
FIG. 1.9 Comparative lengths of the parallels
in relation to the length of a great circle, as shown
by the line designated as 0Lat (the equator).
Note that the sixtieth parallel is half as long as the
equator.
90
-80
-70
-60
-50
-40
-30
-20
-10 C
he would have stated its position on the
earth to within some 10 paces.
The latitude of a place, i.e., its position in
the north-south direction, is determined by
instrumental observation of the vertical angle
at that place between the horizon and a dis-
tant celestial body. The earth is so small rela-
tive to its distance from the stars (including
the sun) that for all practical purposes the
rays of light from a celestial body intercepted
by the earth are parallel. Since the quadrant
of latitude is divided into 90 and since the
curvature along one-fourth of a meridian
great circle also covers 90 of arc, a differ-
ence in arc position along a meridian will be
exactly matched by the change in angular
height of a celestial body. For example, at the
North Pole Polaris is almost overhead; there-
fore its angular height above the horizon is
90, and the latitude of the North Pole is
90 . Similarly, if Polaris could be seen from
the equator, it would appear almost on the
horizon, that is, at an angular elevation
of 0; and at 45 N Lat it appears halfway
between the zenith and the horizon. Figure
1.7 illustrates these relationships. They may
be summarized by stating that the arc dis-
tance in degrees of latitude between two par-
allels on the earth is the difference in the
angular heights above the horizon of a given
celestial body as observed in the meridional
direction at the two points.
The use of stars other than Polaris to de-
termine latitude complicates matters some-
what since corrections must be introduced to
take into account the fact that other celestial
bodies do not lie on the extension of the
earth's axis of rotation, but tables are avail-
able to provide the necessary data.
The longitude of a place, i.e., its position
in the east-west direction, is determined by
observing the solar, or sun, time difference
between that place and some other point of
known longitude. Since the earth rotates
The earth: basic facts and mapping 9
through 360 in approximately 24 hr, it turns
through about 15 in 1 hr, irrespective of
latitude. Consequently, if it is 10:00 A.M. at
one point and noon at another, 30 of longi-
tude separates them. To observe one's local
solar time is relatively easy; noon is the in-
stant when the sun appears to cross the ob-
server's meridian, i.e., when it reaches its
highest point (zenith) in its daily course across
the sky. This instant of time may then be
compared with the solar time of some other
known place by means of a chronometer (an
accurate clock) that is keeping the time of the
other place, or by instantaneous electronic
means. The difference in time may easily be
converted to degrees of longitude at the rate
of 1 hr= 15 Long. Solar time will be further
discussed later in this chapter.
DIRECTION ON THE EARTH
The relative location of places may be
stated in directional terms as well as by iden-
tifying their latitudes and longitudes. The ex-
pression of direction on the earth is somewhat
complex since it is a sphere and its coordinate
system is spherical instead of rectangular.
Earlier it was seen that the path along a
great circle is geometrically the most direct
course between points on the earth, analogous
to the straight line on a plane. Since the
directions at any point on the earth are de-
fined by the orientation of the parallels and
the meridians, and since these are on a
spherical surface, it follows that directional
relationships change from place to place on
the earth. Consequently one. can describe the
"beginning direction" of the great-circle
course from point to point; but elsewhere
along the course the angular relation between
the coordinate system and the particular great
circle will change, unless the great-circle
course is along a meridian or the equator.
Direction from one place to another is speci-
fied by stating the angle between the meridian
10
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
The compass points east of north ( The compass points west of north
20 15' --
FIG. 1.10 Lines of equal magnetic declination (isogonic lines) in the
United States in 1960. Only at points along the agonic line (0 declination)
does the needle of the magnetic compass parallel the meridian.
Elsewhere a correction must be applied to the reading.
((renfralized ftom a map by the f..V. Coast and Geodetic Survey.)
and the great circle at the starting point,
either as a compass bearing, e.g., NE, or as an
azimuth, the angle between the meridian and
the great circle, usually expressed in degrees
reckoned clockwise from north, e.g., NE=
45 az. The precise determination of azimuth
requires astronomical observation to establish
the direction of the reference meridian.
When the direction of the meridian can-
not be determined astronomically, various
mechanical devices or the more familiar mag-
netic compass may be used to find it. The
needle of the compass aligns itself with the
forces emanating from that great magnet the
earth. Unfortunately, however, the positions
of the magnetic north and south poles are not
opposite one another and do not coincide
with the geographical poles. The magnetic
poles are even subject to slight changes of
position. In consequence, only in limited
areas does the magnetic needle parallel the
meridian; at most places the needle rests at an
angle with the meridian. The angle varies
considerably from place to place, and the
magnitude of the variation at any point is
called the compass declination of the point.
Figure 1.10 shows the lines of equal compass
declination in the United States. East of the
agonic line, where declination is nil, the com-
pass has a west declination. In some parts of
the frequented oceans the compass declination
is as much as 30 to 40, and in the polar re-
gions it shows very wide variations.
OTHER SYSTEMS OF DESIGNATING
POSITIONS OR AREAS
The spherical-coordinate system, consist-
ing of latitude and longitude measurements
with their directional derivatives, is the
fundamental basis for establishing relative
location on the earth. Yet for bounding small
areas or locating specific positions on the land
it is often inconveniently complicated, as well
as subject to errors of definition and instru-
mentation. Consequently, in order precisely
to designate parcels of land for administra-
tive purposes, other methods are necessary.
Metes and bounds The world's most
widely used method prevalent in most of
Europe and in some early-settled areas of
North America, for instance is known as
metes and bounds. In this an arbitrary point is
designated as a point of beginning, for ex-
ample, a projecting rock, an iron stake, a
tree, or some other identifiable point on the
land. The land segment is then bounded by
a line extending from the starting point in a
given compass direction for a certain distance,
then in another direction for a specified dis-
tance, and so on back to the point of begin-
ning. This system has often led to dispute
over property lines because the marked points
on the ground have been lost. Moreover, the
stated distances and directions have some-
times been measured inexactly, as in parts of
Texas, where some of the early Spanish land
grants were supposedly stated in such units
of distance as the length of a lariat rope and
how far a horse could walk in a given time.
In most parts of the world the bounding
lines do not enclose rectangular parcels of
land, so that the subdivision of the land does
not produce any consistent pattern of shapes
oriented to the cardinal compass directions.
This lack of coordination is plainly ap-
parent in the road patterns of many long-
settled areas that were not subjected to any
systematic plan of land subdivision prior to
settlement. This may be seen in detailed
maps of parts of North America, such as
New England and Texas. In some localities
The earth: basic facts and mapping 11
of eastern North America the present small
parcels of land are subdivisions of grants
made by European rulers to noblemen or to
the sponsors of settlement projects. In some
sections, such as French Canada and French
Louisiana, the present landholdings do have
a pattern of rectangularity but are very long
and narrow. Their narrow frontage is upon a
river, and their length extends perpendicular
to the river regardless of its course. Even
some of the counties of the Province of
Quebec have much the same shape. They
were established at a time when river front-
age was a most prized possession but the
land of the interior had little value.
Rectangular survey In contrast, this
essentially haphazard practice was not used
in a large part of the United States and much
of Canada, which were settled after a govern-
mental subdivision of the land based on a
rectangular-survey system. This system was
applied to almost all the regions lying to the
west of the earlier-settled eastern seaboard.
In this system the boundaries of public and
private lands are often described in detail by
metes and bounds but in relation to a network
of essentially north-south and east-west lines.
These include selected meridians called prin-
cipal meridians and parallels called base lines
(Fig. 1.11). They have had the effect of
dividing the land into essentially rectangular
blocks. The locations of the blocks are indi-
cated by numbered townships and ranges
(Fig. 1.12).
The ranges are 6-mile-wide strips of land
running north and south, each numbered to
the east or to the west of a particular principal
meridian. Each range is divided into a tier of
townships by east-west lines 6 miles apart,
each township being numbered north or south
from a base line. Each survey township is
thus supposed to be 6 miles square. By this
system any township can be located by refer-
12
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
UNITED STATES
SHOWING
PRINCIPAL MERIDIANS
BASE LINES
FIG. 1.11 The principal meridians and base lines which govern the
land-survey system of most of the United States, except Texas and the
Atlantic Coast states. The fine-dotted lines surround the areas governed
by each principal meridian; some of these areas are large, some small.
FIG. 1.12 The system of designating
government-survey townships by township and
range numbers with respect to given principal
meridians and base lines. The shaded township is
T2N, R3E.
T3N
m
T2N
BASE
LINE
Tl N
Tl S
T2S
R3W R2W RIW RIE 6 R 2E 2 R3E
Ji 8
MILES
ence to its township and range numbers, e.g.,
township 2 north, range 3 east, usually
written T2N, R3E (Fig. 1.12).
The usual survey township is further di-
vided into 36 sections, each approximately
1 mile square, or 640 acres in area, whose
corners are marked on the ground. The sec-
tions are numbered, beginning at the north-
eastern corner and ending at the southeastern,
as shown in Fig. 1.13. In addition, the section
is divided into quarter sections, each con-
taining 160 acres, and these are further
divided into quarters of 40 acres each, com-
monly called "forties." The quarter sections
and the forties are indicated by the points of
the compass (Fig. 1.14).
Because the meridians naturally converge
T8N
36
31
32
33
34
35
36
31
1
6
5
4
3
2
1
6
12 J
7
8
9
J
J^
>
7
13
18
17
]
^
>*
<?
18
TTM
24|
19
20
4
<*
23
r
19
1 /IN
25 Q -
30
29
v^
28
fM
26
25
30
36
31
32
33
34
35
36
31
1
6
5
4
3
2
1
6
T6N
R1W
R1E
R2E
FIG. 1.13 The standard system of numbering
used for the sections within a township. This
township isT7N, R1E.
northward, because straight base lines cannot
also be true east-west, because errors occur
in surveying, and because there are some-
times lakes or streams at critical points, many
corrections and other allowances have had to
be made by men using the rectangular-survey
system to subdivide land. Moreover, the loca-
tions of the section corners and the quarter-
section corners were in the past marked by a
stake, stone, mound, tree, or other device;
but too often these were impermanent fea-
tures, and many of them are now difficult to
locate.
Civil towns or other units of political
administration may or may not coincide with
government townships, which exist for pur-
poses of survey location. In thinly settled
districts the civil towns often are large enough
to include two or more government town-
ships or parts of townships. In other areas
one government township may be divided
into two or more small civil towns. The
boundaries of civil towns, villages, and mu-
The earth: basic facts and mapping 13
nicipalities also are subject to change by
appropriate legislation, but the government-
survey townships remain.
Wherever such a basic survey framework
has been employed it has left an indelible
imprint on the landscape, a pattern of patch-
work rectangularity easily seen from the air
and on air photographs. It is even reflected
in the road maps of much of the United
States and Canada since the minor roads and
even the field boundaries tend to be oriented
with the cardinal directions.
In recent times other kinds of rectangular-
coordinate grid systems have been developed
to designate location on detailed military and
FIG. 1.14 Parts of sections are described and
located by quarter sections, designated by the
compass position of each part in its section. The
40-acre tracts ("forties") within quarter sections
are similarly described and located. For example,
the shaded area (a forty) would be designated as the
NE'A of the SW/4 of, say, Sec 20, T44N, R5E,
followed by either the principal meridian to which
the range number referred or the administrative
district, e.g., county and state, in which the area is
located. A forty may be divided in quarters just as
sections and quarter sections are.
660 1320 2640
Feet
14
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
civil maps, in lieu of the "geographical"
latitude-longitude system. Their character-
istics and uses are explained in technical
manuals and treatises on map use and
cartography.
TIME ON THE EARTH
The movements of the earth relative to the
sun cause, at each place on the earth, a vari-
ety of changes such as those of night and day
and the seasons. The regularity of these
movements provides the basis for the system
of time and the differences of time from
place to place.
There are two distinct ways of reckoning
time, by clock and by calendar. From one
midnight to the next is a calendar day
regardless of the clock time that elapses. On
the other hand, 24 clock hours are also called
1 day, and the records of the two time-telling
systems must sometimes be adjusted or they
will conflict.
Clock time Clock time, as already indi-
cated, is reckoned from the apparent motion
of the sun and is called solar time or sun time,
with noon on any day determined by the
instant the sun reaches its zenith. Because
the earth does not revolve around the sun at
a constant speed, the clock interval between
successive noons at any place varies a bit
during the year. So for convenience an aver-
age is used, called mean solar time. Until
about a century ago each locality kept its
own time; i.e., noon was when the sun ap-
peared directly over the local meridian. But
with the development of rapid mobility this
became inconvenient, and most of the world
adopted a system of standard time.
The standard-time system, in general, allo-
cates to 24 zones running north and south,
each 15 of longitude in width, the mean
solar time of the central meridian of each
zone. All places within a zone then maintain
the same clock time although this may depart
from solar time by as much as % hr. Changes
of clock time are then necessary only when
crossing the boundary of a zone, and each
change is exactly 1 hr. Because the earth
rotates toward the east a timepiece is set for-
ward, e.g., from 12:00 to 1:00, in traveling
east, and backward, e.g., from 12:00 to 11:00,
in traveling west. In practice, these zones
are commonly bounded not by meridians but
by irregular lines, the locations of which are
subject to administrative changes dictated by
local convenience. Figure 1.15 shows stand-
ard-time zones of the United Stages. In the
whole system the 24 zones should each
extend from pole to pole and each differ from
prime meridian (Greenwich) time by an inte-
gral number of hours, but in practice the
arrangement is not quite so simple. Most
countries follow the general plan, but some
have not yet adopted standard time at all, and
a few countries employ the time of meridians
that are not multiples of 15 and therefore
do not differ from Greenwich time by exact
hours.
Calendar time Calendar time is reck-
oned by specifying that one rotation relative
to the sun represents one day, and the year
is the period required for the earth to com-
plete one revolution around the sun, which
is, as previously stated, the time required for
approximately 365^4 rotations. This system
presents no problem if someone remains at
one place on the earth; but if he travels all
the way around the earth he will obviously
either subtract or add one rotation, depending
upon which direction he goes. It will then
become necessary for him either to add or
subtract a day so that his calendar will match
those that have remained at one place. The
The earth: basic facts and mapping 15
FIG. 1.15 Standard-time zones of the United States as of 1960.
(Interstate Commerce Commission.)
total elapsed clock time is, of course, not
affected, but by calendar time a day is a rota-
tion, not 24 hr.
This may be illustrated by imagining an
airplane sufficiently fast to fly in an east-west
direction around the earth in exactly 24 hr.
If the flier starts westward from, say, Chicago
at noon on a Monday the tenth of the month,
his ground speed westward will exacdy cancel
the eastward rotation of the earth. To him
the sun will have no apparent motion; it will
remain in the noon position in his sky for 24
hr. The earth will rotate under him and he
will "return" to Chicago the same (to him)
noon. In other wojds, although 24 hr has
elapsed, he will not have experienced a solar
day. On the other hand, for persons on the
ground a night will have intervened, and it
will be noon of Tuesday the eleventh. The
flier will therefore have "lost" a calendar
day. If he had flown from Chicago eastward
instead, he would have experienced a mid-
night over Spain (6 hr later), noon of another
day over central Asia (12 hr after leaving),
a second midnight over the Pacific Ocean
(18 hr after leaving), and would have returned
at noon of his second calendar day, in spite
of the fact that he had been traveling only
24 hr. According to his solar calendar it
would be Wednesday the twelfth, while to
those who stayed at the starting point it
would be Tuesday the eleventh. The traveler,
therefore, would have "gained" a solar day.
If a person travels slowly and thus cancels
or adds the 1 solar day over a longer period,
the case is the same. Unless he sets his cal-
endar ahead 1 day when making a circuit of
the earth westward or sets it back when
traveling eastward, his calendar will be off by
1 day on his return.
16
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
To avoid the confusion that would result
from individual choice of where to reset the
calendar, an international date line has been
designated. It is located along the 180th
meridian, with some small deviations agreed
upon so that island groups and land areas will
not be divided inconveniently. By designation
of this line as the meridian where the calendar
day "begins," no calendar correction is
necessary except when one crosses it.
MAPS AND MAPPING
MAPS AS ESSENTIAL
SCIENTIFIC TOOLS
Maps, as scientific devices, represent graph-
ically the relative locations of many kinds of
phenomena. They are used in several fields
of learning, especially in earth sciences. For
the student of geography the map is an essen-
tial tool, the medium for recording factual
observations and their derivatives so that lo-
cational relationships and changes from place
to place may be studied.
Maps are so nearly infinite in variety that
they almost constitute a scientific "language"
in themselves. To interpret them readily one
must understand the important types of maps
and their qualities. To that end the student
needs to obtain considerable familiarity with
three fundamental aspects of all maps. These
are (a) the scale, that is, the size of a map
representation relative to the size of that part
of the earth which it represents, (/;) the nature
of the system of projection employed in trans-
forming the spherical surface of the earth to
the flat surface of the map, and (c) the mean-
ings of the various symbols or devices used to
show the things represented on the map.
Scale A reduction of the earth on a
globe is the form of earth representation re-
quiring the least interpretation. The dimen-
sions of the globe may be measured, and
their relation to the dimensions of the earth
may be expressed as a ratio, called the scale
of the globe. For example, the earth has in
fact a diam of about 500,000,000 in., so that
if someone has a large globe with a diam of
50 in., then the ratio of the globe distance
between any two places to the same real dis-
tance on the earth is as 50 is to 500,000,000,
or more simply, as 1 is to 10,000,000, com-
monly written 1 : 10,000,000. This means that
1 unit on the globe represents 10,000,000
units on the earth. The scale is independent of
the units of measure, but the same units must
be used on each side of the scale. The scale
may be expressed as a fraction 1/10,000,000
in this case and called the representative
fraction, or RF for short.
Maps, like globes, always have a scale
relationship to the parts of the earth that they
represent. Usually it is given in the form of
the RF; occasionally it is expressed verbally,
as for instance, "One inch represents one
mile." But most often the scale is shown by
means of a measured line showing the map
lengths of earth units, as in Figs. 1.13 and
1.14.
Maps are sometimes described as being
large-scale or small-scale. The larger the part
of the earth mapped on a given size sheet,
the smaller will be the RF which tells its
scale; that is, the larger will be the number
in the denominator. Therefore, small maps
of large earth areas are called small-scale
maps, and, in reverse, large maps of small
earth areas are called large-scale maps. Any
map showing the entire earth or any major
part of it is termed a small-scale map, while
a topographic map, as described later, is
classed as a large-scale map.
The student should bear in mind one es-
sential difference between the interpretation
of the scale of a globe and that of a flat map.
The scale of a globe, no matter how small,
may properly be applied to it in all parts and
in all directions, but the indicated scale never
applies equally in all directions on a map. (A
reason for this difference will appear below.)
On very large scale maps this inequality may
be ignored, but its importance increases as
the scale of a map becomes smaller.
Globes and map projections Reducing
the earth with maps to help the geographer
see its variations as some other scientists
enlarge their objectives with microscopes
is the job of the cartographer, or map maker.
He may do this in two ways: he may simply
make a reduced global representation of the
spherical earth, or he may make his represen-
tation on a plane surface, such as a sheet of
paper, by a systematic process of transforma-
tion called map projection.
A globe provides a reduced but otherwise
undeformed framework for a map, since the
geometry (earth measure) of the sphere has
been changed only in scale, and direction
and distance relationships remain in strict
proportion to those of the earth. Yet a globe,
in spite of this obvious advantage, has several
disadvantages. Among the more serious are
that (a) only a portion (less than half) of the
globe can be seen at one time, (b) if it is large
enough to show much detail, it is bulky and
unwieldy, (c) its curved surface is difficult to
measure on, and (d) it is expensive to repro-
duce. Most of the mechanical difficulties do
not hold for a plane-surface map. It can be
seen all at once, it is relatively easy to handle
The earth: basic facts and mapping 17
or store, measuring on it is easy, and infor-
mation can easily be printed on its surface.
For these reasons flat maps on plane surfaces
are greatly preferred to curved maps on
spherical surfaces.
In geometric transformation dissimilar sur-
faces are said to be applicable if one can be
bent into another without modifying the
geometric relationships among the points on
the surface. Thus a cylinder or a cone may
be cut and laid out flat to form a plane, and
neither distance nor directional relationships
across the surface will be changed. A spher-
ical surface and a plane surface are not appli-
cable, however. The bending required to
transform one to the other must involve
stretching and shrinking. This unavoidably
results in changing the distance and direc-
tional relationships among points on the
earth represented on a flat map. Yet the other
advantages of a flat map projection listed
above far outweigh the disadvantages pro-
duced by the stretching and compression.
PROPERTIES OF MAP PROJECTIONS
For more than 2,000 years men concerned
with maps have been devising ways of dis-
tributing the changes in distance and direc-
tion on them so that the distance-direction
alterations may be allowed for by the user.
In some instances it can be done in such a
way that for particular uses the distribution
of error becomes a definite advantage, as in
certain kinds of navigational maps. There is
an unlimited number of ways in which the
alterations can be arranged. The majority of
the widely used map projections have one or
more specific useful characteristics, each of
which is called a property. A property is
some attribute of the spherical surface that
has either been strictly retained or usefully
modified in the transformation process. For
18
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
example, a projection may retain on the map
the same relative sizes of areas as on the
earth. There are several important properties,
and some projections can combine several of
them; some projections have no such useful
characteristics.
The only two properties which can exist
all over a projection are those known as (a)
equivalence or equal-area, and (b) conform-
ality or orthomorphism. They are mutually
exclusive; i.e., they cannot exist together in
the same system of projection.
Equivalence A projection is said to be
equal-area when the alteration is so arranged
that at any point the maximum stretching in
one direction is balanced by a reciprocal
compression in the perpendicular direction.
The consequence is that the area of any
region on the map is shownjcorrccdy jnjsla.-
tion to the area of any other region. The
equal-area quality is extremely useful_jn
many aspects of geographical study, foirjf
correct area relationships are np^retained the
sfii9enfis likely to make incorrect inferences
re SH^SJP^~?l^5?l^ s ' Because of the
jnescapable stretching and compression, the
scale jhojvin^linear_or disja^ic^jdatjon^lujgs
will vary from place to place on the projec-
tion.
Over most of an equal-area projection the
scale is different in different directions at
each point, and this condition causes shapes
of areas, even small ones, to be deformed.
Indeed, equivalent projections always deform
shapes, in some instances to a very large
degree. Various equivalent projections arrange
the scale departures in different ways so that
the inescapable deformation of angles (shapes)
may be concentrated in the less used por-
tions of the map. The map reader must
be alert to make allowance for this. Figure
1.16, the equal-area map projection used for
many of the world maps in this book, shows
how the deformation of shape has been con-
centrated in particular areas.
Conformality The property of con-
formality is of particular importance in the
use of such maps as topographic and navi-
gational charts. To obtain this quality the
stretching and compression is arranged in
such a way that, whatever the distance or
linear scale may be at any point on the pro-
jection, it is the same in all directions at that
point. Since it is impossible to have the same
scale at every point on any flat map, and
since on conformal projections the scale
must be uniform in every direction at each
point, it follows that the scale must change
from point to point. The larger the area rep-
resented, the greater this variation will be;
on maps of the whole earth it is not unusual
for the scale to be several times greater at one
place than at another. Consequently, the rela-
tive sizes of areas must vary in different parts
of the projection. In other words, a conformal
projection must exaggerate or reduce areas
relative to one another. When the scale is
consistent in every direction at a point, earth
directions (the compass rose) will be truly
shown. This quality makes conformality use-
ful for maps on which directions from points
are important, as for example, maps used for
navigation, surveying, or plotting wind direc-
tions. Although proper angles will occur at
each point on a conformal projection, the
great-circle directions (and their azimuths or
bearings) between places far from one another
will usually not be correctly shown. Sim-
ilarly, shapes of small areas will be well rep-
resented on conformal projections, but shapes
of large areas will be considerably deformed,
as they are on all projections.
Other properties Another property of
considerable utility is that retained by those
The earth: basic facts and mapping 19
.0 80 100 120 140 160 ISO
GENERALIZED
DISTRIBUTION OF
SHAPE DEFORMATION
I I LEAST
LITTLE
MODERATE
FIG. 1.16 The flat polar equal-area projection used for many of the
world maps in this book, showing the areas where the shape deformation
has been concentrated. The darker the shading, the more the shape
deformation.
projections that are called azimuthal. This is
the quality of showing correct azimuths from
one particular point to every other point.
This can be combined with another property,
equidistance, which is the quality of showing
correct (uniform) scale distances from one
point to all other points. Such projections
are useful in plotting radii and in figuring
distances and directions of travel that follow
great-circle routes, as for example, radio
beams.
There are many other properties or qual-
ities, but most of them are limited to one
projection. Thus there is a projection which
shows all great-circle arcs as straight lines
(gnomonic); another shows rhumb lines, or
lines of steady and true bearing, as straight
lines (Mercator); while several show east-
west directions as parallel anywhere within
the representation. The properties of a pro-
jection may not be indicated in its name, and
one must then turn to a treatise on cartography
to find a description. One should never study
a large area mapped on a small-scale map
without an understanding of the distance-
direction alterations introduced by the
method of projection.
VARIETIES OF MAPS
Maps are employed to show the areal dis-
tribution of many kinds of things, and there
are consequently many kinds of symbols. In
a general way the kinds of maps and their
symbols may be arranged in four groups, but
the groups are neither all-inclusive nor even
quite mutually exclusive. The groups are (a)
maps employed to show areal extent, shape,
or outline, (b) maps for showing patterns of
arrangement, (c) maps intended to convey an
20
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
impression of relative land elevation or sur-
face relief, and (d) maps employed to show
the areal distribution of numerical values of
actual or relative quantity.
In the first group may be included all those
familiar maps containing lines, shading, and
color that show the extent or boundaries of
areas classified upon the basis of some kind
of unity. These may be countries or other
political divisions, or areas of similar geo-
logic formation, climate, landscape compo-
sition, or any other natural unity. In the
second group may be found maps showing
patterns of drainage, city streets and roads,
other means of transportation, communica-
tion, and the relative distribution of towns
and cities. In the third group are the maps
that employ lines or shading arranged to
produce the effect of light and shadow and
thus simulate the land form.
The fourth group includes many kinds of
maps with symbols, such as dots or squares
denoting area, and cubes or spheres indicat-
ing volume. Each of these symbols is intended
to express the existence of some numerical
value in a specific locality on the earth's sur-
face. In a sense such maps function as two-
dimensional, or spatial, graphs. To a degree
their usefulness is in inverse proportion to
the size of the areal units for which the
values are shown. Thus a few dots or squares,
each representing a large unit of value and
covering a large area, show generalities. On
the other hand, many symbols, each repre-
senting a small unit of value and distributed
properly within small units of area, show the
details of a distribution.
Most such maps and their symbols are self-
explanatory; if some are not, their specific
quality may be determined by reference to
the legend which usually accompanies a map.
Care must be exercised by the map reader,
however, not to "read into" a map a greater
degree of accuracy or precision than is war-
ranted by its symbols or scale. Except on
very large-scale maps, all information pre-
sented must be generalized, i.e., simplified in
some way. For example, many of the maps
in this book show distribution of such things
as climatic regions, soil areas, and land-form
differences by shading areas differently or by
separating areas with lines; yet such shading
and lines frequently represent only average
conditions. Similarly, coast lines, civil bound-
aries, roads, wind-direction lines, average
rainfall amounts, and other information must
be greatly simplified on maps. This is neces-
sary for two reasons: first, details cannot be
represented on small-scale maps; second, the
fundamental facts and patterns of distribu-
tions are not so apparent when unnecessary
detail is included.
Isarithms One frequently used carto-
graphic device employed to show distributions
of quantity on maps are the successive lines,
each of which is drawn through all points
that have the same numerical value. Such
lines are called isarithms (Gr. isos, equal +
arithmos, number). When they represent rel-
ative values expressed as ratios, such as the
number of persons per square mile or the per
cent of land in crops, the lines are sometimes
called isopleths or simply isolines.
Isarithms are used to show distributions of
many elements of geography, and these
isarithms are sometimes named by combining
the prefix iso with a term derived from
the type of data. Hence one speaks of iso-
therms (temperature), isobars (air pressure),
isobaths (water depth), and many others.
An isarithmic map is often employed,
especially on large-scale maps, to show the
surface irregularities of the land. Its isarithms
are isohypses (Gr. hypos, elevation), com-
FIG. 1.17 A clay mound in a tank showing
(a) the marked positions of successive water levels
with a 1-in. interval between each, and (b) the
positions of the water levels (contours on the
mound) as viewed from directly above, i.e., as they
would appear when mapped.
monly called contours. A contour is a line
that passes through points that have the same
elevation above sea level on the surface of
the earth. The general idea of contour lines
and the significance of their spacing and
irregularities may be made clear by a simple
illustration. The concepts thus explained
may then be extended to the interpretation of
the patterns of other kinds of isarithms.
If in an open tank we mold, from clay or
some plastic material, an oval mound 6% in.
high that slopes steeply at one end and gently
at the other, and if exactly 6 in. of water is
permitted to flow into the tank, then only
% in. of the mound will protrude above the
water level. With *a sharp point the position
of the water upon the mound can be marked.
The water level can then be lowered by 1-in.
stages and the water position at each stage
successively marked on the mound. The
marks will appear as contour lines on the
The earth: basic facts and mapping 21
mound, the lowest being everywhere 1 in.
above the bottom of the tank, the next 2 in.,
and so on to the sixth, as represented in Fig.
1. 17 a. If the mound is viewed from directly
above, as a map is, the arrangement of the
lines will be that of Fig. 1.176. From such a
pattern of lines certain conclusions may be
drawn which may be universally applied to
the interpretation of contour and other
isarithmic maps. Most important of these are
that where the slope is steep the lines are
close together, and that where the slope is
gentle the lines are more widely spaced. On
our little model the contour lines have a ver-
tical separation of 1 in. This is called the
contour, or isarithmic, interval. The numbers
on the individual lines show the elevations
the lines represent.
Few hills in nature are so smooth as this
mound, and the example may be made more
real by introducing a pair of gullies or
troughs on its side (Fig. 1.1 8a). If the sub-
mergence is then repeated, and the lines re-
FIG. 1.18 (a) The effect that surface
irregularities have on isarithms. (b) Whenever
contours or isarithms cross sloping troughs
(valleys) or ridges (spurs) the apexes of their bends
point upslope and downslope, respectively.
22
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
drawn, the contour lines will enter each
gully, cross its bottom, and come out its
other side. If the pattern of the lines, as
viewed from above, is transferred to a map,
the arrangement will be like that shown in
Fig. 1.186. From the arrangement of these
lines other general conclusions become ap-
parent. One is that when a contour, or any
isarithm, crosses a valley or trough in any
sort of distribution, it does so by a loop, the
closed end of which points in the upslope
direction. Between the two gullies is a ridge.
On the contour map of the mound the con-
tour lines that emerge from the gullies and
pass over the ridge appear to loop so that
their bends point in the downslope direction.
An illustration of these principles is Fig. 1 .19,
which shows "natural" contour lines marked
on hill slopes as a result of wave work per-
formed in a reservoir at different stages in the
lowering of the water level.
MAPS AND PHOTOGRAPHS OF THE
EARTH'S SURFACE
For some 200 years man has been making
detailed large-scale maps of the land which
have helped him slowly extend his knowledge
of the character of the earth's surface. These
maps, called topographic maps, are a pre-
requisite to the scientific study of the earth's
surface.
For a very much shorter time man has also
been able to take precise photographs of the
earth. The air photograph is somewhat like
FIG. 1.19 "Natural" contours on emerging slopes. Wave-cut lines on
slopes result from the intermittent withdrawal of water from an
irrigation reservoir. (Taylor-Rochester.)
The earth: basic facts and mapping 23
FIG. 1.20 Highly generalized map of topographically mapped areas of
the world. The black areas show regions generally covered by medium- and
large-scale topographic maps.
the topographic map since both are record-
ings of existing conditions. By the recogni-
tion of different tones of dark and light and
various tonal patterns on a photograph, one
is able to identify many of the physical and
cultural elements of geography. With the aid
of a stereoscope, overlapping air photographs
may be studied in three dimensions. The un-
generalized and realistic appearance of the
earth's surface in air photographs makes them
a useful aid in revealing and understanding
the complexities of geography. But the air
photograph is not the same as a map, since it
records everything a camera "sees," whereas
the map is a result of a process of human
selection and interpretation.
Many of the world's topographic maps are
made from air photographs by analyzing both
their geometric properties (photogrammetry)
and the earth's image on them (photo inter-
pretation). The air photograph thus consti-
tutes a mapping tool as well as an interpretive
companion of the topographic map. The
detailed knowledge of any part of the earth
is gained, in part, from its maps and its air
photographs; consequently a knowledge of
how well the earth has been mapped and
photographed indicates generally how much
is known about its various parts. (Figures
1.20 and 1.21 show the extent of world
coverage.) Yet it should be remembered that
maps and photographs of some areas are old,
of poor quality, or difficult to obtain, so that
generalizations about man's knowledge of the
earth based on its mapping and photograph-
ing must be made with care.
Almost all of the United States has been
photographed from the air at least once,
many parts of it several times. Most of the
photographs are at a scale of about 3 in. to
1 mile (1 : 20,000); and since the photographs
overlap one another, there is a tremendous
The earth: basic facts and mapping 25
United States were available in 1956. (U.S. Geological Survey.)
number of them. Topographic maps made by
the U.S. Geological Survey and other govern-
mental agencies are now available for more
than half the area of the United States (Fig.
1.22). The standard United States topographic
map includes a quadrangle either of 015' or
07W of latitude and longitude. They are
commonly printed at scales of 1 : 62,500
(approximately 1 in. to 1 mile) and 1 : 24,000
(approximately 2^ in. to 1 mile). Some maps
are printed at other scales. 1
The topographic maps are printed in three
or four colors, each showing a class of in-
formation. In black are generally shown those
features that may be called cultural, i.e., pro-
1 The Map Information Section of the U.S. Geological Sur-
vey, Washington 25, D.C., regularly publishes up-to-date
indexes of the status of topographic mapping and air pho-
tography and the agencies from which the maps and photo-
graphs are available.
duced by man, such as roads, houses, towns,
place names, boundary and rectangular-
survey lines, and the parallels and meridians.
In blue are printed all water features, both
natural and man-made, such as streams,
marshes, drainage ditches, lakes, and seas;
their various subclasses are distinguished by
appropriate symbols also in blue. Areas
covered by timber or woodland are sometimes
shown in green. The contour lines, the num-
bers, and the other symbols related to the
elevation of the land surface are printed in
brown. In recent years tonal shading has been
applied to many topographic maps, including
some of those of the U.S. Geological Survey,
in order to enhance the visual impression of
the terrain. A portion of such a map is
shown in Fig. 1.23.
Each map is provided with a place tide, a
scale, and a statement of the contour interval
26
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
1
F I G . 1 . 2 3 A portion of one of the modern topographic maps of the U.S.
Geological Survey with terrain shading. These are among the best
topographic maps made anywhere.
used on that map. The contour interval em-
ployed is usually either 10, 20, 50, or 100 ft.
On the maps of extremely flat land, intervals
as small as 5 ft, or even 1 ft, are used; but
on maps of rugged mountains, intervals are
sometimes as much as 250 ft. Both the scale
and the contour interval of each map must
be ascertained before correct interpretations
of the map can be made,
Facility in interpreting maps and air photo-
graphs comes only with experience. In the
chapters to follow, the nature and arrange-
ment of many of the land forms to be de-
scribed can be made much clearer and more
realistic if use of the text is supplemented by
selected topographic maps and air photos. It
is hoped that some of these will be available
to the reader, and that he will learn to read
them so that they may contribute to his
understanding of land forms in the natural
environment. A representative list is given in
the appendix.
The earth: basic fads and, mapping 27
SELECTED REFERENCES
Chamberlin, Wellman: The Round Earth on Flat Paper, National Geographic Society, Wash-
ington, D.C., 1947.
Greenhood, David: Down to Earth, 2d ed., Holiday House, Inc., New York, 1951.
Monkhouse, F. J., and H. R. Wilkenson: Maps and Diagrams, Methuen & Co., Ltd., London,
1952.
Robinson, A. H.: Elements of Cartography, 2d ed., John Wiley & Sons, Inc., New York, 1960.
U.S. Department of Defense: Interpretation of Aerial Photographs, TM 5-246, 1942.
CHAPTER 2
The varieties
of surface
form
CHARACTERISTICS OF LAND SURFACES
The earth upon which man lives is char-
acterized by a great and often pleasing variety
of surfaces. High lands and low, level ex-
panses and steep slopes, plains, tablelands,
hill lands, and mountains are arranged in
endless combinations. Because there are so
many types of surfaces, it may seem that they
are distributed over the earth without order
and that an understanding of their nature
and arrangement, or even a systematic descrip-
28
tion of them, is beyond the ability of the
beginning student. Actually, however, the
land surface is quite capable of objective,
specific, and, if desired, quantitative de-
scription.
If many small areas are carefully compared
in order to determine precisely how the land
form of each is unlike that of the others,
there is soon accumulated a long list of
specific differences in the terrain samples.
And if this list is analyzed, it becomes ap-
parent that the many differences may be
grouped under the four major headings of (a)
slope, (b) surface material, (c) arrangement,
and (d) dimensions. That is, the differences
between any two sections of the land surface
may be expressed in terms of these four
major topics.
Slope Slope refers simply to the incli-
nation of the land surface at a particular spot
Normally any section of the surface measuring
a few miles across is made up of many small
bits of sloping land, each one differing from its
neighbors in steepness. Steep slopes, gende
slopes, and slopes of intermediate steepness
may all be present in a single area. However,
there is a great difference between one area
The varieties of surf ace form 29
and another in the predominance of each of
these major slope classes. For example, a
section of the Texas coastal plain near
Houston may be 95 per cent occupied by
very gentle slopes, while in a section of hilly
southwestern Wisconsin only 30 per cent of
the area may be gently sloping, with inter-
mediate and steep slopes occupying the
greater part of the area (Fig. 2.1). It is doubt-
ful that any other bit of information could
tell as much about the fundamental contrast
between these two regions. The figures not
only suggest the contrasting appearance of
the areas but also hint at important differences
in the usefulness of the land.
Surface material Most of the earth's
land surface is covered with relatively fine-
F I G . 2 . 1 An example of contrast in slope, (a) is from the Driftless Area
of southwestern Wisconsin, (b) from coastal plain near Corpus Christi, Texas.
(From U.S. Geological Survey topographic sheets: Boaz, Wis., and Petronilla, Tex.)
30
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
B c
v <*&
u>"-
I I Soil
rock
I . . . '."1 Swamp
Lake
F I G . 2 . 2 An example of contrast in nature of surface material, (a) is
from rolling prairies of northwestern Missouri, (b) from morainic plains of
northern Minnesota, (c) from southern Alaska. (From U.S. Geological Survey
topographic sheets: Bethany, Mo., Ely, Minn., and Seward A-8, Alaska.)
particled mineral matter with some partially
decomposed organic debris mixed in. Where-
ever such soil (using the term in a very broad
sense) does not make up the surface layer, it
is a fact worth knowing. Surfaces of bare
bedrock, of loose sand, of cobbles and
boulders, of permanent ice, and of standing
water are fundamentally different from soil
surfaces, in appearance and feel as well as in
origin and function. The character of the
bedrock many feet below the surface and the
chemical and detailed physical properties of
even the surface layers do not as a rule be-
long in a list of terrain elements. But clearly
the gross physical nature of the surficial ma-
terials cannot be omitted from a terrain de-
scription without running the risk of serious
misrepresentation. It would, for example, be
futile to attempt a characterization of Finland
or of much of northern and eastern Canada
without mentioning almost at the outset that
standing water and exposed bedrock together
probably occupy as much or more of the area
than is covered by soil. The icecap of Ant-
arctica, the sand-dune seas of the Libyan
Sahara, and the great coastal marshes of
South Carolina and Georgia all owe much of
their distinctive character to their unusual
surface materials (Fig. 2.2).
Arrangements Arrangements are the
relative positions of features within an area.
Streams, ridge crests, peaks, areas of gentle
slope, steep bluffs, and exposures of bare
rock are all set upon the land surface in dis-
tinctive groupings that vary from place to
place. Some of these arrangements, or patterns,
may be best seen from an airplane or on a
map. In some regions pattern is one of the
most striking of all characteristics, especially
where it departs from the usual treelike ar-
rangement of valleys or streams and the
divides between them. The remarkably par-
allel arrangement of ridges in the middle belt
of the Appalachians between central Pennsyl-
vania and northern Alabama, the random
dotting of small isolated volcanic hills on the
The varieties of surf ace form 31
C
^^<L_ Ridge crests
F I G . 2 . 3 An example of contrast in pattern of ridge crests and summits,
(a) is in the Driftless Area of southwestern Wisconsin, (b) in the Appalachian
Ridge and Valley region of central Pennsylvania, (c) in an area of volcanic
Cones in SOUth central Oregon. (From U.S. Geological Survey topographic sheets:
La Fatge, Wi\., Orbiwnia. Pa., and Newherry Crater, Ore.)
J 3'
plains of south central Oregon, and the aim-
less maze of lakes, swamps, and streams in
northeastern Minnesota are indispensable to
any meaningful description of these regions
(Fig. 2.3). The patterns distinguish the char-
acter of the terrain; they are highly signifi-
cant clues to the geological history of the
region; and they are clearly reflected in other
geographical patterns, such as those of soils,
of native vegetation, and of agricultural utili-
zation of the land.
Vertical arrangements are also significant,
especially profiles, a profile in this sense being
the change of slope or gradient along a given
line. Included here are such characteristics as
the cross-section forms of valleys; the even-
ness, jaggedness, or presence of deep clefts
in major mountain crests; and the various
changes in gradient of streams from their
headwaters down to their mouths. Regional
contrasts in these are sometimes striking and
important (Figs. 2.4, 2.5). The student of
FIG. 2.4 The continuously high crest line of the Sierra Nevada of
California (a) contrasts sharply with the deeply serrated crest of the Cascade
Range in Washington (b). The openings near the ends of the lower profile
are railroad tunnels. (From Army Map Service series V502: Fresno and Wenatchee
\heeis.)
A.
01234 5 Miles
Vertical exaggeration 3.3 X 1
32
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
Vertical exaggeration 3.3x1
FIG. 2.5 Example of contrasting transverse profiles in areas of high
relief, (a) is from Basin and Range province in Nevada, (b) from Colorado
Plateaus in northern Arizona, and (c) from Rocky Mountains of Idaho.
(From U.S. Geological Survey topographic sheets: Sonoma Range, Nev., Diamond Creek,
Ariz., and Lolo, Idaho.)
earth history finds cross-section profiles of
valleys and divides and the longways profiles
of streams especially valuable, for they may
sometimes be used to determine previous up-
lifts of the earth's crust in an area, earlier
variations in the volume of its streams, and
the effects of the nature of the rocks upon
the processes of erosion. These examples
FIG. 2.6 Example of contrast in texture, or spacing of valleys and
ravines. The patterns are similar, but the textures are strikingly different,
(a) is from Badlands of southwestern South Dakota, (b) from central
Missouri. (From U.S. Geological Survey topographic sheets; Cuny Table East, S. Dak.,
and Nehon, Mo.)
A
Valleys and Ravines
The varieties of surf ace form 33
VERTICAL EXAGGERATION 2x1
1 2
Miles
FIG. 2.7 Contrasting local relief in two areas of rough lands, (a) is from
Missouri Ozarks, (b) from Appalachians in central West Virginia.
(From U.S. Geological Survey topographic sheets: Round Spring, Mo., and Bald Knob, W. Va.)
will suggest that profiles of the terrain may
relate also to other aspects of geography, in-
cluding utilization of the land by man.
Dimensions Dimensions give scale to
the characterization. Without a knowledge of
such numerical values as the height of
ridges, the width and depth of valleys, the
spacing of streams, and the size of patches of
gently sloping land, it is impossible to visu-
alize a landscape that is being described.
Important among the dimensions in the
horizontal plane are the spacings of valleys,
ridges and streams, and the widths of patches
of gentle and steep slope, of bodies of water,
and of patches of particular kinds of material.
Areas similar in other characteristics are
sometimes strikingly different in horizontal
dimensions (Fig. 2.6). Areas in which widths
and spacings of features are relatively large
are spoken of as coarse-textured; those in
which horizontal dimensions are small are
fine-textured.
In the vertical direction, the dimensions of
the terrain of a limited area are given by
various expressions of local relief, or differ-
ence in elevation. For a general expression of
local relief, the difference in elevation between
the highest and lowest points in the small area
is sometimes used. Or a figure may be used that
indicates the average or prevalent height of
crests above the adjacent valley bottoms in
the area. The relief along crest lines, on local
uplands, and along valley floors or streams
may also be of interest. Local relief is a char-
acteristic of considerable descriptive value,
suggesting at once something of the scale of
features and the degree of irregularity within
the area being considered (Fig. 2.7). If the
local relief in an area is only 50 ft, it is evi-
dent that the surface must be either nearly
flat or marked by only small roughnesses.
But a local relief of 5,000 ft immediately sug-
gests a landscape of considerable grandeur,
though without specifying what form its great
features may take. When combined with data
on slopes and profiles, local relief is one of
the most revealing of all generalized expres-
sions of terrain character.
TYPES OF LAND SURFACES AND THEIR OCCURRENCE
THE CONCEPT OF
TERRAIN TYPES
In the preceding paragraphs it has been
suggested that a land surface, even a com-
sidering it in terms of specific characteristics.
Many can be described in quantitative terms,
others with reasonable precision by words or
diagrams. A complete and systematic descrip-
tion of the land surface of an area employs
plex one, can be effectively analyzed by con- all available techniques verbal, numerical,
34
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
cartographic, and pictorial in order to
speak objectively, precisely and clearly. With
such specific and systematic information, it
becomes possible to compare, characteristic
by characteristic, any one bit of the earth's
surface with any other bit.
When large numbers of terrain samples
from all over the world are so compared,
certain combinations of major characteristics
occur again and again in widely separated
places. Given these similarities, a number of
distinct types of terrain that may be recog-
nized whenever they occur, and that together
make up the entire surface of the continents,
may be defined at least in the general terms
of a few major characteristics. They are com-
parable to the climatic types defined in later
chapters.
MAJOR CLASSES OF
LAND SURFACES
The scheme of land-form types to be used
here is based upon similarities and differences
with respect to three major characteristics:
relative amount of gently sloping land, local
relief, and generalized profile. On the basis
of the first two characteristics alone we may
distinguish among (a) plains, having a pre-
dominance of gently sloping land, coupled
with low relief, (b) plains with some features of
considerable relief, also dominated by gently
sloping land but having moderate to high
local relief, (c) hills, with little gently slop-
ing land and with low to moderate relief,
and (a) mountains, which have little gently
sloping land and high local relief.
The second group, plains with some fea-
tures of considerable relief, may be further
subdivided on the basis of whether the exist-
ing large amount of gently sloping land
occurs in the lower part of the profile or in
the upper part. If most of the gently sloping
land lies at relatively low levels, with steep
slopes rising above it, the surfaces may be
designated plains with hills or mountains. If,
on the other hand, most of the nearly level
land lies relatively high, with canyon walls or
long lines of bluffs (escarpments) drop-
ping down from it, the surfaces may be called
tablelands. If the relief is slight or if the
amount of gently sloping land is not large,
this profile distinction is less fundamentally
significant, so it is not used here as a basis
for subdividing plains, hills, and mountains.
Figures 2.8 and 2.9 give examples of the
principal terrain classes. Figure 2.10 shows
schematically how the classes are defined.
It must be fully realized that within each
of these five major classes of land surfaces,
which have been defined in terms of only two
or three characteristics that seem particularly
important to visualization or to utility, there
exists a vast variety, based upon differences
in other characteristics. Some plains, for in-
stance, are conspicuously flat and swampy,
others are rolling and well-drained, and still
others are simply broad expanses of smooth
ice. Similarly, some mountains are low,
smooth-sloped, and arranged in parallel
ridges, while others are exceedingly high,
with rugged, rocky slopes and great glaciers
and snow fields. The subdivision that has
been outlined is intended to bring out only
the most striking contrasts among land sur-
faces and to provide a general basis for sys-
tematic discussion of land surfaces and their
origin as well as the general surface character
of the various continents.
WORLD PATTERN OF
LAND FORM
DISTRIBUTION OF LAND SURFACES
Plate 3 shows the distribution of the
major land-form classes over the earth. In
The varieties of surface form 35
FIG. 2.8 Examples of the three smoother
classes of terrain: (a) rolling plains in
southwestern Iowa; (b) Canyon de Chelly National
Monument in northeastern Arizona, a remarkably
clear-cut example of a tableland; (c) Hopi Buttes
near Winslow, Arizona, a plain with hills and
mountains, [(a) Soil Conservation Servur; (b) and
(c) Speme Air Photon]
36
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
FIG. 2.9 Examples of the two rougher classes of terrain: (a) an
extensive hill land, the Lammerlaws, in the southern part of South Island,
New Zealand, and (b) high mountain country in the Alaska Range.
[(a) White\ Aviation Photograph; (b) Knit If. Stone.]
The varieties of surf ace form 37
addition to the five classes defined above,
several significant subclasses have been
shown. The conspicuously flat plains have
been distinguished from the more irregular
ones, the mountains of particularly high re-
lief have been separated from the rest, and
the few broad icecaps of the world have
been shown by an entirely separate symbol.
The following table, which was derived
directly from Plate 3, shows that the major
types of land surfaces are neither equal in
total extent nor evenly distributed among the
continents. The more irregular types of plains
are especially widespread, suggesting that
conditions favoring the development of such
surfaces have been common in late geologic
time. On the other hand, the generation of
tablelands and flat plains requires sets of
circumstances that have not occurred so
widely. It will be seen later that each of these
kinds of surfaces demands rather specific and
limited circumstances to arise at all.
FIG. 2.10 How the principal classes of terrain
are defined and how they are related to each other.
COMPLEXITY OF THE WORLD PATTERN
The world's land-form pattern is undenia-
bly complex. However, a study of Plate 3 and
other maps of surface form reveals broadly
systematic arrangements and general similari-
ties and variations among the continents that
help to reduce the apparent chaos.
Percentage of Continental and World Land Areas Occupied
by Major Land-surface Types
Australia
Eurasia Africa New Antarctica World
nmcrica nmcnca *-* \ \
Zealand
Flat plains
7
18
2
1
4
5
Rolling and
irregular plains
30
29
30
44
51
31
Tablelands
6
14
3
5
1
5
Plains with hills
or mountains
9
7
10
22
19
11
Hills
15
8
11
11
12
10
Low mountains
9
13
21
13
12
14
High mountains
16
11
23
4
1
13
Icecaps
8
100
11
Percentage of
world area
16
12
36
20
6
10
100
38
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
A useful starting point in considering the
world pattern is the cordilleran belts, the
great bands that contain most of the world's
major mountain systems together with various
basins and lands of lesser roughness. The
principal cordilleran systems form a nearly
continuous ring about the Pacific Ocean
Basin and thrust a long arm westward through
southern Eurasia to the Atlantic. Sometimes
they are described as a group of three arms
radiating from a "knot" in the Pamir region
of the Afghanistan-U.S.S.R. frontier (Fig.
2.11). Thus both of the Americas and Eura-
sia have long cordilleran segments running
along one side of the continent, forming for
each an immense "backbone" to which the
less rugged remainder of the continent is at-
tached. Africa and Australia lack such well-
marked cordilleran bands, though in each of
them there is a relatively rough zone running
from north to south through the eastern part
of the land mass.
In the Americas and Eurasia there is,
toward the Atlantic margin, secondary rough
land, less rugged than the cordilleran belt
opposite it. Between the two rough belts lie
the most extensive plains of these three con-
tinents. In Africa and Australia the pattern is
more patchy, with large areas of plains inter-
mingled with areas of moderate roughness
comparable to the secondary rough lands of
the cordilleran continents. The Antarctic
Continent is so largely covered by ice that
little is known of the form of its bedrock sur-
face except that it is in part ruggedly moun-
tainous. It is believed possible that the
"continent" beneath the ice may actually
involve more than one land mass.
INDIVIDUAL CONTINENTS
North America North America is a
roughly average continent in the proportional
occurrence of the various types of land form.
Along its western side is a broad and com-
F I G . 2.11 The great cordilleran belts of the world are all interconnected.
They may be considered as three great arms radiating from the Pamir knot,
two of them embracing the Pacific Ocean Basin, the third reaching westward
across southern Europe.
CORDILLERAN BELTS
|H CORDILLERAN BELTS
DC
['.'."".'.1 OTHER HIGHLANDS
plex cordilleran belt that occupies most of
Alaska, more than a quarter of Canada and
the United States, and all but an interrupted
east-coastal strip in Mexico and Central
America. In the United States and northern
Mexico, where it achieves a width of about
1,000 miles, the cordillera is made up of
loosely linked mountain strands separated by
extensive basins and tablelands, considerable
parts of which do not drain to the sea. North
and south of this section it becomes narrower
and more continuously rough.
The extensive secondary rough land of
North America includes the Ozark-Ouachita
and Appalachian-New England areas, irregu-
lar margins of the great Canadian Shield, and
the major portion of the Arctic Islands. Most
of this area is made up of hills and low
mountains, but in the eastern Arctic the rug-
gedness becomes extreme and icecaps be-
come prominent. Greenland accounts for
nearly one-tenth of the world's area of icecap.
The North American plains lie between
these two bands of rougher land, with a fur-
ther extension along the Atlantic Coast in the
southeastern United States. The interior
plains, with significant exceptions, are irregu-
lar and rise gradually toward the west, reach-
ing elevations of 3,000 to 6,000 ft at the base
of the Rocky Mountains. The northern sec-
tions are notable for their abundant lakes
and swamps. The southeastern plains are low
and frequently marshy near the coast, rising
and becoming better-drained and more
irregular inland.
South America South America is simi-
lar to North America in having a western
cordillera, secondary rough lands to the east,
and extensive plains between. However, the
nature of the three parts is strikingly different.
The South American cordillera, the Andes,
is higher, more continuous, and much nar-
The varieties of surf ace form 39
rower than that of the northern continent, its
greatest width being no more than 500 miles.
Except in the extreme north and south there
are no real breaks in the mountain wall, and
in the central section the elevation of the
divide continuously exceeds 10,000 ft for a
distance of 2,000 miles. In the widest central
section are broad basins at great elevation,
strongly resembling those of the high uplands
of Tibet.
The secondary rough land is in two sec-
tions, separated by the lower Amazon plains.
The northern and smaller section, known as
the Cuiana Highlands, is largely a loose
array of groups and ranges of low mountains
rising from the plains. The much larger
Brazilian Highlands are a great platform that
rises gradually from the Amazon lowlands
toward the southeastern margin, where it
drops abruptly to the sea. Most of the rough
part of this section is along the high south-
eastern edge; the interior parts are upland
plains and tablelands.
The plains of South America occupy
nearly half the continent and form the major
part of the drainage basins of its three greatest
river systems, the Orinoco, the Amazon, and
the Paraguay-Parana. They lie at low eleva-
tions and contain (especially the southern
basin) larger areas of flat land than are found
in North America. The plains reach broadly
to the Atlantic near the river mouths, and on
the west abut directly, at low elevation,
against the foot of the Andes. Only in the
extreme south is there a more elevated sec-
tion, the Plateau of Patagonia, a counterpart
for the High Plains of North America.
Eurasia Eurasia is by far the largest, the
roughest, and the most complex of the con-
tinents. Less than one-third of its area is plain,
and that is split into numerous pieces. Like
the Americas, it may be regarded as having a
40
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
cordillera, a secondary rough land, and inter-
vening plains, but these are vastly different
in form, position, and proportion from those
in the New World continents.
The Eurasian cordillera covers nearly half
of its land mass. In the west, from Spain to
western Iran, it is of moderate width, and is
composed of loosely linked mountain systems
separated by broad, low basins, some drowned
by the sea, and broken by numerous low,
transverse gaps such as the valley of the
Rhone River and the straits of the Bosporus
and Dardanelles. Two strands of mountains,
completely separated from the rest by the
narrow straits of the Mediterranean, form the
Atlas Mountains of northwest Africa. In the
Middle East the basin levels become higher
and the low breaks disappear. In and just
beyond the Pamir knot the mountains surge
to extreme heights and begin the fanwise
spreading that carries the cordillera into all
parts of eastern Asia. This section of the con-
tinent, with its tangled ranges and huge inter-
montane basins, is the most complex and
extensive cordilleran area in the world. The
belt even continues into the sea along the
rugged island chains of Indonesia and Japan
and hurdles the Bering Sea to join with the
backbone of the Americas. Along the northern
margin of the cordillera are a number of areas
of hills, low mountains, and tablelands, the
largest being the extensive upland of central
Siberia.
The principal plain of Eurasia occupies
much of the northwestern quarter of the con-
tinent. Like the plains of South America this
great wedge of smooth land lies at low ele-
vations, even to the very foot of the cordil-
leran ranges. As in North America the north-
ern part of the plain contains many lakes and
swamps. The flat lowland of northwestern
Siberia is the most extensive swampland in
the world. The dry southeastern part of the
plain does not drain to the open ocean but
to the immense salt lakes called the Caspian
and Aral Seas. In addition to this most ex-
tensive plain, Eurasia possesses several others
of considerable size, most notably those of
Iraq, of north India and Pakistan, and of
north China and Manchuria.
The secondary rough land of Eurasia is
small and divided, occupying the Scandina-
vian Peninsula and much of the British Isles 1 .
It is largely open hill-and-low-mountain
country, though in Norway it achieves con-
siderable elevation. The Ural Mountains form
a curious and unique isolated north-south
band of rough country in interior Russia.
The Arabian and Indian Peninsulas are an
element in Eurasian geography that has no
counterpart, for location, in the Americas.
These peninsulas, lying beyond the cordil-
lera, are both tilted platforms, highest on the
west, that bear some resemblance to the
Brazilian Highlands and are even more closely
related to sections of Africa. The Arabian
Peninsula may in all respects be considered
a detached fragment of Saharan Africa, just
as the African Atlas is a detached strip of
Europe. There is little to distinguish its sur-
face from that of northeastern Africa, which
faces it across the Red Sea. Peninsular India
is higher and more irregular, much like
southeastern Brazil and parts of west central
Africa.
Africa Africa differs from the three con-
tinents already discussed in that it has no true
cordilleran backbone. High mountains are
scarce. Surfaces smoother than hill lands
make up nearly three-quarters of the total
area. From the southern Sahara southward,
Africa may be regarded as a series of broad,
shallow basins separated by somewhat higher
swells or thresholds. Generally speaking, the
basin surfaces are largely plains, while the
swells are commonly hill-studded plains or
are carved into hills or low mountains. The
long line of swells that traverses eastern
Africa from north to south is especially high
and in many places rugged and broken. The
detached block of Madagascar is somewhat
similar to the rougher sections of this swell.
This central and southern part of Africa is
moderately elevated (principally to the south
and east), and the outer swells drop with
varying degrees of abruptness to the sea or to
relatively narrow coastal plains.
Saharan Africa is generally lower than the
southern part of the continent. It is largely
plain and low tableland, with several areas of
hilly or mountainous terrain. The trough of
The varieties of surf ate form 41
the Red Sea and the rugged swell adjacent
to it separate the Sahara from the similar
Arabian peninsula.
Australia Australia somewhat resembles
northern Africa, being largely low-lying and
smooth-surfaced. Along the eastern margin of
the continent runs a swell of moderate eleva-
tion and roughness, the closest approach to
an Australian cordillera. As in northern
Africa, most of the interior is dry, and con-
siderable areas of it do not have through
drainage to the sea.
By way of contrast, New Zealand is dis-
tinctly cordilleran, with predominantly rough
terrain that also characterizes the Indonesian
and other islands north and northeast of Aus-
tralia along the same general structural line.
CHAPTER 3
How
surface form
develops
TIME AND PROCESS
The shortness of man's span of life led
ancient peoples into a false idea of the per-
manence of the natural features of the earth's
surface. Since little change could be seen,
even over a space of several generations of
mankind, it was natural to think of the land-
scape as essentially "given" and unchanging.
Only along certain river courses and sea coasts
and near unusually active volcanoes could
significant changes he noticed, and these
42
could be regarded as exceptions, perhaps as
willful acts of the gods.
But even now, when something is known
of both the vast reaches of geologic time and
the extreme changeability of land surfaces,
man's short life is a poor yardstick with which
to measure and understand the rates of earth-
shaping events. It is like trying to express the
distance from the earth to the moon in
inches: the resulting numbers are so large
they are meaningless. It is difficult to realize
how much can be accomplished over millions
of years of geologic time by the almost imper-
ceptibly slow processes that can be seen at
work today. Yet a simple computation shows
that a canyon 1,000 ft deep could be cut in
no more than a million years by a stream
eroding its bed at the modest rate of just over
1 in. per century. In the history of the earth's
surface development, there has been abundant
time for slow processes to alter the surface
greatly and repeatedly. The lands in their
present form are in no way permanent but
represent only a momentary stage in a long
and complex history of change.
The natural processes that have produced
and are producing alterations of the terrain
How surf ace form develops 43
are in a general way known and understood,
though there is much yet to be learned about
the details of their workings. It is convenient
to group them into two major sets: (a) those
that move, or locally change the composition
of, the rocky crust of the earth (tectonic
processes), and (b) those that move material
about from place to place over the surface,
picking up here and depositing there (grada-
tional processes). These two sets of happen-
ings go on at the same time and, as will be
seen, commonly work in opposing directions.
The land surface at any moment in earth his-
tory therefore indicates the existing state of
affairs in a never-ending war between tec-
tonics and gradation.
CHANGES IN THE CRUST
Nature of the crust The rocky outer
shell of the earth was originally termed the
"crust" of the earth because it was thought
to enclose a completely molten interior.
Although it is now believed that most of the
interior is not liquid but very dense solid
material, the term has been retained to refer
to the thin outermost layer composed of the
familiar types of rocks that occur at the sur-
face (Fig. 3.1). These crustal materials are
much less dense than those of the layers be-
neath. The crust probably averages less than
20 miles in thickness, and is much thinner
beneath the ocean floors than under the
continents.
Especially important to the development
of surface form is the fact that the crust,
although it seems very rigid, is actually not
so in relation to the tremendous forces that
attack it. Furthermore, the layers immediately
beneath it, which are evidently extremely hot
and dense solid-rock materials, are capable
of very slow flowage or other deformation,
much like an exceedingly stiff or viscous
FIG. 3.1 Internal structure of the earth. The
heavy outer line represents the crust, too thin to
be shown otherwise.
20 mi.
PHYSICAL GEOGRAPHY
F I G . 3 . 2 A portion of a folded rock structure that has been exposed in
a stream valley. (U.S. Geolv^ual Smvey.)
liquid. This combination of relatively weak
crust and somewhat unstable underpinning
makes possible the deformation of the crust,
provided there are forces strong enough to
produce it. Since there is abundant evidence
that the crust has in fact been warped,
buckled, and shattered, it is clear that such
forces do exist though little is yet known of
their nature or causes.
Movements of the crust Examination
of the existing structure and arrangement of
rock layers at or near the surface of the earth
indicates clearly that over the long reaches
of geologic history the crust has been sub-
jected to almost every conceivable sort of
bending, breaking, uplift, and depression.
Rock strata originally laid down as horizontal
sheets of sediment on the shallow sea floor
are now found gently inclined, warped into
broad domes and basins, or thrown into
gigantic wrinkles such as would be obtained
by jamming an immense carpet against an
unyielding wall (Fig. 3.2). In places the crust
has broken under the stress, and the sections
on the two sides of the break have slipped
vertically or horizontally relative to one
another. When bending or breaking, sections
of the crust have been raised or lowered, so
that, for example, rocks containing fossil sea
life may be found thousands of feet above
the present sea level. Almost everywhere
rocks are more or less shattered as a result of
having been subjected to severe stresses at
some time in the near or distant past.
Examples of crustal movement A few ex-
amples may give a clearer picture of the
variety of crustal movements that can occur.
The American Middle West is an excellent
case of relatively simple and gentle warping
of the crust. Here between the Appalachian
and Rocky Mountains, the rock strata, mostly
ancient marine sediments, have been cast into
a series of shallow domes and basins. Rarely
are the beds inclined at angles of more than a
few degrees. However, because of the long
distances involved, a single stratum may ap-
pear at the surface in one locality, only to
descend to a depth of several thousand feet
below the surface a few hundred miles away.
Much of the warping appears to have oc-
curred long ago in geologic time. Even con-
sidering its gentle deformation, this area must
be regarded as one of the more stable parts
of the earth's crust.
The central strip of the Appalachians dis-
plays much more intense disturbance of the
crust. Rock strata similar in age and nature
to those in the Middle West have been folded
into a remarkable series of long, nearly paral-
lel wrinkles measuring thousands of feet in
height and several miles from crest to crest.
In many mountainous areas, such as the
Alps, the Himalayas, and parts of the Rockies,
folding of the rocks has been still more
vigorous and so complicated by fracturing of
the crust as to produce a jammed and broken
structure of bewildering complexity (Fig. 3.3).
In some of these areas it is estimated that the
crust has been shortened by several tens of
miles by horizontal compression and buckling.
How surf ace form develops 45
In some places folding appears to have
been less significant than breaking and dis-
placement (faulting) of the crust. Prominent
fractures (faults) develop, and over a long
period of time vertical or horizontal slippages
may occur repeatedly along them. By this
means large blocks of the crust may be
raised, lowered, or moved horizontally tens,
hundreds, or thousands of feet relative to
adjacent sections (Fig. 3.4). The towering
east face of the Sierra Nevada Range of Cali-
fornia owes its great height to large-scale up-
ward movement of the mountain block along
a series of faults that follow the eastern base
of the range (Fig. 3.5). Many of the smaller
ranges of Nevada and western Utah are raised
or tilted fault blocks, as is the high Wasatch
Range that rises immediately behind Salt
Lake City. In eastern Africa an extensive
series of trenchlike valleys has been formed
by movements along parallel faults in such
fashion that long strips of the crust have been
left depressed between higher blocks to
either side. The Red Sea and the Dead Sea
occupy northerly parts of this valley system
(Fig. 3.6).
It should not be thought that large-scale
folding or faulting occurs in single swift
FIG. 3.3 Simple and complex deformation of the crust, (a) shows
simple open folding in the Appalachian Ridge and Valley region in West
Virginia and Pennsylvania, (b) shows combined folding and compressional
faulting in the Rock Mountains of southeastern Idaho, [(a) After U.S.
Geological Survey Geological Folio 1 79; (b) after U.S. Geological Survey Professional
Paper 238.]
A. Simple folding
ig^fTU'
R. Comnlex fnldinff and faultinff
46
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
FIG. 3.4 Development of a normal fault in
stratified rock: (a) the strata before faulting;
(b) fault, showing direction of displacement and
the fault scarp (cliff); (c) modification of the fault
scarp by erosion. (V. C. Finch.)
cataclysms. Instead the movements proceed
very slowly, sometimes intermittently or even
jerkily. During historical times it has been
possible to record some crustal movements,
but none has been extreme in size. The coast
of the Baltic Sea, for example, has been
rising at a rate of about 3 ft per century,
while the outer part of the Mississippi River
Delta in Louisiana is apparently sinking at a
similar rate. On the other hand, displacement
along fault lines is often sudden, the result
of an abrupt giving way to stresses that have
FIG. 3.5 The rugged eastern face of the Sierra Nevada in California is
a dissected fault scarp. Mount Whitney, at right, rises nearly 8,000 ft above
the gentle slopes in the foreground. (Spenct Air Photos.)
built up over a considerable time. These
sudden movements often produce strong
vibrations, or earthquakes, that may travel
long distances through the crust. Yet observed
fault displacements in single earthquake-
producing movements rarely exceed a few
inches, and never more than a few tens of feet.
In the violent San Francisco earthquake of
1906, the maximum observed displacement
was 21 ft, indicated by the horizontal offset
of a road. It must be remembered, of course,
that however small or slow folding and fault-
ing are, the time available for their operation
is great. It is clear that processes of the order
indicated by these examples can produce
immense changes in the crust if they are
enabled to proceed over a period of a few
million years.
Molten material in the crust Nor-
mally both the crust and the layers of mate-
rial beneath it are in the solid state in spite
of the high temperatures that prevail below
the surface. Yet repeatedly during geologic
history, large masses of material in the deeper
crust or immediately beneath the crust have
become molten and forced their way toward
the surface. The complex reasons why these
molten masses develop are not well under-
stood, though the masses are known to come
into being most frequently in areas of active
crustal deformation.
The upward movement of the molten
materials and the various phenomena that ac-
company that movement are collectively
known as vulcanism. Cases are given off,
water in the ground is boiled into steam, and
the molten rock forces its way upward, partly
by melting the rocks above it and partly by
passing through fractures. The various rising
materials sometimes reach the surface, caus-
ing the often spectacular events known as
volcanic activity (extrusive vulcanism). Most
How surface form develops 47
of the molten material cools and hardens
again into solid rock before reaching the
surface (intrusive vulcanism).
In extrusive vulcanism much rock material,
as well as quantities of gases and steam,
is forced out onto the surface of the earth.
Some is emitted rather quietly as molten rock
or lava; some is vigorously blown out in
solid form, in particles varying from fine dust
to large boulders. Some volcanoes character-
istically erupt in a series of explosions, often
of tremendous force. This happens when the
vent of the volcano has been sealed over by
quick-hardening lavas, allowing extreme
pressures to build up underneath. Vesuvius
is a familiar example of an explosive volcano.
F I G . 3 . 6 An immense system of fault valleys
(grabens, or rift valleys) extends through much of
east Africa and neighboring areas.
(From Machatschek and others.)
THE AFRICAN
RIFT VALLEY
SYSTEM
48
FIG. 3.7 The volcano Parfcutin, Mexico, in
violent eruption. Typical steep-sided cinder cone.
(American Museum of Natural History.)
Krakatau, in Indonesia, and Katmai, in
Alaska, have created two of the greatest ex-
plosions of human record: each mountain
nearly destroyed itself by blasts of unbeliev-
able violence. As would be expected, the
percentage of solid material, or ash, in the
products ejected by explosive volcanoes is
high (Fig. 3.7). On the other hand, many
volcanoes emit slow-cooling lavas that have
less tendency to plug the vents. Such erup-
tions are quieter and often produce a much
lower percentage of ash. The volcanoes of
Hawaii are of this type.
Effects of crustal disturbances Both
crustal movement and vulcanism produce
two quite different sets of results that are im-
portant to the development of surface form.
First, each immediately and directly pro-
duces irregularities in the surface. But further,
each invariablv leaves behind an arrangement
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
of diverse rock materials in the crust, a rock
structure, upon which die surface-sculpturing
forces will thereafter have to work. In the
long run the second effect is the more far-
reaching. The surface irregularities are soon
destroyed or altered beyond recognition by
the sculpturing processes, while the rock
structures, reaching deep into the crust, may
persist for eons, always modifying, through
their variations in resistance, the activities of
the sculpturing processes that are at work
upon them.
Geologic structure, indeed, is important to
the development of land form largely because
of differences in the rate at which the various
structured rock materials yield to sculptur-
ing. Some rocks are much more resistant
than others, and hence will continue to stand
firm when neighboring materials have been
eroded away. Areas where resistant rocks are
at the surface will tend to remain as heights
and ridges, while weak-rock areas will be
eroded to valleys and lowlands.
The pattern of strong and weak rocks at
the surface is largely set by the structure, and
therefore by the history of crustal disturbance.
Areas where the crust has been stable nor-
mally show simpler structures and simpler
patterns of surface geology than areas where
the crust has been strongly deformed or in-
vaded by molten materials. Layered rocks
that have been tilted or folded will occur at
the surface in bands of varying thickness and
pattern,' with different bands displaying dif-
fering degrees of resistance. Intrusive rocks,
those that have cooled and hardened from a
molten state without reaching the surface,
will occur in masses, sheets, and fingers of
varying sizes, that commonly differ from the
surrounding rocks in resistance. They can
affect the course of surface sculpture only
when the rocks overlying them have been
How surface form develops 49
stripped away (Fig. 3.8). This may be long
ages after the vulcanism which placed them
occurred.
Although movements of the crust are
largely responsible for the major differences
that exist in the elevation of the surface, the
forms that occur on the elevated or depressed
sections are often not direct results of crustal
disturbance. For as soon as crustal disturb-
ance begins to produce any irregularity of
the surface, the sculpturing forces go to work
on it, cutting into the raised sections and
depositing in the low. Therefore by the time
the crustal movement is completed, the sur-
face may have been powerfully modified by
surface sculpture, and may bear little resem-
blance to the surface form that would exist
had there been no erosion or deposition. A
glance at Fig. 3.3, for example, reveals a
striking lack of correspondence between the
form of the present mountain surface and the
shape of the underlying folds.
Only where crustal disturbance has been
so recent and so rapid that the sculpturing
forces have been utterly unable to keep pace
do* land forms occur that can be said to be
distinctly tectonic in their origin. Some of
the more striking examples of this are (a)
volcanic cones, produced by the accumula-
tion of ash and lava about an active vent, (b)
extensive lava plains, formed by the emission
of quantities of highly fluid, slow-cooling
lavas, (c) fault scarps, which are cliffs resulting
from large vertical fault displacements, and (rf)
smooth domes, swells, and similar features,
usually small, that indicate relatively rapid
and recent folding. Usually even these forms
show a certain amount of gullying and other
modification by streams, glaciers, or gravity.
Strongly sculptured counterparts of them are
much more common.
But technically produced surface forms,
F I G . 3 . 8 A vertical sheet of intrusive rock
(a dike) that stands in relief because it is more
resistant to erosion than the rocks on either side
of it. Near Spanish Peaks in southern Colorado.
(U.S. G?fll<>Mal Sun<ry.)
like all others, are, by geologic time scales,
short-lived. It is probable that even a great
mountain range can be destroyed by the sur-
face forces in no more than 20 to 40 million
years, and the major portion of the destruc-
tion would occur in the first few million.
Rock structures, on the other hand, as long
as they are not stripped away by surface
erosion, will last indefinitely. The folded
structure of the Appalachians is perhaps 250
million years old, that of the Scottish High-
lands perhaps twice as old. Numerous gen-
erations of surface forms have developed and
vanished on these structures, but always their
development has been strongly affected by
the nature and arrangement of the underlying
rocks, that is, by the structure that was
formed so long ago.
World pattern of crustal disturbance
There are vast differences from place to place
over the earth in how much the crust has
been disturbed in past times and in how
active it is now. Some areas appear to have
suffered strong and repeated disturbance over
a long span of geologic time and are still
50
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
FIG. 3.9 The principal earthquake regions of the world. (After Uet.)
active. Others have not undergone anything
more severe than mild warping for hundreds
of millions of years. Still others indicate, by
their disordered structures involving ancient
rocks, that they were active zones in times
past, but show no evidence of recent
deformation.
The location of areas of past deformation
may be determined from geological maps that
show disordered structures of various ages.
Areas of present disturbance can be located
by the occurrence of earthquakes (Fig. 3.9)
or volcanic activity (Fig. 3.10), by significant
changes in level along coasts, and also simply
FIG. 3.10 The principal volcanic regions of the world. (After Karl Sapper, Vulkankunde..)
by the occurrence of mountains or other
features of high relief (Plate 3). These maps
all show the areas of greatest disturbance in
late geologic time to be especially concen-
trated in the cordilleran bands that encircle
the Pacific Ocean Basin and extend across
southern Eurasia. In earlier geologic time
several other areas were active, notably parts
How surf ace form develops 51
of the Atlantic borderlands. Especially stable
sections of the crust include central North
America, interior South America, parts of
northern Europe, and much of Africa and
Australia. The reasons for this pattern of
crustal disturbance are not well understood
as yet and will not be discussed here.
CHANGES AT THE SURFACE
THE IDEA OF
SURFACE SCULPTURE
Any process that can pick up soil or rock
material at some point on the earth's surface
and move it somewhere else can thereby
change sculpture the surface form. Where
the picking up (erosion) occurs, the surface
is lowered; where the material is subsequently
laid down, the surface is raised. In a com-
plex combination, such erosion and deposi-
tion work upon surfaces and structures modi-
fied by crustal disturbances, and the diverse
forms of the land surface are produced.
The specific natural agents that are able to
move surface materials are several: running
water (both in streams and in thin unchan-
neled sheets), glacier ice, the wind, waves,
and currents in lakes or the sea, and gravity.
Gravity, an erosional and depositional agent
in its own right, is also significant in the
working of the other agents. The result is
that when material is picked up and moved
by any agent, it most commonly comes to
rest at a lower elevation than where it began.
The long-run effect is for higher parts of the
surface to be lowered by erosion and low
sections to be raised by deposition, thus re-
ducing the over-all surface irregularity. The
surface may be locally roughened for a while
by the cutting of erosional valleys in uplands;
but eventually, through the widening of the
valleys and the lowering of the high ground
between them, the surface becomes relatively
smooth. It is for this reason that the agents
working at the surface are often called the
gradational agents.
BREAKDOWN OF
ROCK MATERIALS
Processes of rock breakdown None of
the gradational agents is able to accomplish
much in working against solid, massive bed-
rock. All are far more effective if the material
to be moved is in the form of relatively fine
grains or particles. But even coarse chunks
are more easily moved than unbroken rock.
Therefore the processes by which rock is
rotted and broken, collectively referred to by
the somewhat misleading term weathering)
are very important preliminaries to the work
of surface erosion.
There are two interrelated groups of proc-
esses that contribute to rock breakdown. One
is simply mechanical breaking of the rock, or
disintegration. The other is chemical altera-
tion of the rock substances, or decomposi-
52
FIG. 3.11 Jointing in granite. Joint planes
commonly occur in sets, all the members of which
have the same directional trend. The sets may be
vertical, inclined, or horizontal.
(U.S. Gfologual Survey.)
tion. The two go on at the same time and
actually aid one another; for on the one
hand, cracking of rock makes for easier pene-
F I G . 3.12 Angular rock debris covering
surface of ground at 11,000-ft elevation in the
Beartooth Range, Montana. At these high altitudes,
low temperatures favor mechanical weathering by
freezing water but inhibit chemical weathering.
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
tration of chemical agents (chiefly water), and
on the other hand, decomposition rots and
weakens rock masses so that they can be
more easily broken.
Mechanical breaking Rocks are broken
mechanically in several ways. Much breaking
occurs far beneath the surface as a result of
stresses produced by crustal deformation,
heating, cooling, and compaction. Thus most
rocks are found to be shot through with
small cracks (joints), and some are quite
shattered, by the time they are exposed to
the surface by erosional stripping (Fig. 3.11).
Then near the surface the rocks are attacked
by other agents. Tiny plant roots invade
crevices and pore spaces and crack the rocks
by the tremendous pressures they exert as
they grow. Water freezes in similar openings
and exerts breaking pressures as it expands
upon changing to ice. Bits are loosened from
the surface by the impact of moving frag-
ments on stream beds, by the blasting of
wind-blown sand, by slides and avalanches,
and by extreme heating under forest fires or
where lightning strikes. Some maintain that
extreme day-to-night temperature changes,
especially in desert areas, may cause enough
irregular expansion and contraction to strain
rocks to the breaking point, but this is de-
batable. Mechanical breakup therefore may
be found occurring everywhere, though most
intensively where strong crustal deformation
has taken place, where tree roots are abun-
dant, and where alternate freezing and thaw-
ing is frequent (Fig. 3.12).
Chemical decomposition Rocks are chem-
ically decomposed principally by water con-
taining in solution various substances, notably
carbon dioxide, that increase its chemical ag-
gressiveness. These substances are in part
given off by plant roots, in part released dur-
ing the decay of organic remains on and in
surface form develops 53
the soil, and in small part carried down by
rain water from the atmosphere. One would
naturally, and correctly, expect chemical de-
composition to be most rapid where water is
abundant, and least active where water is
scarce or is frozen much of the time. High
temperatures keep water unfrozen and also
favor rapidity of chemical reaction. Hence
the humid tropics should be most favored
realms for chemical decomposition, while in
the deserts, the polar areas, and the high-
mountain zones decomposition should be
slow. The truth of this is demonstrated by
the prevalence, in humid regions, of a cover
of fine-particled soil and partially decomposed
rock at the surface (Fig. 3.13). But in dry
and very cold areas such a cover is usually
patchy and thin, and the angular fragments
produced by mechanical breaking are more
in evidence, not because they are more abun-
dant there than elsewhere, but simply because
they are not covered by finer debris.
Rock resistance There are many kinds
of rocks, and they react variously to the at-
tacks of the disintegrating and decomposing
agents. These differences stem from the
chemical composition of the rock materials
and the size and arrangement of the grains or
particles of the rock.
Rocks are made up of particles of various
substances called minerals, each of which
has its own well-defined chemical composi-
tion and physical properties. Some minerals
are soluble or otherwise unstable chemically
under conditions that are common near the
earth's surface. These, of course, are especially
liable to decomposition. Other minerals are
highly stable chemically under normal sur-
face conditions and thus resist decomposi-
tion. Some minerals are physically hard and
difficult to break. Others are easily crushed
or split.
-- '
1% " " '
_
f ,
>
M i . 3 . 1 3 bearocK grading up war a into
weathered rock and soil.
Since rocks differ in the minerals of which
they are composed, they differ in their
resistance to attack. Those that are made up
largely of unstable minerals are easily decom-
posed, while those that contain chiefly stable
or hard minerals resist breakdown. Also im-
portant are the arrangement and size of the
mineral grains. In some rocks the grains are
intricately interlocked and closely spaced, so
that breakage is resisted and water penetra-
tion is slow. In others the particles are loosely
cemented or are in the form of poorly joined
plates or sheets, so that they readily break or
split. Water penetrates easily into porous
rocks, thus speeding the rotting process.
Though space does not permit a lengthy
discussion of specific rock types and their re-
sistance, a few examples will illustrate the
subject. The common rock limestone is made
up largely of the mineral calcite, which under
surface conditions is quite soluble. Thus
limestone decomposes rapidly in humid areas.
In dry regions, however, chemical attack is
less active, and the dense, tight texture of
many limestones tends to protect them against
disintegration, so that in the desert limestone
is often a relatively resistant rock. Granite is
54
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
made up of coarse grains of the minerals
quartz and feldspar. Quartz is hard and chem-
ically very stable. Feldspar, on the other
hand, is hard but decomposes fairly readily.
Thus in humid climates the feldspar grains
decompose to become clay, and the quartz
grains fall apart but retain their identity as
grains of sand. Even in dry regions granite,
being somewhat porous, is easily penetrated
by what little water there is. This causes
slight rotting on the surfaces of the feldspar
grains, which weakens the bonds that hold
the rock together and furthers disintegration.
Sandstones are usually composed of grains
of quartz cemented by some other substance.
If this substance is a weak material, such as
clay or iron oxide, the sandstone may disin-
tegrate readily. Or if the cement is a soluble
substance, such as calcite, the sandstone is
easily rotted. Some sandstones have been
cemented by silica, chemically the same as
quartz, and these (called quartzites if the
cementing is especially firm) are perhaps the
most resistant of all common rocks, chem-
ically and mechanically and in all envi-
ronments.
The importance of differences in rock re-
sistance as they affect surface sculpture has
already been suggested. Where nonresistant
rocks outcrop at the surface, rock breakup is
relatively rapid, and erosion and valley de-
velopment can proceed swiftly. In areas of
resistant-rock outcrop, materials small enough
to be moved are produced only slowly and
sparingly. Erosion is hindered, the cutting of
valleys and general lowering of the surface
lags, and the area comes in time to stand
above its surroundings because of the staunch-
ness of its foundation. This is differential or
selective erosion, an important control of sur-
face sculpture.
THE ACTUAL MOVING OF
SURFACE MATERIALS
PICKING UP, CARRYING, AND DROPPING
It is convenient, for purposes of study, to
divide the work of any sculpturing agent into
three parts. The first, erosion, includes the
detaching and picking up of material from its
original position. The second, transportation,
is the carrying of the material from the place
of erosion to the place where it is to be
deposited. The third, deposition, is the laying
down of the material at the end of the trans-
portation line. To a degree this division of
processes is somewhat artificial, for it is often
difficult to say precisely where one ends and
the next begins. It is also difficult to draw a
fine line separating rock weathering and
erosion. However, each of the processes is
governed by its own set of laws, and for this
reason it is valuable to consider them as dis-
tinct from one another.
Each of the sculpturing agents works in its
own way, with its own peculiarities and
therefore with its own results. Thus, for ex-
ample, running water has a strong tendency
to become narrowly channeled, while glacier
ice, especially if very thick, is less subject
to channeling, and wind and gravity are still
less so. Hence running water is the prime
producer of well-defined valleys; the other
agents work more broadly. The sizes of
particles that will be picked up, transported,
and deposited by running water or by wind
depend closely upon the velocity at which
either of these agents is moving. This makes
for marked place-to-place differences in
erosive power, selectiveness in sites of deposi-
tion, and a high degree of sorting by size
among the deposited materials. Ice and
gravity, on the other hand, have no particular
mechanism for sorting materials by size. Their
deposits are notably jumbled mixtures of all
sizes of particles and chunks. Because of such
differences, it is important to consider the
work of the agents separately.
THE WORK OF RUNNING WATER
Occurrence and importance A con-
siderable part of the water that falls on the land
surface as precipitation runs downslope across
the surface in response to the pull of gravity.
While it may start moving as a thin sheet of
water on the slopes, it is always seeking the
lowest place and the easiest line of flow. There-
fore it soon becomes concentrated in well-
defined channels, which progressively join
with other channels to form larger and larger
streams. Because there are few parts of the
earth's surface where there is no running
water, there are few landscapes that do not
show the effects of its sculpturing activity. It
must be rated, along with gravity, as one of
the most important of the sculpturing agents.
Water erosion Apart from the work of
waves, which will be considered later, water
erodes by four principal means: (a) the im-
pact of raindrops striking against a bare soil
surface, (b) the striking against the stream
bed of solid particles that are already being
carried, (c) the force of eddying currents in
the moving mass of water, and (d) the dis-
solving of material with which the water
comes in contact. High velocities of flow,
such as will develop where there are steep
gradients and deep waters, increase the force
of particle impact and also the amount of
eddying, and hence increase erosional power.
A cover of vegetation on the surface protects
the soil against raindrop and particle impact
and decreases the speed of flow, thereby
How surface form develops 55
tending strongly to protect the surface against
erosion. Rains of the heavy-downpour type
are productive of great amounts of surface
runoff* in a short time, and thus are especially
favorable to erosion.
Other factors affecting the erosive power
of running water are the size, cohesiveness,
and solubility of materials available for it to
move. As the size decreases from boulders
through gravel to very fine sand and coarse
silt, the ease of erosion becomes steadily
greater. Fine silts and especially very fine
clays, however, are surprisingly difficult to
erode, because of the strong tendency of the
tiny, flat particles to cling tightly together.
Hence conditions especially favorable to
rapid erosion are heavy downpours of rain,
surfaces bare of vegetation, steep gradients,
and materials that are soluble or that are
largely of fine-sand-coarse-silt size. Least
favorable are infrequent and gentle rains, a
thick cover of vegetation, flat surfaces, and
either unusually coarse or exceedingly fine
materials.
Transportation by running water
Particles that have been dislodged from the
stream bed are transported in several ways
(Fig. 3.14). Materials too heavy to be raised
from the bottom may simply be rolled or
shoved along by the force of the current and
the impact of other particles. Somewhat
smaller grains are thrown up into the current
and carried downstream until they settle,
strike the bottom, bounce up again, and so
proceed by a series of leaps. The finest par-
ticles are so light that they can be kept off the
bottom entirely by the force of the churning
eddy currents. Material in solution is, of
course, indistinguishable from the water itself.
In terms of the mode of transport it is pos-
sible to distinguish a bed load that is rolled
56
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
Finest particles
carried in
suspension
Coarser particles
bounced along
near bottom
Coarsest material
shoved and rolled
along bottom
FIG. 3.14 Ways in which solid materials are transported by running water.
or bounced along the bottom, a suspended
load carried in the body of the stream, and
a load carried in solution.
The ability of a stream to transport eroded
material depends upon the volume of water
and upon the speed of flow. Naturally a large
stream has a greater carrying capacity than a
smaller one. But with a given amount of
water, a difference in the speed of flow has
a profound effect upon the amount and size
of material that can be moved in the various
ways. As the velocity increases, the amount
of solid material that can be transported in-
creases manyfold, and larger and larger par-
ticles are shifted from the bed load up to the
suspended load. A sluggish stream may move
clay as suspended load, and small amounts
of silt and fine sand as bed load. A swift
stream, on the other hand, may move even
coarse sand in suspension and may be able
to roll sizable boulders along the bottom. It
should be noted that although clay is rather
difficult to erode, it is the easiest of all solid
materials to transport. Also, the carrying of
material in solution is not affected by velocity,
but only by the chemical condition of the
water.
Deposition by water Deposition is
simply the end of transportation. Wherever
the carrying power of a stream is reduced
below the degree necessary to handle the
existing load, some of that material will drop
out. Since carrying power depends upon
speed and volume, deposition results from a
reduction of either speed or volume. Slowing
of a stream occurs at places where the gra-
dient becomes less or where the stream flows
into a lake or the ocean, both common occur-
rences. Decrease in stream volume in a down-
stream direction is less common, but often
occurs in dry areas. There a stream may
receive little nourishment from the surround-
ing desert, while losing much water through
evaporation to the atmosphere or soaking
into the ground.
As carrying power is reduced, the first
materials to be discarded are the coarsest
parts of the bed load, followed by the re-
mainder of the original bed load and the
coarser particles from suspension. The finer
suspended load, especially clay, will remain
in suspension even at very low speeds. Dep-
osition of clay requires that the water be
virtually at rest.
Development of valleys Because run-
ning water is so readily channeled, much of its
erosional activity is concentrated along nar-
row lines of flow. The result is that the prin-
cipal work of water erosion is the cutting of
valleys. To be sure, thin sheets of water
moving down slopes also do some stripping
of surface soil, but under natural conditions
How surf ace form develops 57
this work is small compared with that accom-
plished by the channeled flow.
In stream beds the erosional force is di-
rected chiefly downward, so that valley
deepening is the principal erosional effect.
However, such downcutting cannot go on
indefinitely. The mouth of a stream cannot
be lowered as long as the sea or lake level
does not change, and the rest of the stream
bed cannot be cut below the level of the
mouth, or baselevel. The result is that as ero-
sion continues, the gradient of the stream
becomes gentler, first near the mouth and
then successively farther and farther upstream
(Fig. 3.15). But as the gradient decreases,
the speed of flow, and hence the cutting
power, also decreases, so that erosion be-
comes progressively less active and eventually
almost ceases, except perhaps for solution.
The lower reaches of the stream normally
reach this condition first, the headwaters last.
In an actively eroding stream there may be
significant irregularities in gradient, with
falls and rapids intervening between gender
stretches, as a result of different rates of ero-
sion on rocks of differing resistance (Fig.
3.16). In time, however, these features are
evened out and the gradient becomes smooth.
As a valley is being deepened, runoff water
-Ultimate level of erosion
FIG. 3.15 Idealized development of a stream
profile. Lower reaches of stream achieve gentle
gradients first, resulting in concave profile.
flowing down its sides becomes channeled
and cuts secondary or tributary ravines that
flow into the main valley. As time goes on
these become more numerous and extend
themselves headward into the upland areas
between the principal valleys (Fig. 3.17). If
runoff water is abundant, this may happen
rapidly, but if runoff is slight because of
dryness, flatness, or porous and absorbent
surface materials, tributary development may
be very slow.
Stream deposits Deposits left by streams
take the form of thin sheets of material, usu-
ally rather well sorted by size and distinctly
layered. Any stream-deposited material is
called alluvium. If the stream is flowing in a
well-defined valley, the alluvium is deposited
in the valley bottom as a long, flat strip that
becomes thicker and broader as deposition
continues. In time of flood the stream will
often spread out over the entire strip, for
FIG. 3.16 Effects of rock resistance upon stream profile. Weaker rocks
erode more rapidly and allow stream to achieve gentle gradients, while
resistant outcrops, because they yield more slowly and are undercut from
below, develop steep gradients, rapids, and falls.
-Rapids
Rapids and falls
58
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
F I G . 3 . 1 7 As a gully grows in length by headwdul er ubiun, tributary
gullies branch from its sides and grow in like manner. (F. W. Lehman^
g. Railroad Company.)
which reason such a deposit is called a
floodplain. If deposition occurs where the
stream is not in a valley, the deposit will take
the form of a broad, flat, fan-shaped sheet,
across which the stream channel may shift
rather freely. Such a deposit, if made on dry
land, is called an alluvial fan (Fig. 3.18). If
laid down in water at the mouth of a stream,
where it is normally flatter and more marshy,
it is known as a delta.
Alluvial materials, except for clays and
very coarse debris, are easily reworked and
re-eroded, especially at floodtimes, when the
streams are particularly powerful. For this
reason stream channels on alluvial surfaces
are continually being altered and shifted. If
the stream is a steep one that is depositing
sand and gravel, it tends to develop a wide,
shallow channel, choked with many sand
bars and low islands (braided channel). If
the stream is sluggish and is flowing on silty
materials, it tends to develop a very winding
(meandering) course, with looping bends that
are continually changing form and are occa-
sionally cut off from the main stream (Fig.
3.19). Between these two extremes are many
intermediate types. Channels on alluvial fans
and on deltas commonly branch in a down-
stream direction, spreading out from the head
of the deposit. Alluvial- surf ace streams will
be more fully discussed in the next chapter.
THE WORK OF GRAVITY
Gravity as an earth-moving agent
Streams and glaciers flow in direct response
to the pull of gravity, and the ways in which
they move earth materials are conditioned by
the fact that gravitational force is always
present everywhere. Indeed, the force of
gravity is well known as a contributing factor
in the operations of all other gradational
agents. But the work of gravity as an essen-
tially independent gradational agent in its own
right is much less familiar, and its importance
was in the past grossly underestimated.
Just as gravity's pull urges water or ice to
flow, so also does it exert a continuous down-
ward pull on the layer of weathered and
broken materials that blankets the surface.
That layer, of course, is rarely a fluid mass;
it is rather an agglomeration of rigid particles
and chunks of various sizes, each particle
being supported by those beneath it. But if
in any way the support is undermined or
weakened or if the material of the layer be-
comes lubricated, as it may through satura-
How surface form develops 59
tion with water, then gravity can act. The
surface mantle moves downslope, either par-
dele by particle or in large masses. Such
movements are collectively referred to as
mass movement or mass wasting.
How and where mass movements
occur Removal of support for the surface
materials is commonly caused by active ero-
sion at the foot of the slope (or of a section of
the slope). This may be accomplished by a
swift downcutting stream, a valley glacial
tongue, a stream on an alluvial plain widen-
ing its channel during floodtime, or by waves
cutting at the base of a coastal bluff. By all
of these processes slopes become oversteep-
ened, support for the upper part of the slope
becomes insufficient, and that upper part
slips down.
F I G . 3 . 1 8 A small and steep alluvial fan in Nevada. The apex of the fan
lies at the mouth of the gully from which the fan material was eroded.
(Jo/in C. Weaver.)
60
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
ON
F I G . 3 . 1 9 A sluggish stream on a floodplain commonly exhibits a pattern
of shifting meanders. A stream moving and depositing quantities of sand
and gravel often develops a complex system of intertwined, shallow,
sand-choked channels. Streams cutting on bedrock characteristically show
relatively narrow channels of irregular pattern. (From U.S. Gwhgual Survey
topographic sheets: Fairbanks D-1 , Alaska, Fairbanks C-7, Alaska, and Fairbanks A-4,
Alaska.)
One circumstance favorable for this is
present when the lower part of the slope is
based upon rock materials that are weaker
and more easily weathered and eroded than
those above. This results in oversteepening,
sometimes to the point where the resistant
rocks above come to stand in cliffs, or may
even overhang the receding slope beneath.
Another favoring factor is the presence of
slanting joints or other fracture planes in the
rocks, slippery clay layers in the subsoil, or
other such features: all may serve as surfaces
along which the "hanging" hillside material
may break away or slip down. Often nowadays
man plays a part by making excavations in
places where they undermine slopes.
Saturation of the ground is especially likely
to occur and cause mass movement as a result
of long-continued rains or the melting of
quantities of snow. It is not surprising, then,
to find that actual flowing of the surface
mantle occurs rather frequently in areas that
have a pronounced rainy season and in areas
that experience a rapid seasonal thaw, but
infrequently elsewhere.
Kinds of mass movements Mass move-
ments differ from one another in the amount
of material moved, the sizes of particles
involved, and the form and speed of the
movement.
On steep slopes the motion is likely to be
rapid, sometimes extremely so, and may
involve materials of every conceivable size,
including immense boulders. These rapid
movements may involve a single rock or a
huge mass of material. They are often pro-
duced by the gradual erosional undermining
of a steep slope, on the side of which is poised
a mantle of soil and weathered or jointed
rock. Often the dislodgment of the particle
or mass is triggered by some particular occur-
rence such as a heavy rain, the entry of water
into a joint or a separation between layers as
a lubricant, a slight earthquake, or some
excavation by man. The particle or mass
breaks loose, falls, rolls or slides down the
slope, and comes to rest near the base. Some
How surface form develops 61
such landslides and rock falls achieve tre-
mendous size and may be highly destructive
(Fig. 3.20). Most, however, are small and
inconspicuous.
Saturation of soil by heavy rain or melting
may result in actual flowage of the soil like a
thick liquid. Commonly the saturation be-
comes significant only in a limited area on a
given slope, and usually in a clayey subsoil
layer. Eventually the material of the layer be-
comes highly plastic, and a patch or tongue
of it begins to flow, usually at a slow but vis-
ible rate, carrying the surface layers along
with it. Occasionally such earthflows are
very large, but usually they are small and
move only a few yards before losing much
of their water content and coming to rest
(Fig. 3.21).
In high-latitude and high-altitude areas,
FIG. 3.20 The Gros Ventre landslide of 1925, near Jackson Hole,
Wyoming, produced an immense scar on the mountainside and temporarily
dammed the creek flowing in the valley below. (U.S. Forest Service.)
62
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
FIG. 3.21 Earthflow resulting from rain saturation of the ground on a shaly hill slope in
eastern Ohio. The lower slope shows turf bulges resulting from flowage beneath the sod; the upper
Slope shOWS tension Cracks. (U.S. Soil Conservation Srtvttr.)
the deeper ground may remain frozen for
some time after the surface layers have thawed.
The water from melting cannot escape down-
ward by infiltration, so the surface layers be-
come so wet as to be jellylike. The soil may
then ooze slowly downhill over an entire ex-
tensive area of sloping land. This process is
known as solifluction.
The most widespread, continuous, and
hence most important of all forms of mass
movement is, oddly enough, the slowest and
least evident of all. This form, called creep,
is not a single process, but rather the sum
total of all processes by which individual soil
particles can be moved a fraction of an inch
downhill. There are many such causes. The
filling of cracks, burrows, or root cavities
comes mostly from the uphill side. The
growth of frost crystals lifts particles and
then upon melting permits them to settle
downhill. Soil expands or swells when it is
wetted, heated, or frozen, and contracts
again when it dries, cools, or thaws. Such
expansion and contraction is greatest in the
downhill direction because of gravity. Soil is
forced downhill by the prying action of
wind-blown trees and shrubs or by the
weight of walking animals. In these and
other ways the soil on all slopes is slowly
and steadily moved downward, grain by
grain. Though the movement itself is imper-
ceptible because of its slowness, its results
are visible in various forms (Fig. 3.22).
Importance of mass movement Aided
only by unchanneled rain wash, mass move-
ment must accomplish all of the gradational
activity there is on the slopes and uplands
that lie between stream beds. And since these
have a total area many times as large as that
of the stream beds themselves, the accom-
plishment is great.
The swifter and more localized forms of
FIG, 3.22 Common evidences of creep:
(a) moved joint blocks; (b) trees with curved
trunks; (c) downslope bending and drag of
fractured and weathered rock; (d) displaced posts,
poles, etc.; (e) broken or displaced retaining walls;
(f) roads and railroads moved out of alignment;
(g) turf rolls downslope from creeping boulders;
(h) stone line near base of creeping soil.
^C. F. S. Sharpe, Landslide's and Related Phenomena,
Columbia University Press, Mew York, 1934. Reproduced by
permission of author and publishers.)
mass movement, such as landslides and earth
flows, usually leave behind obvious marks on
the surface. Normally there is a sunken scar
on the upper slope where the material has
broken loose, and at the lower end of the
scar a jumbled, humpy accumulation of the
debris that has come down (Figs. 3.20, 3.21).
Where prominent cliffs occur, chunks and
blocks often break loose singly, and fall and
roll to the base of the slope. Often they ac-
cumulate there in large quantities, forming
what is known as a talus slope (Fig. 3.23).
These rapid and concentrated forms of
mass movement are important chiefly in
mountainous and hilly areas where steep
slopes prevail. In many of those areas they
undoubtedly account for a large part of the
transfer of debris from the slopes to the valley
bottoms, where it may be carried away by
streams or ice tongues. It is because of their
concentrated occurrence that they are less
important, in total result, than the more un-
obtrusive creep, which occurs everywhere a
How surface form develops 63
weathered mantle lies on even a gentle
slope.
Creep, however, does not produce well-
defined landforms. It urges the entire mantle
downslope. Instead of forming scars and
localized accumulations, it serves to drive
back the entire expanse of a slope. The ma-
terial that has crept down may be carried
away by streams or other transporting agents,
or it may accumulate in a thickening sheet,
gentling the lower part of the slope and
masking it against further weathering and
erosion. Creep probably has most relative
importance in humid areas that have a thick
mantle of weathered material and a well-
established cover of vegetation. Under such
conditions creep can proceed at a significant
rate, but surface erosion outside of the stream
beds is negligible. Here, then, creep may be
the chief means of modifying the slopes be-
tween streams. On the other hand, where the
F I G . 3 . 2 3 An extensive talus slope at the foot
of a cliff. Bear tooth Range, Montana.
64
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
vegetation cover is sparse or open, surface
erosion by rainwash is much more important,
though creep will still occur.
THE WORK OF MOVING ICE
Definition of glaciers Glaciers are not
simply inert masses of ice and snow but
rather accumulations of ice so thick that they
actually move in response to gravity. The
movement is much in the manner of the flow
of very viscous liquids, like the traditional
"molasses in January." Rates of movement
of glaciers rarely exceed a fraction of an inch
per day, though exceptional rates of several
feet per day have been recorded.
Since ice must become 150 to 200 ft thick
before it will begin to flow, glaciers form
only where there is the possibilty of an un-
F I G . 3.24 Nysne Glacier, Peary Land,
northern Greenland. Note the collecting basins
from which the glacier tongue flows and the
surface markings that indicate the flowing
movement. Ridges of ice-deposited debris
(moraines) border the ice tongue, and a braided
stream of meltwater flows across the sand it has
washed out from the ice margin.
(Geodetic Institute, Copenhagen, copyright.)
usual accumulation of ice carrying over from
year to year. If more snow falls during a cold
season than can be melted during the follow-
ing summer, then the unmelted residue is
added to the accumulation of the next year.
The old, buried snow changes gradually into
solid ice under the effects of compression
and partial melting and refreezing. In a rel-
atively short time a great thickness can be
built up. Such circumstances are most often
encountered in areas having unusually heavy
winter snowfall but short and cool summers.
Antarctica and Greenland have most of
the existing ice-covered area of the earth.
Elsewhere glaciers are confined to the moister
and colder mountain regions. Dryness and
summer heat are enemies of glacier develop-
ment, and many high mountain ranges and
even some large areas within the polar circles
have no glaciers because of snowfall insuffi-
cient to last out the summer.
The ice in a glacier will always move, in
the main, downslope and away from the
center of thickest accumulation, following
the paths of least resistance (Fig. 3.24). As it
spreads beyond the region of accumulation
into neighboring areas of lower elevation,
warmer and longer summers, or less snowfall,
its outer margins are attacked by melting.
The ice will continue to spread until its edge
reaches the point of balance between the rate
of movement and the rate of melting. There-
after, as long as conditions do not change,
the edge of the glacier remains in the same
place, though the ice is in continuous move-
ment from the source to the edge. If climatic
conditions change so that the supply of ice is
lessened or melting is increased, the glacier
begins to shrink under the attacks of melting.
If melting is decreased or the ice supply is
increased, the edge of the glacier advances
until it reaches a new point of equilibrium.
How surf ace form develops 65
FIG. 3.25 Extent of former continental glaciers in North America and
Eurasia. (After Flint.)
Former continental glaciers If glaciers
had not once been more extensive than they
are now, they would be of little interest as
sculptors of the land, for the surfaces beneath
the glaciers are effectively hidden. However it
is well known that at times during the last
million years glaciers of tremendous size have
spread over large parts of the Northern Hem-
isphere continents. In North America they
originated to the east and west of Hudson
Bay and covered, at one time or another, all
of Canada and the northeastern and north
central United States. In Eurasia they devel-
oped in the Scandinavian highlands and
spread over most of northern Europe and
northwestern Siberia. Most of eastern Siberia
and much of Alaska were not glaciated in
spite of their coldness, probably because of
insufficient snowfall (Fig. 3.25).
Outside of the areas of these continental
ice sheets there was, at the same time, a gen-
eral expansion of glaciers in high mountain
valleys all over the world. The Rocky Moun-
tains in the western United States, for ex-
ample, which now are almost bare of glaciers,
were heavily glaciated, much as the Alps and
high Himalayas are now. There were no con-
tinental glaciers in the Southern Hemisphere
except on Antarctica because there are no
large land masses in the upper middle lati-
tudes where they could have grown.
Why such immense glaciers developed
during this great ice age, or Pleistocene
period, is not at all clear. Unquestionably
climatic changes in the direction of cooler
summers and greater snowfall were involved,
but the reason for these changes lies in the
realm of theory.
Even the course of glacial history is most
imperfectly known. It is generally believed
that ice sheets formed, spread, fluctuated,
and finally melted away several times (pos-
sibly four) during the Pleistocene period.
The last major expansion reached its maxi-
mum not far from 20,000 years ago, did not
finally disappear from the northern edge of
the United States until perhaps 8,000 years
ago, and reached approximately its present
state only about 5,000 years ago. This last
glaciation fell well within the period when
man had become widely established over the
earth, and must have had profound effects
upon his existence. By the time the last ice
sheet had vanished, history had reached the
early stages of the sedentary civilizations of
Babylon and Egypt.
The effects of glaciation were widespread
and complex. The surfaces actually covered
66
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
by the ice were modified by erosion and
deposition, and were also evidendy consider-
ably depressed by the weight of the ice, rising
again when the ice melted. Valleys and plains
adjacent to the ice received deposits of debris
carried from the glacial edge by meltwater.
Throughout the world sea levels were greatly
lowered as more and more water became tied
up in the ice sheets, only to rise again as the
glaciers wasted away. Accompanying the
whole affair was a complex series of climatic
changes, affecting not only the glaciated areas
but much of the rest of the world as well.
The study of all of these events and their
history and interrelation is still in its infancy,
and undoubtedly there are yet many fascinat-
ing discoveries to be made.
Erosion by glaciers The investigation
of how glaciers erode and deposit is greatly
complicated by the fact that it is almost im-
possible to see just what is happening under-
neath glaciers now in existence. Much of our
notion of how glaciers rework the surface is
inferred from the forms left behind on sur-
faces from which the ice has lately melted
away. Hence our knowledge has grown slowly,
and there are strong differences of opinion,
especially with regard to how glaciers erode.
Apparendy glaciers can erode in three ways.
First, and probably the most important by
far, is the process known as plucking or
quarrying. In this the plastic ice molds itself
about particles of the weathered mande or
blocks of bedrock and then drags them out
of place as the ice mass moves on forward
(Fig. 3.26). Quarrying is most effective where
the surface materials are loose or jointed. A
second erosional technique is that of grind-
ing or abrasion. Quarried rocks that are
partly imbedded in the lower surface of the
ice are dragged across bedrock outcrops like
grains on a giant sheet of sandpaper, scrap-
ing and gouging as they go. Grooved and
polished rock surfaces testify to the work of
this process, though it is no doubt much less
effective than quarrying. Third, and least
important, is a bulldozerlike shoving effect at
the front of the ice. Probably this process is
significant only locally, as, especially, where
the ice edge readvances over loose heaped-up
debris dropped earlier.
Undoubtedly the greatest effect of glacial
erosion is the stripping of the weathered
mantle from the surface over much of the
area covered. There is also active quarrying
of strongly jointed or conspicuously weak
bedrock. Projecting crags are removed or
reduced in size. Bottleneck valleys oriented
FIG. 3.26 How a glacier erodes by plucking and abrasion.
Abrasion by debris
dragged over surface
How surface form develops 67
Material melting from surface >
and edge of ice
Lodgement of material beneath ice
FIG. 3.27 Near its edge, an ice sheet suffers melting on both upper
and lower surfaces. Thus some of the debris it contains is lodged beneath
the ice, and the rest is deposited at the edge.
in the direction of ice movement seem espe-
cially liable to strong erosion. But generally
speaking the extensive and thick continental
ice sheets were not strongly channeled, so
that their erosional work was inclined to be
patchy, producing irregular depressions
rather than integrated valleys. Erosion by
glacial tongues in mountain valleys is, of
course, confined to the bottom and lower
sides of the valleys.
Transportation by glaciers Glaciers
are highly competent transporting agents,
able to carry material of all sizes, including
immense boulders. The debris eroded by the
glacier itself is concentrated near the base of
the ice. But valley glaciers may also carry
quantities of material that has been dumped
onto their surfaces by mass movement or
washed down the valley sides. This material
is concentrated near the surface, though some
may reach considerable depth in the ice be-
cause of later covering by snow.
Deposition by glaciers A glacier de-
posits its load by melting away from it. Melt-
ing occurs toward the edges or outer margin,
of the ice, and works both upward from the
ground and downward from the upper sur-
face (Fig. 3.27).
As a result of melting on the lower surface,
the debris carried in the lower part of the ice
is lodged beneath the glacier. Melting down-
ward from above exposes more and more
debris on the surface of the ice, so that some
mountain- valley glaciers are almost com-
pletely obscured near their lower ends by a
thick cover of rock and sand. This debris is
deposited along the edges of the glacier as
the ice melts.
There is no mechanism for selectivity in
either the transporting or depositing process.
Therefore glacial deposits are commonly
jumbled mixtures of material varying in size
from clay to huge boulders (Fig. 3.28). By
this they can usually be easily distinguished
from water-laid deposits, which almost always
show some degree of sorting and layering
(Fig. 3.29).
Glacial deposits are called moraines. Those
believed to have been laid down by lodgment
beneath the ice are ground moraines; those de-
posited along the ice edge are marginal
moraines. The material itself is called till
Moraines will be found throughout the
glaciated area, for at some time or other
every part of the area covered by the ice will
have been in the vicinity of the glacial margin.
However, within this area, the till will have
been unevenly distributed in patches, heaps,
ridges, and blankets of unequal thickness.
The deposits will normally be thickest in
valleys, "downstream" from sources of easily
eroded material that furnish quantities of till,
68
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
F I G . 3 . 2 8 An exposure of glacial till showing the unassorted clay,
pebbles, and boulders of which it is composed. (Wisconsin Geological Survey.)
and in the outer parts of the glaciated area.
Deposits will be thin or absent on hilltops,
in areas of especially resistant rock, and in
the source regions of the ice.
Meltwater flowing from the ice margin
may carry out quantities of finer debris, to
deposit it as alluvium across plains or in
valley bottoms. Such deposits, called out-
washy are not strictly glacial, but outwash
and till deposits are both commonly included
under the general term of glacial drift.
Till deposition modifies in varying degree
the landscapes over which it is laid down.
The resulting forms depend upon the thick-
ness and stoniness of the till and upon the
form of the underlying surface. Characteristic
moraine features will be discussed farther on,
but one point may be mentioned here. The
tendency of moraine deposition to be irreg-
ularly heaped and to be concentrated in
valleys has the effect of obscuring old drain-
age lines and of leaving behind an irregular
surface of rises and depressions without any
well-organized valley system. Thus glacial
deposition tends to destroy existing stream
channels and to produce irregular, "pock-
marked" surfaces. It was noted above that
glacial erosion has somewhat similar effects.
Because there is a significant difference in
age between glacial deposits from the early
Pleistocene and those from the late Pleisto-
cene, there is also a considerable difference
in the freshness and degree of preservation
of the surface features that the ice produced.
Those formed by the later ice sheets are gen-
erally clearly defined and little-altered. Those
formed during the middle and early Pleisto-
cene are commonly so changed by stream
erosion and mass movement as to be unrecog-
nizable. Only the distinctive character of till
deposits reveals the glacial history of these
"old drift" areas.
THE WORK OF WIND
Where and how wind works The wind
is a much less important producer of surface
forms than are water, gravity, and ice. This
is true principally because the wind can erode
only under certain limited conditions. In par-
ticular, the wind is almost powerless to erode
unless the surface is nearly bare of vegetation,
and then can erode only if the surface ma-
terial is fine and dry. For this reason the work
of the wind is confined to the deserts or
semideserts and to those few areas in humid
regions, such as beaches, river beds at low
water, and, nowadays, plowed fields, where
there is little plant cover.
Where it is able to work, the wind erodes,
transports, and deposits in much the same
manner as running water, except that there is
little channeling. Erosion is accomplished by
the force of eddy currents near the surface
and by the impact of particles already being
carried (sand-blasting effect). The wind moves
material by rolling or bouncing it along the
ground or by carrying it in suspension.
Deposition occurs where surface irregularities,
including vegetation, check the speed of the
wind near the ground, or where the wind
velocity decreases simply because of the
atmospheric-pressure pattern. Rain falling
through the dust-laden air will often carry
most of the suspended material down with it.
The wind can rarely move material larger
than coarse sand. Sand is carried as "bed
load" of the air stream, that is by rolling or
bouncing, and seldom rises more than a few
feet above the ground. Silt and clay can be
carried in suspension, and thus may reach
great heights and may travel long distances.
Fine red soil traceable to the plains of western
Oklahoma has, for example, been observed
to fall on the decks of steamers in the Atlan-
How surface form develops 69
tic. Because it remains close to the ground,
sand usually does not move far from its
source region. Finer material, on the other
hand, may be spread as a thin blanket over
a huge expanse downwind from its place of
origin.
Wind erosion Erosion by the wind, like
that by ice, tends to be widespread or patchy
rather than channeled. Thus it may lower the
surface rather uniformly over a broad area
without producing any pronounced surface
forms. On occasion it scours out shallow de-
pressions in favored places where the vegeta-
tion has been destroyed, where the material
is especially loose and fine, or where the
velocity is increased by a natural botdeneck.
A common occurrence is for the wind to
winnow out the finer particles from mixed
surface material, leaving behind a coarse-
textured gravelly or stony cover (Fig. 3.30).
Wind deposition The deposition of
fine suspended material is so broad and un-
FIG. 3.29 A cut through an outwash plain,
showing sand and gravel washed free of clay and
rudely stratified according to size.
(Wisconsin Geological Survey.)
70
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
FIG. 3.30 The pebbles and sand-blasted rock
fragments of an unusually coarse wind-sorted
surface cover on the floor of Death Valley,
California. (Eliot Blackwelder.)
concentrated that it modifies the surface
strongly only if it continues for a long time
and leaves a very thick layer. Most of the ex-
tensive deposits of silty, unlayered, buff-
colored, limy material called loess, common
in the middle western United States, eastern
Europe, and north China, are believed to
have originated as wind-blown silt.
By contrast, sand is usually deposited in
heaps rather than smooth sheets, and thus
sand deposition does produce distinct land
forms. These features, called dunes, are com-
mon in some desert regions and along many
coasts.
F I c; . 3 . 3 1 A wave breaking. (F. P. S
THE WORK OF WAVES AND CURRENTS
Occurrence Like that of winds, the
work of waves and currents is narrowly con-
fined. Along the thousands of miles of the
world's coasts and lake shores, however, this
work is a prime factor in shaping the surface
forms. Though waves and currents also occur
in the open seas, they can erode only along
the shore and in shallow water where their
activity can reach to the bottom. It should be
emphasized that the currents being discussed
here are not the slow, large-scale, world-wide
drifts set up by prevailing winds and water-
density differences. These are instead local,
relatively swift movements that occur along
the shores and in narrow inlets as a result of
winds and tides. Such currents may reach
velocities of several miles per hour, and are
quite able to pick up and move loose sand or
fine debris.
Wave erosion The largest part of the
actual erosion, however, is accomplished by
waves. When a wind-driven wave enters
shallow water, it drags on the bottom,
steepens to a sharp crest, and finally breaks
or pitches forward in a plunge of water and
foam (Fig. 3.31). In large storm waves tons
of water (and sometimes sand and rock as
well) are flung forward and downward, strik-
ing against the shore or the shallow bottom
with tremendous violence. Under favorable
conditions the erosive force generated is prob-
ably as great as any found in nature. The
water flung toward the shore is then drawn
again seaward by gravity, flowing beneath
oncoming waves as an undertow current.
These currents are sometimes strong enough
to move out quantities of material loosened
by the breaking wave.
Where the water maintains a depth of 10
to 20 ft close to the shore, waves will not
break until they are almost to the land. Their
How surface form develops 71
erosional force is then spent against the land
itself, driving it back and cutting sea cliffs. If,
on the other hand, the water becomes shallow
far out, the principal wave breaking occurs a
long distance from shore and the force of
erosion is expended against the bottom,
deepening the water at that point and casting
some of the loosened debris up ahead of the
breakers in the form of a bar.
Transportation and deposition Ma-
terial dislodged by waves or fed into the
water by streams is carried by wave-generated
or tide-generated currents either outward into
deeper and quieter water, or along the shore
until it reaches a sheltered spot. That carried
outward is deposited to form a shelf of sed-
iment that tends to build slowly seaward.
Fine material may be carried far out and
spread thinly over vast areas of deeper water.
Material is carried along the shore chiefly by
zigzag in-and-out movements produced by
FIG. 3.32 The zigzag path of a pebble under
the combined forces of oblique waves, undertow,
and longshore current.
waves that strike the coast at an oblique
angle (Fig. 3.32). In storms these currents
may be remarkably strong, and quantities of
sand may be moved along until they reach
the sheltered waters of a bay or the protected
lee of an island or projection of the coast.
There the sand is dropped in the form of a
bar or beach.
SELECTED REFERENCES
Finch, V. C., G. T. Trewartha, A. H. Robinson, and E. H. Hammond: Physical Elements of Geog-
raphy, 4th ed., McGraw-Hill Book Company, Inc., New York, 1957.
Flint, R. F.: Glacial and Pleistocene Geology, John Wiley & Sons, Inc., New York, 1957.
Gilluly, J., A. C. Waters, and A. O. Woodford: Principles of Geology, 2d ed., W. H. Freeman &
Company, San Francisco, 1959.
Leet, L. D., and S. Judson: Physical Geology, 2d ed., Prentice- Hall, Inc., Englewood Cliffs,
N.J., 1958.
Lobeck, A. K.: Geomorphofagy: An Introduction to the Study of Landscapes, McGraw-Hill Book
Company, Inc., New York, 1939.
: Geological Map of the United States (with text), C. S. Hammond fc Co. Inc., New
York, 1941.
Longwell, C. R., and R. F. Flint: Introduction to Physical Geology, John Wiley & Sons, Inc.,
New York, 1955.
Sharpe, C. F. S.: Landslides and Related Phenomena, Columbia University Press, New York,
1934.
Thornbury, W. D.: Principles of Geomorphology, John Wiley & Sons, Inc., New York, 1954.
Wooldridge, S. W., and R. S. Morgan: An Outline of Geomorphology: The Physical Basis of
Geography, 2d ed., Longmans, Green & Co., Ltd., London, 1959.
CHAPTER 4
Plains
ORIGIN OF SMOOTH SURFACES
Plains were defined in Chap. 2 as surfaces
consisting predominantly of gentle slopes,
with low relief. The detailed features of
plains result from recent operations of the
various surface-sculpturing agents, but for
plains to exist at all a specific set of con-
ditions able to produce low relief and much
gentle slope must have existed sometime in
the past.
High relief is the result of either crustal
disturbance that has raised parts of the sur-
face above their surroundings or the cutting
of deep valleys in an upland surface. But for
valleys to be cut deeply, the upland surface
must be far above the baselevel of erosion,
and the usual way it has gotten there is by
72
uplift of the earth's crust. So it is generally
true that high relief requires preceding crustal
disturbance and that without it relief will be
low. As has already been suggested, many of
the world's most extensive plains lie in areas
where the crust has been stable in late geo-
logic time.
However, low relief can occur in areas of
disturbed crust if a broad surface has been
uplifted so recently that valley cutting has
not yet gone very far. Much of the upland of
central and southern Africa fits this condi-
tion. Also, some depressed sections of the
crust have low relief because they have served
as receptacles for extensive and smoothing
deposition. Examples are the Central Valley
of California, the Ganges Plain of north India,
and the Mesopotamian plains of Iraq in the
Middle East.
For two reasons gentle slopes usually ac-
company low relief. One is that on low-
relief surfaces streams usually have gentle
gradients and therefore cut down slowly.
This gives ample time for surface erosion and
mass movement to keep most valley sides from
Plains 73
becoming very steep. Second, with so little
valley deepening, valley floors often begin to
widen out while there is much upland still
uncut by tributaries. Where this has happened
most of the surface is occupied either by valley
floors or by smooth uplands and is therefore
predominantly gentle in slope, even if the
valley sides happen to be steeper than average.
PLAINS SHAPED BY RUNNING WATER
STREAM-ERODED PLAINS
Characteristics and varieties The
majority of plains owe their surface de-
tail chiefly to the erosional work of streams,
aided by slope wash and mass movement.
The distinguishing characteristic of these
plains is widespread integrated systems of
stream valleys. The differences among indi-
vidual stream-sculptured plains are definable
in terms of the size, spacing, cross-section
form, and pattern of these valleys and of the
divides between them.
Differences in tributary development also
provide an excellent basis for making dis-
tinctions among stream-sculptured plains.
Some plains are crossed only by a few major
valleys, with broad, almost uncut uplands
between them. Tributaries of any consider-
able length are few, though there may be
fringes of short ravines along the sides of the
principal valleys. A plain having these char-
acteristics is sometimes said to be "youthful,"
not because of its age in years, but because
reduction of the upland has hardly more
than begun (Fig. 4. la).
On other stream-cut plains the landscape
is occupied by a close network of valleys and
FIG. 4.1 The ideal stages in the development
of a land surface by stream erosion, from youth
(a) through maturity (b) to old age (c). The dashed
white line indicates the baselevel toward which the
streams are working. (V. C. Finch.)
tributaries. The original upland surface is
gone, or nearly so, and the surface is made
up largely of sloping valley sides (Fig. 4.16).
74
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
A rolling, dissected plain of this kind is
termed "mature," for the valley system is
fully established, though much gradational
work still remains to be done.
These differences in degree of tributary
development are partly a matter of the
amount and effectiveness of surface runoff.
Some surfaces pass quickly into maturity,
while others remain youthful almost indefi-
nitely. Characteristics favoring slight runoff
and therefore slow tributary development and
the maintenance of youth are marked flatness
of the upland, porous and absorbent surface
material, a protective blanket of vegetation,
and light or gentle rains. On the other hand,
a sloping upland, fine and impervious surface
material, sparse vegetation, and torrential
rains favor much runoff, rapid tributary
growth, and quick achievement of maturity.
A good case of a persistently youthful sur-
face is the High Plains of western Kansas,
Oklahoma, and Texas. Here the original sur-
face was extremely smooth, the material is
rather porous, there has been a dense cover
of sod, and the climate is semiarid. The area
has been exposed long enough for one to ex-
pect that erosion would have made consider-
able headway, but instead valleys are few,
and most of the surface remains as if un-
touched (Fig. 4.2). Somewhat similar condi-
tions exist in the plains of southern Russia.
On the other hand, rolling, typically mature
plains occupy large sections of the American
Middle West, especially in southern and
western Iowa, northern Missouri, and eastern
Kansas and Nebraska (Fig. 4.3). The more
advanced erosion here is probably due not to
greater age, but to a more rolling initial sur-
face, easily eroded surface material (much of
it loess), and, possibly, to more frequent
heavy rains.
Valley form and size Stream-sculptured
plains also exhibit great differences in the
cross-section forms of their valleys, especially
F I G . 4 . 2 The remarkably smooth surface of the High Plains in southwestern Kansas, an old
depositional plain upon which stream erosion has made little headway.
Plains 75
FIG. 4.3 The rolling surface of a mature plain in northwestern Missouri.
in the width that the valleys have achieved.
Thus, for example, in the southern half of
Illinois valleys are narrow and relatively
steep-walled, while in much of southwestern
Missouri, central Oklahoma, and north cen-
tral Texas valleys are extremely wide, their
smooth floors occupying the major portion of
the land area.
Like tributary development, the width of
valleys in plains is largely controlled by time
and the rate of erosion (Fig. 4.4). As erosion
proceeds, valley walls are worn back by
wash and creep, and the valleys widen at the
expense of the higher ground between them.
Valley widening, like tributary development,
is strongly affected by the amount of surface
runoff and the ease of erosion of the surface
material. If there is little runoff, valley wid-
ening will have to depend almost wholly on
creep, and thus will be slow.
Even in youthful and in mature plains
there may be some large valleys that have be-
come quite wide, though by definition valley
floors occupy a small percentage of the total
area of such surfaces. But once the tributary
net is complete and the original upland de-
stroyed, then valley widening and reduction
of the height of divides become the chief
erosional processes. Valley floors expand
until they make up the larger part of the area.
Divides become smaller, lower and less con-
spicuous, and finally the entire surface is re-
duced to a low level and erosion practically
ceases. Such a surface is said to be in the
stage of "old age," and is given the rather
misleading name peneplain, which means
almost a plain (Fig. 4.1c). Small hills or
mountains that are the last remnants of dis-
appearing divides are common features on
plains in advanced stages of erosion. From
F I G . 4 . 4 A stream that cuts down rapidly
relative to the rate of valley widening develops a
narrow, steep-sided valley. Slow downcutting
permits valley widening to open up a broad, flaring
profile.
Valley widening slow relative to
valley deepening
Valley deepening slow relative to valley widening
76
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
FIG. 4.5 Spencer Mountain, a monadnock on the partially redissected
old-age erosion surface of the Appalachian Piedmont, near Gastonia,
North Carolina. (V. C. Finch.)
the example of isolated Mount Monadnock
in New Hampshire, the name monadnock is
given to these last vestiges of former height
(Fig. 4.5).
Patterns of streams and divides As a
general rule, tributary streams develop at
open acute angles to the main stream. This
gives to the stream system a branching pattern
FIG. 4.6 Random dendritic stream patterns develop where there are no
strong local contrasts in rock resistance (a). If the original surface has a
pronounced slope, the branching pattern is drawn out in a downslope
direction (b). (From Army Map Service series 1:250,000: Charleston and Moab sheeh.)
like that formed by the limbs of a tree. If the
surface on which the streams develop is
gently inclined, the branches may be spread
broadly as on an oak, while if the inclination
is steeper, they may be drawn out lengthwise
as on a poplar. But they are still treelike, or
to use a technical word that means the same,
dendritic (Fig. 4.6).
If the stream pattern is not dendritic, it
usually means one of two things. Either (a)
there was some striking peculiarity of the
slope on which the streams formed, as when
streams radiate from a center like a new
volcanic cone or a dome, or (b) there are
strong local variations of rock resistance.
Effects of rock resistance The effect
of local variations in rock resistance is simply
to favor the development of valleys in places
where the rock materials are most easily
Plains 77
weathered and eroded. The resulting valley
pattern will not be dendritic but will instead
conform to the pattern of rock weakness.
This is most strikingly developed in hilly
and mountainous lands, where relief is high,
and features are on a large scale (Fig. 4.7). Yet
even on plains some streams have achieved an
unusual angular pattern by developing along
prominent systems of joints where weather-
ing is rapid. Roughly parallel stream patterns
also form sometimes on plains that cut across
the edges of upturned rock layers of con-
trasting resistance.
More important for plains, though, are the
effects of rock structure upon the broader
pattern of major divides and valley systems.
Broad patches or bands of relatively resistant
rock tend to stand out as areas of higher
ground in which valleys are narrower and
FIG. 4.7 Stream patterns affected by structure: (a) parallelism resulting
from erosion on parallel bands of rock of contrasting resistance; (b)
angularity imposed on dendritic pattern by erosion on strongly jointed rocks.
(From Army Map Service series 1:250,000: Charlottesville and Lake Champlain sheets.)
r
78
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
Escarpment i
Undissected
Dissected
FIG. 4.8 Form and structure of cuestas. Example at left is sharp and
regular. Dissected form at right is more typical, especially in humid regions.
less advanced in their development than else-
where. Patches and strips of weak rocks ad-
vance more rapidly toward old age, and be-
come lowlands or elongated valley systems.
The development of alternating strips of
higher, rougher land and lower, smoother
land is very common, occurring almost any-
where that there are gently inclined sedimen-
tary strata of varying resistance. Central and
southeastern North America, northern France,
and southern Great Britain are among the
regions in which this type of development
occurs. The higher and rougher belts, devel-
F I G . 4.9 Northern France is a structural basin
in which the rock strata dip generally toward Paris
from all sides. Erosion on this structure has
produced rings of cuestas with escarpments facing
outward. (V. (,. Finch.)
oped where the resistant strata outcrop, are
termed cuestas. Usually they have quite
abrupt escarpments along their higher margins
(Figs. 4.8, 4.9).
Erosional plains in dry lands Ero-
sional plains in dry lands differ from those in
humid areas chiefly in the extent to which
certain kinds of features occur. These differ-
ences from the humid areas stem from slow
weathering, streams that flow only when it
rains, and a surface unprotected by vegeta-
tion in dry lands.
Most of the sculpturing there takes place
during the rare heavy rains. Absence of vege-
tation and the prevalent thinness of the
weathered mantle favor runoff*, and the bare
surface is readily attacked by erosion.
Quantities of debris are stripped from the
slopes and carried down the many ravines
and gullies into the valleys and basins below.
But the material is usually dropped without
being carried far, for once the rain ceases
and the stream leaves the area, the stream
dwindles, losing its carrying power.
Thus it is that desert plains are likely to
have unusually rocky and roughly gullied
slopes and uplands, and on the other hand,
lowlands that are more or less thickly covered
with alluvium and therefore smooth. These
features give desert plains their distinctive
character.
KINDS OF
WATER-LAID PLAINS
General characteristics Most of the
world's smoothest plains are the surfaces of
extensive deposits laid down by water. Some
of these deposits have been dropped by
streams along their courses or at their mouths.
Others have accumulated upon the floors of
lakes or on shallow sea bottoms, and have
become exposed through a change in water
level or an uplift of the land. In all kinds,
the materials are normally well-sorted,
layered, and loose.
Such plains are flat, but they are not truly
featureless. Stream channels are nearly always
present. Usually there are also various slight
swells and depressions, and the latter often
contain shallow lakes or marshes. Streams
shift their channels readily in the loose sedi-
mentary materials, so that scars of abandoned
channels are almost as characteristic as active
streams.
Floodplains Alluvial deposition is re-
sponsible for the flat bottomlands that are so
characteristic of the floors of gentle-gradient
valleys. Sluggish flow and, in some instances,
decreasing volume render the streams in such
valleys incapable of carrying all the sediment
load fed to them by their tributaries, and
excess sediment is deposited along the valley
bottom as a floodplain. Under normal low-
water conditions a stream on a floodplain is
confined to a definite and somewhat wander-
ing channel. But during periods of heavy
runoff this channel may prove inadequate to
carry the vastly increased discharge from the
tributaries, and the stream then overflows its
banks and spreads in a thin sheet over much
or all of the floodplain surface.
Sandy materials and braided channels
Stream channels on alluvium are extremely
Plains 79
changeable because of the ease with which
the loose material may be eroded (Fig. 3.19).
A relatively swiftly flowing stream, able to
move sand and gravel, has a tendency to de-
velop a very wide, shallow channel which
the stream entirely covers at high water. In
the channel are many shifting sand bars,
around which are branching and rejoining
threads of deeper water and swifter current.
At low water only the deeper threads of the
channel carry water; the rest of the bed is
exposed (Fig. 4.10). This is the classic
braided channel, widely seen in sediment-
charged streams emerging from mountain
valleys, flowing from the margins of glaciers,
or passing through areas of loose, coarse,
sedimentary materials. Excellent examples of
braided channels are provided by most of the
larger streams of the High Plains east of the
Rocky Mountains, such as the Platte, the
Arkansas, and the Canadian Rivers.
Silty floodplains and meandering channels
At the other end of the scale are the silty
floodplains of slow-flowing streams. Here the
stream channel is narrower and more sharply
defined, and usually exceedingly sinuous, or
meandering (Fig. 4.11). Its loops and bends
change form rather rapidly; individual loops
are often cut off entirely by the stream's
breaking through the narrow neck of land at
the base of the curve.
During floods silt-depositing streams of this
type characteristically drop the major part of
their load immediately as the waters leave the
deep channel and begin to spread across the
plain. This results in the formation of slightly
raised strips of ground along the sides of the
channel. These low swells, called natural
levees, provide the highest, best-drained, and
most useful land on the floodplains (Fig.
4.12). The lower lands behind them are
poorly drained and sometimes permanently
80
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
FIG. 4.10 The braided channel of the Rio Grande in northern New
Mexico. During flood the entire belt of channels and sandy islands will be
covered with water. (Spence Air Photos.)
swampy. The scars of abandoned channels
and cutoff meander bends, each with its own
natural levees, are common features over most
of the floodplain surface. The great plain of
the lower Mississippi is the most extensively
FIG. 4.11 The floodplain of a meandering
stream, showing cutoffs and scars formed by
shifting of the channel. Laramie River, Wyoming.
(J. R. Balsky, U.S. Geological Sunxy.)
studied floodplain of the silty type, but
smaller examples are numerous.
Between the wide braided channels on the
one hand and the narrower channels that
freely meander on the other are many inter-
mediate types, usually showing rather wide,
shallow beds with numerous sand bars, swing-
ing but not looping courses, and an absence
of natural levees.
Use of floodplains Because they are flat,
are developed on loose and relatively fine
F I G . 4 . 1 2 A cross-section diagram illustrating
typical floodplain features. The highest ground is
along the natural levees.
Natural levees
Active
Backswamp\channel
Plains 81
material, and have easy access to water, flood-
plains are often eagerly sought as agricultural
lands. Sometimes, though by no means
always, their soils are more fertile than the
older soils on the neighboring uplands. How-
ever, floodplain agriculture is always beset by
the problem of floods, with their destructive-
ness to crops, buildings, and livestock. Even
excepting actual floods, much of the land is
likely to be permanently swampy or subject
to waterlogging by heavy rains. While these
problems can be attacked by various flood-
control and drainage programs, such measures
are expensive and not always worth the cost
and effort they involve.
Alluvial terraces After a floodplain has
been formed, the stream's gradient, its volume
of flow, or the amount of sediment load being
fed into it may change in such a way that the
stream starts eroding once again. It will then
cut down into its earlier deposit, leaving only
shelflike remnants along the valley sides. Such
alluvial terraces are very common. In some
valleys several terrace levels may be seen, in-
dicating that the stream has repeatedly
changed its activity from deposition to ero-
sion (Fig. 4.13).
Alluvial terraces have many of the advan-
tages of floodplains, without the dangers of
flood and poor drainage. Therefore they often
serve as agricultural lands or as the sites of
towns or of transportation routes. However,
they tend to be less well-watered and, because
their soils are older, less fertile than the bot-
tomlands themselves.
Deltas Deltas, as previously stated, are
the plains formed by alluvial deposition at the
mouths of streams. Here the stream's velocity
is checked as it enters the body of standing
water. The sediment load is dropped on either
side of the principal line of flow and often also
in a bar opposite the open end of the channel.
FIG. 4.13 Development of alluvial terraces by
renewed downcutting in an older deposit of
alluvium. Natural levees border the present stream
Course. (V. C. Finch.)
By continued deposition the delta grows both
outward and laterally. Often the channel
divides around the bar at its mouth, and each
branch extends itself seaward as the delta
grows. In this way the stream acquires nu-
merous branching outlets, known as dis-
tributaries, and the delta becomes complex
and fan-shaped (Fig. 4.14).
Most large deltas are composed chiefly of
silt and fine sand, and show some of the
characteristics of silty floodplains. Natural
levees are often well-developed, and meander-
ing channels are common. Swift streams en-
tering a lake or the sea may build sandy
deltas. These commonly have relatively steep
gradients and display braided and shifting
channels. They are essentially alluvial fans
built in the sea instead of on the land.
Some deltas reach great size. Those of the
Mississippi, the Nile, the Volga, and the Gan-
ges, for example, all exceed 100 miles in
width (Fig. 4.15). That of the Yellow River
(the Hwang Ho) is a plain more than 300
miles wide. Many deltas, though large, are
not conspicuous on the map because they are
built in the ends of large arms of the sea. The
Colorado, Sacramento-San Joaquin, and
Tigris-Euphrates deltas are examples (Fig.
4.16).
Not all streams form deltas. Many lack the
necessary sediment load. Others enter the sea
where the water is too deep or wave action
so strong that the sediment is spread broadly
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
-'7 Bar developing,
; / initiating formation
Early stage
of distributary
development
FIG. 4.14 Characteristic features of a delta plain.
over the sea bottom without building up to
the surface. The St. Lawrence and the Congo
both deposit most of their load in interior
basins and reach the sea with too little sedi-
ment to produce deltas in the deep waters
that they enter.
Deltas, like floodplains, are sometimes use-
F I G . 4.15 Delta outlines and channels.
ful agricultural lands because of their flat-
ness, their water supply, and often their
fertility. But, they are also subject to floods
and often contain much swampy land (Fig.
4.17). The natural levees may offer the only
delta land that can be used without diking,
draining, or pumping. Yet useless delta land
Plains 83
can be reclaimed, as the Netherlands has
shown. Most of that country lies on the great
combined delta of the Rhine and the Maas
Rivers and several smaller streams. Much of
it was originally either swamp or shallow sea
floor. Now, by means of centuries of dike
building, pumping, and flushing out of salt,
huge areas of highly productive land have
been virtually created there by man (Fig.
4.18). Indeed, the Netherlands serves as a re-
markable example of what can be done in
reclamation of deltas when the demand for
land is sufficiently intense and the initiative
and the necessary technical knowledge are
available.
Alluvial fans The alluvial fan is similar
to the delta as a spreading form developed
by a stream dropping its load because of an
abrupt checking of the velocity. But with the
alluvial fan the break in speed is the result of
a sudden decrease in the stream's gradient:
most fans are formed by streams emerging
from steep mountain canyons onto plains or
flat valley floors. The material dropped is
largely the coarser bed load of gravel, sand,
and sometimes coarse silt. The large frag-
ments are dropped first, near the mouth of
the canyon.
Like most other sand-moving streams, those
that form alluvial fans commonly have wide,
shallow, braided channels. They continually
choke themselves with debris and shift from
one side to the other, so that the deposit
takes on the shape of a spread fan (or half
of a low, flat cone) with the apex at the
mouth of the canyon. The surface of the fan
bears the marks of many diverging sandy
channels, most of them dry (Fig, 4.19).
Piedmont alluvial plain Some alluvial
fans are only a few yards across; the broadest
ones extend out several tens of miles from the
mountain front. At the foot of an elongated
F I G . 4 . 1 6 An arm of the sea once reached
through San Francisco Bay into the heart of the
Central Valley of California. The combined delta of
the Sacramento, San Joaquin, and other rivers that
drain the valley has been built in the head of this
embayment, far from the open sea.
mountain range many fans may develop side
by side, eventually growing together to form
an extensive gently sloping alluvial apron or
piedmont (foot-of-the-mountain) alluvial
plain (Fig. 4.20). Most of the southeastern
part of the Central Valley of California is a
plain of this type. Much of Los Angeles
stands on a similar plain that has been built
out into the sea from the mountains north of
the city. Alluvial fans and aprons surround
most of the small ranges of Nevada and
western Utah.
The High Plains that stretch eastward
from the Rocky Mountains of Colorado and
New Mexico into Kansas, Oklahoma, and
84
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
P| Old land
FIG. 4.17 I he Mississippi River Delta has fringing areas of salt-marsh
grass and reeds, belts of wooded swamp, and strips of tilled levee lands.
Note that the* levee lands grow narrow downstream and disappear. (V. C. Fiwh.)
FIG. 4.18 The extent of reclaimed land in the
Netherlands in relation to the area of the Rhine
River delta.
CD Old land
a
sea
sta
2S3 New tends
from
the 2tt
Texas owe their remarkably smooth upland
surface to alluvial deposition. During suc-
cessive uplifts of the mountains great quan-
tities of alluvium were eroded and spread
eastward as a piedmont alluvial plain of
enormous size. More recently the streams
crossing the plains have begun to erode and
have cut valleys in the surface, but only at
wide intervals, so that there are still broad
expanses of smooth upland remaining (Fig.
4.2). A similar plain, but almost uncut, lies
to the east of the Andes in eastern Bolivia,
western Paraguay, and western Argentina.
Lake plains and coastal plains The
areas of plain which are former lake bottoms
or portions of shallow sea bottom from which
water has disappeared are many. Lakes are
short-lived, by the scale of geologic time, for
they are inevitably subject to filling by
washed-in sediment or to draining by down-
Plains 85
F I G . 4 . 1 9 I nese large alluvial Tans at tne eastern root OT tne sierra
Nevada of California are beginning to merge, forming a piedmont alluvial
plain. (Spemr An Photos.)
cutting of the outlet. Other lakes disappear
as a result of increasing dryness of the
climate. So it is not surprising that there are
many lake beds not now occupied by lakes.
Similarly, small uplifts of the land or lower-
ings of the sea level, both of which have
occurred frequently, expose strips of surface
that were formerly parts of the sea bottom
just offshore.
On lake bottoms and coastal sea bottoms
sediment is likely to be spread broadly and
evenly, sometimes to a considerable thick-
ness. Thus their surfaces tend to become
quite smooth, with only a very gentle slope
away from the shore. Certain plains of this
origin are among the most featureless sur-
faces in existence (Fig. 4.21). Because they
are usually low-lying, stream erosion is slow
and shallow. Swamps or shallow lakes may
occur in the slightly lower sags in the sur-
face. Around the margins of the plain there are
often beaches, terraces, or other features that
indicate the position of the former shorelines.
The outer part of the southeastern plains
of the United States, along the south Atlantic
and Gulf Coasts, is one of the more exten-
sive examples of a surface lately emerged
from the sea. It is low, flat, sandy, and
swampy, with only feeble stream erosion
(Figs. 4.22, 4.23). The area farther inland
F I G . 4 . 2 A piedmont alluvial plain is formed
by the growing together of many extensive alluvial
fans at the foot of a mountain range.
FIG. 4.21 The extremely level surface of a
glacial-lake plain near Saginaw, Michigan.
(V, C. Finch.)
was also once covered by the sea, but has
been exposed long enough and is now far
enough above baselevel that erosion has
made strong headway. Many other coasts of
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
the world have strips of emergent sea bot-
tom along them, but most of these are rela-
tively narrow. The margins of Hudson Bay,
especially the southwestern, show remark-
ably broad belts of low, flat, poorly drained
land marked by innumerable parallel lines of
raised beaches. These are apparently the re-
sult of a rather rapid but irregular rise of the
land following the melting of the last con-
tinental glacier.
The exposed floors of former lakes are
also common, and some are very extensive.
Some of the largest known are in North
America. Large parts of northwestern Min-
nesota and the eastern Dakotas, and most of
southern Manitoba are occupied by a flat
plain that represents the bottom of a huge
lake, larger than any of the present Great
Lakes, that existed near the end of the Pleis-
tocene period.
F I G . 4 . 2 2 The flat, marshy surface of the Florida Everglades, part of a
relatively new plain not yet dissected by streams. (V. C. Finch.)
Plains 87
F I G . 4 . 2 3 A flat but well-drained section of the West Gulf Coastal
Plain in Texas. (V. C. Finch.)
This body of water, which is known as Lake
Agassiz, came into being when the north-
ward-flowing drainage of the area was
dammed by the edge of the melting ice sheet.
At present the surface is nearly featureless,
with the Red River cutting slightly into the
lowest part and several sets of low beach
ridges visible around the margins. There are
several smaller plains of similar origin in the
north central part of the continent (Fig.
4.24).
Another set of lake plains exists in the
Basin and Range province of Nevada and
surrounding states. These represent the beds
of lakes that existed in the extensive struc-
tural basins of that area during times when
the climate was moister than it is now. This
probably occurred more than once during
the late Pleistocene and possibly at least once
since the Pleistocene. Two of these lakes,
named Bonneville and Lahontan, occupied
extensive areas in western Utah and western
Nevada, respectively. Great Salt Lake is a
shrunken remnant of Lake Bonneville, and
the Bonneville Salt Flats, famous for auto-
mobile speed trials, are part of the former
lake floor.
Among the larger lake plains elsewhere in
the world are part of the smooth elevated
basin of the Congo River, and several basins
along the southern margin of the Sahara
Desert, including that containing the remnant
Lake Chad.
FIG. 4.24 Map of the plain of glacial Lake
Agassiz, with the plains of other ice-margin lakes
that existed at various times during the wastage of
the Wisconsin ice sheets. Also shown are some of
the spillways through which these lakes drained
when their normal drainage was blocked by the ice.
88
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
PLAIN SURFACES AFFECTED BY ICE,
GROUND WATER, AND WIND
KINDS AND FEATURES OF
GLACIALLY MODIFIED PLAINS
Glacial modification of the surface
Nearly a third of the continental area of the
world was occupied by ice at one time or
another during the Pleistocene period. How-
ever, not all of that area is now characterized
by distinctly glacial land forms. Only in
those sections covered by ice in late Pleis-
tocene time (called Wisconsin in this conti-
nent) are the glacially produced features
well enough preserved to give distinctive
character to the surface. The older glaciated
surfaces have been so modified by running
water and mass movement that their glacial
history can be inferred only from the pres-
ence of material recognizable as till.
Surfaces glaciated in Wisconsin time are
shown in Fig. 3.25. The land forms of these
areas are quite varied, and not all of the
FIG. 4.25 Areas of late glacial deposition
commonly display numerous lakes, swamps, and
wandering streams.
areas are plains. However, there do exist
throughout recurrent types or associations of
features that reflect the patchy, irregular, un-
channeled erosional and depositional activity
of the great ice sheets. A significant part of
that activity involved virtual obliteration of
the old stream courses and the production
of a surface in which there were many shallow
enclosed depressions and few continuous
valleys. For this reason lakes, swamps, and
aimlessly wandering streams are found almost
everywhere in the lately glaciated country,
though they are rare in stream-eroded land-
scapes. It is these features that are the most
obvious and impressive novelties to a person
coming into glaciated country for the first
time (Fig. 4.25).
Most of the area of glaciated plains is
dominated by depositional features. Only in
scattered sections is the drift so patchy or so
lacking that bedrock surfaces scoured by
erosion are broadly exposed. These appear
to be chiefly in areas where the bedrock is
especially resistant and where the original
surface was relatively rough.
Till surfaces Since the major portion
of the glaciated surface is depositional, it
will be well to consider some of the char-
acteristic features of drift-covered surfaces.
These assume many forms because of dif-
ferences in (a) the roughness and shape of
the surface on which the drift was laid down,
(b) the thickness of the drift, (c) the amount
of rock, sand, and clay in the drift, and (d)
the specific local conditions of deposition.
Effects of till deposition Except where the
original surface was unusually smooth, the
deposition of till usually had the effect of
making the surface smoother than it was.
Till was deposited thickly in the valleys and
only thinly on the hilltops. If the original
terrain had slight relief, the till might com-
pletely obscure the older forms, producing
an entirely new surface. If, on the other
hand, the original terrain was hilly or the
drift rather thin, the old hilltops may be
still visible as elements in the landscape,
even though they are thinly covered with till
(Fig. 4.26). This kind of partial control of
the surface form by preglacial bedrock
features is common in the northern United
States, Canada, and Scandinavia. Where it
occurs, lakes and swamps are especially
likely to be found along the larger old-
stream valleys that are now only partly filled
by drift. An excellent example is the chain
of lakes at Madison, Wisconsin.
Where the deposition of till has been the
controlling factor in shaping the landforms,
the surface is ordinarily smoothly undulating
to moderately rolling. Steep and angular
slopes are rare; features are usually rounded,
and bedrock outcrops are few (Fig. 4.27).
Some of the clayey or silty till surfaces, such
as those in northeastern Illinois, north cen-
F I G . 4 . 2 7 The undulating surface of a till plain. (
, C Bitfttl ot stffijoth swtofi fcy i
F I G . 4 . 2 6 The effects of glacial deposition
vary with the roughness of the buried surface and
the thickness of the drift. (V. C. Finch.)
tral Iowa, and the eastern Dakotas, are among
the smoother sections of the continent. Stony
and gravelly till sustains steeper slopes and
hence normally displays more irregular sur-
faces than fine till.
Distinctive surface features of till
deposits Among the more distinctive
features of till surfaces are the marginal
moraines, the strips of thicker drift that ac-
cumulated along the edge of the ice sheet
when the margin remained long in one place.
Here debris was dropped as at the end of a
srows/w Geological Survn.)
90
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
FIG. 4.28 Small kettle ponds surrounded by boulder-strewn knobs in a
marginal moraine near Whitewater, Wisconsin. (V. C. Fimh.)
great conveyor belt, partly by the ice melt-
ing from around it, partly by being washed
over the edge of the glacier by melt water.
Marginal moraines usually appear in the
landscape as bands of somewhat rougher,
higher, and often stonier land than that to
FIG. 4.29 The unusually rough, knobby
surface of the Kettle Moraine, in eastern Wisconsin.
(John R. Raiutall.)
either side (Fig. 4.28). Some are narrow,
low, and inconspicuous; others are broad
strips of knobby, pitted, lake-strewn country
that can hardly fail to be noticed by the
traveler. In the United States one of the
most conspicuous is the high, rugged Ketde
Moraine of eastern Wisconsin, formed be-
tween two tongues, or lobes, of the late
Pleistocene glacier (Fig. 4.29). Another is
the broad series of moraines in western Min-
nesota that forms a belt of unusually rough
knob-and-kettle surface 25 to 50 miles in
width. An important group of European
moraines can be traced as a belt of irregular
lake-dotted country that extends from Den-
mark through north Germany and Poland to
the vicinity of Moscow. Narrower but very
prominent moraines also cross southern
Finland from west to east.
Many till plains exhibit elongated ridges
and grooves that extend in the direction of
ice movement. In some areas these take the
form of low, streamlined hills of drift that
are called drumlins (Fig. 4.30). The origin
of such features is not well understood,
though it probably involved the riding of
the ice over old moraine deposits or over
material lodged beneath the ice by melting.
Recent investigations, especially in Canada,
have disclosed that linear features of this
kind are much more widespread than was
formerly thought.
The lakes and swamps that are common
to most till surfaces are simply collections
of water in the low sags and depressions in
the drift surface. They are most abundant
where the surface is especially rough because
of marginal-moraine deposition or the pres-
ence of an irregular preglacial surface under-
neath. Most of the lakes are shallow. Nearly
all are destined for early disappearance be-
cause of filling by sediment and swamp
debris as well as rapid lowering of the out-
lets by erosion of the unconsolidated drift.
Outwash surfaces Outwash surfaces
can occur both within and beyond the limits
of the area covered by the ice. Meltwater
streams may carry debris tens or hundreds
of miles from the ice margin to deposit it in
Plains 91
the form of floodplains along established
valley bottoms or as fans and sheets across
smooth plains.
With one significant exception* the features
of outwash surfaces are those common to
other alluvial plains of comparable type.
The surfaces are usually conspicuously flat
and exhibit various patterns of stream
channels (Fig. 4.31). Since the finer silts and
clays are normally carried in suspension
clear out of the area of origin, the outwash
deposits in the vicinity of the ice margin are
usually sands and gravels, reasonably well
stratified. A peculiarity of some outwash
surfaces is the existence of sizable depres-
sions, often rather steep-sided, and usually
containing lakes or swamps. These are not
normal alluvial features, but are found prin-
cipally in outwash plains laid down within
the glaciated area. During wasting away of
the glacier, individual masses of stagnant ice
are left in front of the retreating glacial
margin, to be largely or completely buried
in outwash. Eventually the mass melts away
from beneath the deposit, and the surface
sags down into the space it occupied.
Outwash deposits are widespread about
F I G . 4 . 3 A drumlin on the till plain of central New York. Its shape
indicates that the ice movement was from right to left. (U.S. Geological Survey.)
92
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
FIG. 4.31 The nearly flat surface of an outwash plain. (Wisronsin
the former glacial margins and throughout
the glaciated areas. A few areas have received
unusually thick and extensive deposits be-
cause of conditions that favored the pouring
of large amounts of meltwater into a re-
stricted space. Southern Michigan, for ex-
ample, received outwash from glacial tongues
to the west, north, and east, and is notable
for such deposition (Fig. 4.32). The Baltic
Coast of Denmark, Germany, and Poland
received quantities of outwash that was
trapped between the ice front to the north
and higher ground to the south.
Glaciated surfaces with little drift
Some glaciated surfaces have little drift.
They occur principally in the rougher parts
of the crystalline rock regions of Canada,
Scandinavia, and Finland. In these areas the
thin preglacial soil cover was extensively re-
moved, especially from the higher spots, and
the resistant bedrock yielded little new
drift. Locally, shallow basins were excavated
in the bedrock, and lakes now occupy them.
Patchy drift was deposited in the low spots,
but especially on the uplands the naked
bedrock has been left exposed (Fig. 4.33).
These bare rock surfaces were in many
places crudely smoothed by the action of the
ice, and often show grooves or scratches that
indicate the direction in which rocks were
dragged across them by the glacier. Joints,
fault lines, and other bands of weaker ma-
terial show especially clearly because they
have been etched out by selective erosion.
Ice-scoured surfaces of this kind are gen-
erally of low human utility because of their
thin, stony, and patchy soils, their irregular
surfaces, and the large amounts of standing
water. Rapids and falls provide some useful
water-power sites, and valuable mineral de-
posits have been discovered in some of these
areas of ancient rocks, but for the most part
they offer little to man.
PLAINS AFFECTED BY
UNDERGROUND SOLUTION
How solution affects the surface Rel-
atively limited areas of the world's plains
owe their surface form primarily to the work
of agents other than running water or mov-
ing ice. The most significant are those show-
ing the effects of subsurface solution and
those modified by wind action.
Plains 93
THE PRINCIPAL
GLACIAL DEPOSITS
IN THE
GREAT LAKES REGION
OF THE
UNITED STATES
Marginal moraines
Outwash plains and
valley trains
==} Glacial lake deposits
Undifferentiated drift of
earlier glaciations
| | Driftless regions
GENERALIZED FROM A MANUSCRFT
MAP OF THE GLACIAL GEOLOGY OF
NORTHEASTERN UNITED STATES COM-
PILED BY KARL 6RAETZ AND F. T.
THWAITES. UNIV. OF WISCONSIN, 1933.
FIG. 4.32 The pattern of arrangement of the drift deposits in the Great
Lakes region. (Reproduced by permission of F. T. Thwaites.)
Subsurface solution is highly significant dissolving the rock. In time the mass be-
only in regions underlain by relatively pure comes honeycombed with cavities, some of
limestones. Water in the ground makes its them of cavernous size. This weakens the
way along joints and between layers of the structure of the rock, and parts of it collapse,
limestone and enlarges these openings by If breakdown occurs near ground level, the
94
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
F I G . 4 . 3 3 The rounded uplands and rock basins of an ice-scoured
surface in northern Canada, where vegetation is scant. Note the different
elevations Of the lakes. (Royal Canadian Air Force.)
surface sinks down, forming a depression, or
sinkhole (Fig. 4.34).
Solution features Sinkholes are of all
sizes, but those in plains areas are usually
no more than a few feet or at most a
few tens of feet in depth, and from a few
yards to a large fraction of a mile in diameter
(Fig. 4.35). They may have openings at the
bottom through which surface water can es-
cape to underground passages. But some-
times the opening becomes stopped with clay
and other debris, so that a small lake is
formed.
These depressions are the most distinctive
feature of topography in which solution has
played a part. Where they are closely spaced
and small, the surface strongly resembles
morainic topography. In addition, however,
solution areas commonly have strikingly few
surface streams. A small stream will run on
the surface for a short distance and will then
disappear into a sink opening or small hole,
pursuing the rest of its course underground.
In many depressions and valleys streams
emerge from caves or springs, sometimes
diving again into the ground some distance
downstream.
Sizable areas of solution-marked plains
FIG. 4.34 Sinkholes and their relation to solution cavities beneath the surface. (V. C. Finch.)
occur in northern and northwestern Florida;
smaller areas are numerous and widespread.
Several highly spectacular solution land-
scapes of high relief exist in various parts of
the world, most notably in western Yugo-
slavia, but these cannot be classified as
plains.
PLAIN SURFACES
SHAPED BY THE WIND
Occurrence and characteristic ero-
sional features Wind-shaped plain sur-
faces are largely confined to the dry parts of
the world. Even there the effects of wind
action are usually less important than the
work of water. Except for the few great seas
of sand dunes, wind-produced features are
mostly minor details on surfaces sculptured
principally by water.
Wind erosion produces three kinds of
noticeable features. Some broad, shallow,
enclosed depressions that cannot be other-
Ptains 95
wise explained are attributed to wind erosion
resulting from destruction of the vegetation
by some local cause. These features, called
blowouts, are common in arid and semiarid
regions. Also attributable to the wind are
the various polished, etched, and curiously
formed bedrock features produced by sand-
blasting during strong winds. The most ex-
tensive products of wind erosion are the
gravel plains left behind by the selective re-
moval of fine particles from the surface layers
of mixed alluvium. This "desert pavement"
occupies broad areas in most of the world's
dry lands (Fig. 3.30).
Depositional features The only pro-
nounced surface forms directly attributable
to deposition by the wind are sand dunes.
They have developed chiefly from thick
alluvial deposits of lowland basins and in
some instances from the residual weathering
products of sandstones (Fig. 4.36).
Where sand is thick and abundant, dunes
commonly form as series of great waves sim-
F I G . 4.35 Limestone plain with numerous small sinkholes, some
containing ponds. Near Park City, Kentucky, south of Mammoth Cave.
(IL Ray Scott, National Park Concessions, Im., (ourtesy Kentucky Geological Sun>ey.)
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
F I G . 4 . 3 6 The billowing, wind-rippled forms of one of the small patches
of sand dunes in the American desert. Death Valley National Monument.
(George A. Grant, National Patk Service.)
ilar in form to waves in the sea. However,
where strong winds may come from various
directions, these waves are distorted into
pyramids and other more complex forms.
Where the sand is less plentiful, dunes are
usually separated from one another, some-
times forming almost perfect crescents
(barchans) with the horns pointing downwind.
FIG. 4.37 Common types of sand dunes. In
all the examples shown, the prevailing direction of
strong winds is from left to right.
Long, peaked ridgelike forms (seifs) are also
common (Fig. 4.37). Many dunes, especially
the smaller ones, are moved along progres-
sively by the wind, not as a mass but grain
by grain. Roads, irrigated fields, and even
towns, in the Sahara and elsewhere, have
been engulfed by moving dunes.
Perhaps because of the striking forms and
movements of dunes, it is a common belief
that most deserts are largely covered by them,
but this is not true. Dunes are rather rare in
the Americas, and even in the Old World
they occupy a minority of the desert lands
(Fig. 4.38).
Dunes also occur as features of sandy sur-
faces in humid lands, especially adjacent to
beaches. Usually, however, they do not move
far from their place of origin, but tend to be
confined by the growth of vegetation.
The deposition of wind-blown loess (page
70) may serve to cover the land with an
Plains 97
PRINCIPAL
SAND DUNE AREAS
OF THE
WORLD
FIG. 4.38 Extensive areas of sand dunes are largely confined to the
deserts of the Eastern Hemisphere continents.
extensive blanket of silt several inches or
even many feet thick (Fig. 4.39). But the
smooth-surfaced loess accumulations, unless
they are unusually thick, do not greatly
change the form of the preexisting terrain,
unlike sand deposits. Because of the relative
ease with which loess is eroded, and because
FIG. 4.39 Loess deposits are widespread in
the United States. (From C. F. Marbut. U.S. Department
of Agriculture. )
of its propensity for maintaining steep slopes,
areas of thick loess are nonetheless com-
monly rather strongly dissected by streams,
and in places have become quite picturesque
(Fig. 4.40).
F I G . 4 . 4 An eroded and slumped hillside in
deep loess in central Nebraska.
ill CHAPTER 5
Surfaces
rougher
than plains
HILLS AND MOUNTAINS
ORIGIN AND
DEVELOPMENT
Basic nature; tectonic background
Hill and mountain lands are distinguished
from other types of surfaces by the fact that
the majority of their slopes are too steep to
be described as gentle. In addition, most hill
lands have local relief of several hundred
feet, and the relief of mountains is still
higher.
98
As previously mentioned, high relief can
develop only if parts of the area are built or
carried far above their surroundings by crustal
deformation or vulcanism, or if the entire
area is brought so far above the baselevel of
erosion that streams can cut deeply into the
surface. In either instance tectonic activity
of considerable strength is necessary. So it
is no surprise to find that the world's moun-
tains occur in strongly disturbed parts of the
crust.
Furthermore, a considerable amount of
the disturbance must have occurred late in
geologic time; otherwise the surface irregu-
larities would already have been smoothed
out by gradation. In most mountain areas
major crustal disturbance appears to have
taken place several times, with long intervals
between. In these instances the present
mountains are simply the latest of several
generations that have existed in one place.
Thus, for example, the first (and strongest)
crustal deformation in the Appalachian
Mountain region took place some 250 million
years ago, but the mountains produced at
that time were soon destroyed. An unknown
number of generations of mountains have
come into existence and been destroyed in
the area since then, and the present ones are
not more than a few million years old. The
latest disturbance was a broad, rather modest
upwarping of the old complex structures,
with carving out of the existing forms by
differential erosion.
Steep slopes naturally accompany high
relief. Streams that have far to go in order to
reach baselevel commonly achieve steep
gradients and cut down rapidly, producing
steep-sided, often canyonlike valleys. As
suggested earlier, a predominance of steep
slopes is rather rare in areas of low relief,
where slow downcutting is more the rule.
Hills of low relief usually require unusual
conditions for their development, conditions
that favor concentrated runoff and especially
swift erosion. Thus such hills are especially
likely to develop in dry areas, limestone
areas, and loess areas.
It is worth emphasizing that extensive
areas of hills are rarely if ever simply worn-
down mountain lands. Where mountains
have been worn down to moderate relief,
their valleys have ordinarily been so widened
by erosion and mass wasting that only widely
Surfaces rougher than plains 99
separated clumps or lines of high-standing
remnants are left. Broad expanses of hills
such as the Ozarks and the hills of eastern
Kentucky and West Virginia have been
formed by the cutting of a close network of
narrow valleys in extensive masses of the
crust that have been only moderately up-
lifted. The relief has not, in late geologic
time, been appreciably larger than it is now.
Sculpturing The sculpturing of rough
lands is accomplished by the same agents
that shape the surface features of plains. It is
therefore not necessary to recapitulate the
principles of surface development but only
to consider their application to these surfaces
of higher relief and predominantly steeper
slopes.
Mountain and hill lands are all carved out
of masses of the crust that have been lifted
up or built up significantly by crustal defor-
mation or vulcanism. The forms taken by
the various ranges, groups, ridges, valleys,
and peaks depend first upon the nature of
the gradational agents and the form and
geologic structure of the mass on which they
have to work. Most mountain forms are
erosional; that is, they are produced by such
processes as stream cutting, mass movement,
and glacial quarrying. Only the occasional
uneroded fault scarps, volcanic cones, lava
flows, and alluvial features are exceptions.
CHARACTERISTICS
STREAM ERODED FEATURES
Valley forms and patterns The ma-
jority of streams in rough lands are still cut-
ting down rapidly. Their gradients are steep
and irregular, with many rapids and falls.
Because valley deepening is going on actively,
there is no chance for valley floors to open out;
so the cross sections are commonly V-shaped.
Where the rocks are especially resistant to
100
*' J u . 5 . l I he Black uanyon of Gunnison
River, Colorado, a narrow gorge cut by an
active stream in resistant rock. (W. 7.' Lee, U.S.
Geological Survey.)
weathering, valley widening is so slow in
comparison to deepening that a slitlike gorge
develops (Fig. 5.1). Where rocks are weaker
and stream gradients gentler, valley sides are
much less steep, provided there is enough
surface runoff or mass movement to wear
them back.
In those rough lands where erosion has
proceeded for a long time, the principal
streams have attained gentle gradients and
their valleys have begun to widen and to
develop open floors. Many of the tributary
valleys, however, are still steep and V-shaped.
Eventually the continued widening of the
principal valleys tends to parcel the rough
land into separated masses and spidery
ridges. Such areas are likely to offer much
more usable land and somewhat easier paths
of access than those in which erosion is less
far advanced.
As in plains, the pattern of valleys and
ridges will be dendritic unless there are pro-
nounced local variations in rock resistance
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
or peculiarities in the arrangement of original
slopes (Fig. 5.2). However, because com-
plex rock structures and irregular arrange-
ments of slopes on the original uplands are
almost the rule in mountainous areas, depar-
tures from the dendritic pattern are common.
And because the relief is great, these pecul-
iarities are more obvious and more significant
than they are in plains (Fig. 4.7).
Slopes, peaks, and ridges The steep
slopes of hills and mountains favor rapid
wash and active mass movement on the valley
sides. Landslides, avalanches, and sudden
floods are not uncommon. On especially
steep, rocky mountainsides sizable blocks of
rock, dislodged by weathering and ice wedg-
ing, roll or fall down the slopes, accumulat-
ing in steeply inclined piles or fans called
talus or scree (Fig. 3.23). Hillside soils are
characteristically thin and stony because of
the continual stripping of the surface by
erosion.
Most peaks and ridges are simply what
has been left after the cutting of adjacent
valleys. Their pattern is determined by the
pattern of the streams. Sometimes this
arrangement is strongly conditioned by varia-
tions in the resistance of the rocks, with
ridges and peaks upheld by outcrops of un-
usually resistant rock. In a few mountain
areas many or all of the individual high
peaks are volcanic cones.
In erosional mountains, as in stream-eroded
plains, upland divides are continuous and
high during the earlier stages of development.
In time, however, the divides become nar-
rowed to relatively sharp crests and from then
on are subjected to irregular lowering.
Notches appear at the heads of major valleys
and then at tributary heads (Fig. 5.3). As the
number and depth of the notches increases,
the divides change from continuous high
Surfaces rougher than plains 101
ridges to lines of well-defined peaks separated
by lower gaps or passes. The existence of
such gaps may be highly important to the
location and relative ease of transport routes
through a mountainous area.
Some areas of hills or mountains are re-
markable for the close spacing and extreme
sharpness of their gullies, ravines, and ridges.
Such intricacy of sculpture is fairly common
in dry lands, where the surface is not pro-
tected by vegetation and gullying is the rule.
In a few regions of the world there are hill
lands of almost incredible ruggedness and
complexity, The famed Badlands of the
western Dakotas are an excellent example
(Fig. 5.4). These have been carved out of
weakly cemented, easily eroded sandy silts in
a semiarid climate. Small-scale features of
the same sort can often be seen on dirt fills
and old mine dumps that have been allowed
to remain for years without a plant cover.
Effects of structure The form and the
rock structure of the upraised mass of the
crust can have strong effects upon the moun-
tains or hills that are carved from it. These
effects are seen sometimes in the form and
pattern of major features and sometimes in
relatively minor details of slope and crest.
Where crustal disturbance or volcanic out-
pouring has occurred very rapidly and
recently, the shape and pattern of the up-
lifted section of the crust is clearly visible in
the form and outline of the mountain mass
itself. One of the most striking examples is
to be seen in the Sierra Nevada of California.
This great range is sculptured from an im-
mense tilted fault block nearly 400 miles
long. The precipitous fault scarp, in places
more than 8,000 ft high, faces eastward, and
the tilted surface of the block slopes more
gently toward the west (Fig. 3.5). Both
slopes have been deeply cut by streams and
FIG. 5.2 Aerial view in the Allegheny hill region
of West Virginia shows it to be a stream-dissected
upland with a dendritic valley pattern. (John L. Rnh.
(btithw of the "GmRwphual Review," Amentan
(irtifriapfiual Sot iti \ of New York.)
valley glaciers, but the general form and the
size of the range are those of the block.
Such fault-block ranges are not uncommon,
though few are as large as this one. Other
ranges and groups of mountains and hills
show forms and outlines produced directly
by rapid and recent folding or doming.
More commonly, however, uplift occurs
so slowly that by the time it is completed the
upraised mass has already been strongly
eroded, so much so that the resulting moun-
102
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
FIG. 5.3 Brown Pass, a saddle-shaped notch in Glacier National Park,
Montana. The white arrow point touches the crest of the pass, which crosses
the Continental Divide. Note talus slopes at foot of cliffs at right.
(National Park Service.)
tains or hills bear little direct relationship to
the form of the uplift except in general extent
and outline. In these cases the complete struc-
ture of the mass can be deduced only by
inference from what is left of it. Often the
amount eroded away is larger than the bulk
of the mountains or hills that remain.
It was mentioned earlier in this chapter
that in most rough lands there is evidence
that uplift has occurred repeatedly and that
the present mountains and hills are not the
first ones to exist. In these situations it is
almost a rule that the earlier disturbances
were the most violent and complex, and that
the later ones tended to be simply broad
general uparchings of relatively modest
height. But these upwarped masses are still
rooted on the older and much more com-
plex structures, which not infrequently
include rocks of high resistance. As a result,
differential erosion becomes very important
in determining patterns of ranges and valleys
within the broadly upwarped area.
Some of the most remarkable examples of
the effects of structure are found in the
Appalachian Highlands of the eastern United
States. The present mountains are the result
of erosion on a broadly upwarped section of
the crust in which are some very old and
complex structures. The western part of the
highlands is underlain by nearly horizontal
rock strata. Since, within a limited area, there
are not great differences in rock resistance,
the valley patterns are dendritic, and the
entire area is carved into a jumble of hills
and low mountains (Figs. 2.90, 5.2). The
easternmost section of the highlands is de-
veloped on a highly complex structure, but
one in which the ancient rocks involved do
not vary much in resistance. Here again the
mountains display a typical dendritic pattern
(Fig. 5.5). Between these two areas, however,
is a long belt of layered rocks that were in
ancient times thrown into a remarkable series
of long, parallel wrinkles. The layers now
,outcrop in parallel bands and vary greatly
in resistance. Erosion on this structure has
etched out valleys along the bands of weaker
rock, leaving the edges of the resistant
Surfaces rougher than plants 103
strata standing in relief as long ridges (Figs.
5.6, 5.7). Somewhat similar features are
found in the Ouachita Mountains of Arkan-
sas and in the Jura of eastern France.
Less striking, though not less significant,
effects of structure are common. Thus, for
example, most of the ranges of the Rocky
Mountains of Wyoming and Colorado owe
their present height above their surroundings
to the superior resistance of their rocks,
which are the roots of earlier, folded moun-
tain structures.
Rock structure has many effects upon the
smaller features of rough lands, just as it
does in plains. Resistant outcrops stand out
as ridges and peaks or form ledges and cliffs
on the hillsides (Fig. 5.8). Joints, fault lines
and nonresistant outcrops become sites of
rapid erosion and therefore serve to localize
F 1 . 5 . 4 The intricately dissected terrain of the Badlands of western
South Dakota.
104
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
FIG. 5.5 The Irregular, peaked ridges and eroded basins of the
southern Blue Ridge, Great Smoky Mountains National Park, North Carolina.
(National Park Service.)
valley cutting. Scarcely a slope, a crest, or a
stream course fails to show at least some
effect of the underlying rock structure.
FIG. 5.6 Series of diagrams in chronological
sequence to illustrate the erosional development of
linear ridges. (V. c. Finch.)
GLACIAL FEATURES
Mountain glaciers Large areas of hill-
and-low-mountain lands in northern North
America and northwestern Europe were
completely overrun by the continental ice
sheets of the Pleistocene period. The higher
mountains in those areas, as well as else-
where in the world, supported large glacial
tongues in their valleys; indeed, many still
do. Both continental and valley glaciers had
significant effects upon the land forms, and
the still-existing glaciers are significant forms
in themselves.
From their broad, snow-covered areas of
accumulation on the upper slopes and
especially in the valley heads, mountain
glaciers extend far down the valleys into the
zone of melting. The tiny, almost extinct
glaciers of the western United States rarely
exceed a mile in length, but in the Rocky
Mountains of Canada and in the Alps glacial
Surfaces rougher than plains 105
tongues 5 to 10 miles long are common.
Some of the largest valley glaciers are in
southern Alaska and the Himalayas, where
lengths of 20 to more than 50 miles are re-
corded (Fig. 5.9).
The upper parts of mountain glaciers are
often concave in form, with snow-covered,
sometimes smooth surfaces. Toward the lower
ends, however, the snow cover melts away in
the summer, exposing a rather rough surface
deeply slashed in places by open cracks or
crevasses, especially at sharp turns in the
valleys and where the gradient abruptly in-
creases. Wastage of the ice by melting and
evaporation uncovers masses of rock debris
^contained within the glacier, so that the
lower" ends of many glaciers are almost ob-
scured by a cover of rubble.
Effects of continental glaciation
Mountain and hill areas that have been over-
ridden by continental ice sheets appear to
have suffered much the same kinds of modi-
fication experienced by glaciated plains. The
ice tended, in general, to smooth the surface
by eroding away crags, projecting peaks, and
small spurs, and by depositing drift in the
valleys and ravines (Fig. 5.10). In a few
places, where valleys were oriented in the
direction of glacial flow, the ice was able to
erode conspicuously in moving through the
bottleneck. As in plains, the drift-clogged
valleys are often the sites of numerous lakes
and swamps. The northern Appalachians
and the Adirondacks in New York State, the
uplands and mountains of New England, and
most of the Laurentian highlands of eastern
Canada are rough lands that have been
modified by overriding continental glaciation.
Effects of valley glaciation While
broad ice sheets have the general effect of
smoothing the topography, valley glaciers
have quite the opposite effect, for their work
FIG. 5.7 Winter aerial view along one of the
parallel ridges of the Appalachian Ridge and Valley
region in Pennsylvania. The resistant stratum that
forms the ridge dips sharply to the left. The
Susquehanna River cuts through the ridge,
forming a water gap. (Famhild Aerial Survyi, Inc.)
is restricted to the valleys themselves. Within
each valley they work vigorously, clearing
weathered rock and talus from the valley
bottom and sides, plucking in jointed bed-
rock, scouring on rock projections, and dump-
ing their transported load farther down the
valley and along the sides of the ice tongue.
Thus the walls of glaciated mountain valleys
are commonly steeper and freer of debris
than are those of typical stream-cut valleys.
Valley floors often descend in a series of
106
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
FIG. 5.8 View northward along the eastern front of the Rocky Mountains
near Denver, Colorado. Massive crystalline rocks at extreme left. In center are outcrops
and long hogback ridges on steeply inclined sedimentary strata. Mesa in
right background is capped by an old lava flow. (T. s. levering, U.S. Geokgual Survty.)
FIG. 5.9 Head of Susitna Glacier in central Alaska. Several tributary glaciers, descending
from high snowfields, join to form a large glacial tongue in the foreground. Note the many
cirques at the glacier heads, the crevasses at sharp turns and in steeper sections of the
glaciers, and the prominent medial and lateral moraines in the foreground. (Bradford Wuhburn.)
Surfaces rougher than plains 107
FIG. 5.10 Keuka Lake, one of the Finger Lakes, rests among the
smooth-sloped glaciated hills of western New York. Contrast with
Fig. 16.5 (XYSPIXConururce, Albany, JV.Y.)
FIG. 5.11 Head of a glaciated mountain valley. A large cirque in
background, with precipitous rock walls and a small remnant glacier.
Characteristic stepped-down valley profile with lakes and waterfalls.
(Hilttnan, from Glacier National Paik.)
108
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
steps, and are marked by lakes strung like
beads along a cord, some of them occupying
shallow depressions in the bedrock, some
dammed by moraines. Rapid weathering and
wearing back of the bare, steep valley walls
may reduce intervening divides to jagged,
knife-edged ridges. Many valley heads look
as though they had been excavated from the
mountain masses by tremendous scoop
shovels. Such steep-walled valley heads are
termed cirques (Fig. 5.11).
This combination of abrupt slopes, sharp
peaks and ridges, much exposed rock,
numerous lakes, and prominent moraine
ridges gives to valley-glaciated mountains a
particularly spectacular quality, which is
enhanced if there are still large glaciers and
snow fields present (Fig. 5.12). It is true that
many of these characteristics are sometimes
found in mountains that have been subjected
to especially vigorous stream erosion but
never glaciated. But the recurring combina-
F I G . 5.12 Valley-glaciated mountains of the alpine type. Note the large
abandoned lateral moraine in the foreground, indicating that the glacier was
once larger than it is now. Mount Athabasca, Alberta, Canada.
(Canadian I'acijii Railway Co.)
don of such features, and most significantly
the presence of lakes and moraines, is the
special mark of mountain glaciation. Nearly
all of the truly great and high ranges of the
earth display glacial features, some of modern
development, some dating from the Pleisto-
cene. Those ranges in which the glacial
features are especially rugged and in which
living glaciers still exist are often termed
alpine mountains.
VOLCANIC MOUNTAINS
Occurrence While many of the world's
rough lands are carved out of rocks that
have originated through vulcanism, rel-
atively few mountains have actually been
constructed primarily by volcanic activity.
The map of volcanic regions shows that
volcanoes are largely confined to Central
America, parts of the Andes, certain of the
Atlantic islands, Italy and Sicily, eastern
Africa, and the island chains of the western
and northern Pacific. The majority of the
great mountain systems, including the
Rockies, the Alps, and the Himalayas, are
nonvolcanic, although there is evidence of
local activity in past times. Yet nearly all of
the volcanic areas do lie in the mountain
belts. The same strong crustal disturbances
that have led to the development of the
great mountain systems have undoubtedly
favored the formation of molten pockets
near the base of the crust and have provided
the zones of weakness through which these
materials have escaped to the surface.
Volcanic cones Strictly speaking, the
truly volcanic mountains are the cones that
have been built up by the accumulation of
lava and ash about eruptive vents. Such
cones form as essentially isolated features,
ranging in size from insignificant hillocks to
magnificent peaks thousands of feet high and
Surfaces rougher than plains 109
several miles in diameter. As a general rule
the cones formed by explosive eruption in-
volving ash are steep-sided, while those
formed by the effusion of slowly hardening
lavas are very broad and gentle. Paricutin
(Fig. 3.7) and Vesuvius are examples of the
former, the Hawaiian volcanoes of the latter
(Fig. 5.13). However, the majority of cones
are actually made up of layers of both lava
and ash, and so are intermediate between the
two extreme varieties. Fujiyama, the beauti-
ful dormant volcano of central Japan, is a
composite cone of this type (Fig. 5.14).
Fresh volcanic cones are usually smooth
and symmetrical in form and display one or
more well-defined craters, but erosion soon
destroys this perfection. Once the volcano
has ceased to be active, its crater is breached
by ravines and its flanks become scarred and
irregular. Mount Hood, Mount Rainier, and
the other great peaks of the Cascade Range
in the northwestern United States have all
been dissected and roughened in varying
degree by the work of both streams and
glaciers (Fig. 5.15). Eventually, extinct
cones become so reduced that they can
scarcely be recognized for what they are.
Occasionally destruction of a cone is hastened
by explosion or collapse that destroys the top
of the cone, leaving an immense depression
(caldera) larger than any normal crater.
Crater Lake in Oregon occupies a caldera of
this kind (Fig. 5.16).
In some areas volcanic cones are so closely
grouped in clusters or lines that they form
mountain ranges or groups by themselves.
The high-peaked and almost continuous
range that extends the entire length of the
island of Java, for example, is made up
almost entirely of volcanic cones. Much more
commonly, however, the cones are incidental
features in areas of complex mountains formed
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
F 1 G . 5 . 13 The great "shield" volcanoes on the island of Hawaii. The
summit of Mauna Loa (13,018 ft), with several craters, in the foreground;
the broad cone of Mauna Kea (13,784 ft) in the background. The largest
crater shown is 2 miles long and nearly 2 miles wide.
(Official U.S. Navy photograph.)
F I G . 5 . 1 4 The symmetrical cone of Fujiyama
rises more than 12,000 ft above Suruga Bay and its
bordering alluvial plains. (If. Suite.)
chiefly by crustal deformation. Thus, for ex-
ample, the cones of the Cascade Range and
of the Andes have been constructed on top
of or among mountains formed by the erosion
of complex upraised masses of lava, ash, and
various older rocks.
Erosional mountains developed on
volcanic materials Where thick and ex-
tensive accumulations of lava and ash have
been upraised and deeply dissected, the re-
sulting mountains, though composed of vol-
canic materials, are actually erosional features
and do not owe their form directly to the
vnlranir activitv. Much of the San Juan
Surfaces rougher than plains 111
FIG. 5.15 Mount Hood, Oregon, is an extinct composite volcanic cone,
deeply eroded by water and ice. (U.S. Forest Service.)
FIG. 5.16 Crater Lake, Oregon, occupies a deep caldera formed by the
collapse and destruction of the upper part of a great volcanic cone. Wizard
Island (foreground) is a younger cone formed later. (National Park Service.)
112
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
FIG. 5.17 I he Abajo Mountains in eastern
Utah. This mountain group is a dissected intrusive
mass (laccolith) about 6 miles in diameter and
rising nearly 4,000 ft above the uplands of the
Colorado Plateaus. (W. ftom, U.S. Geological Survey.)
Mountains of southwestern Colorado, the
most extensive mass in the Southern Rockies,
is carved in this way out of old volcanic
debris.
Small intrusions sometimes form domelike
bulges on the surface. These too are attacked
by erosion and are sculptured into isolated
peaks or small groups of mountains. Navajo
Mountain, northeast of the Grand Canyon,
is such a feature as yet but slightly dissected.
The Henry Mountains, La Sal Mountains,
FIG. 5.18 Agathla Peak, a great volcanic
neck in northeastern Arizona. (American Museum of
Natural Histmy.)
and Abajo Mountains of eastern Utah have
been more deeply carved into rugged moun-
tain groups (Fig. 5.17). Because of the
superior resistance of their rocks, the fillings
of volcanic vents and the last remnants of
small intrusive domes sometimes remain
standing as striking towers after the surround-
ing materials have been eroded away
(Fig. 5.18).
SURFACES COMBINING SMOOTHNESS AND HIGH RELIEF
GENERAL NATURE
Large areas of the continents are occupied
by surfaces in which much gentle slope and
relatively high relief are combined. These
surfaces would, indeed, be plains, were there
not certain widely spaced features on them
having steep slopes and considerable height
or depth. The distinction has already been
drawn between those surfaces in which relief
is provided chiefly by canyons or cliffs fall-
ing away below a smooth upland (tablelands)
and those in which spaced hills or moun-
tains rise above an extensive plain (plains
with hills or mountains).
Since smooth surfaces usually imply a
dominance of gradational processes and since
high relief cannot occur without significant
uplift of a part of the crust, it is clear that
these rough-and-smooth surfaces are combi-
nations in origin as well as in configuration.
Commonly the smooth plain and the inter-
rupting irregularities must be accounted for
by quite separate series of events.
TABLELANDS
Origin Tablelands, with their relatively
smooth uplands and deep, steep-sided valleys,
are like youthfully dissected plains in which
the valleys are unusually deeply cut. Such
deep cutting requires that the original sur-
face shall have been lifted far above base-
level. Thus most tablelands have originated
as plains of various kinds that have in rel-
atively late time been uplifted or built up far
enough to permit deep dissection by streams.
However, preservation of the upland sur-
face against dissection during such deep
cutting requires that tributary development
be unusually slow. Hence the conditions that
retard tributary growth (a) flatness, (b)
porous and absorbent surface materials, (c) a
solid vegetation cover, (J) light rains, and
(e) resistant surface strata are even more im-
portant to the existence of tablelands than to
the maintenance of youthful plains. It is not
mere coincidence that most tablelands are
found in rather dry areas. Nor is it surpris-
ing that many tablelands are capped by
nearly horizontal strata of unusually resistant
rock or by sheets of porous gravel.
Characteristics The detailed features of
tablelands are chiefly products of recent
gradational activity and are not fundamentally
different from features already discussed in
connection with plains and rough lands. The
uplands may be typical stream-eroded plains
the details of which depend upon rock struc-
ture and history. Many, having developed
under dry climates on nearly horizontal rock
strata of varying resistance, show features
typically produced by those conditions.
Narrow, steep- walled ravines, low cliffs and
ledges, small, flat-topped mesas, and rocky
terraces combine to give such uplands the
Surfaces rougher than plains 113
appearance of being miniatures of the larger
tablelands of which they are a part. Other
uplands show features of alluvial or glacial
deposition. Still others are capped by lava
flows or ash deposits.
Canyons and cliffs are so characteristic of
tablelands as to demand special attention.
The existence of canyons is favored by the
very factors responsible for tablelands in gen-
eral. Late, strong uplift of the surface favors
rapid downcutting. The conditions that favor
preservation of the upland also inhibit valley
widening. So the valleys become deep but
not wide, and canyons and gorges are the
result. Their excessively slitlike appearance
is, of course, something of an illusion; rarely
are they as deep as they are wide. (Fig. 5.19).
The lines of cliffs (escarpments) that form
the margins of many tablelands usually origi-
nate either as canyon sides or as fault scarps,
though some are formed by differential erosion.
They are worn back by erosion and mass move-
ment but retain their steepness. In time they
may retreat many miles from their original
position. As they do so, the tableland surface
becomes smaller and smaller and eventually
disappears entirely. Usually the escarpment
is highly irregular, with many ravines and
projections (Fig. 5.20). Often fragments of
the tableland are detached from the main
mass by erosion and form spectacular flat-
topped hills (mesas or buttes) and columns
in front of the escarpment. Such features are
called outliers (Fig. 5.21).
The upland surfaces of tablelands are in
many places smooth enough to be easily
traversable, even to serve as agricultural land.
However, they are often rather dry, and may
be too far above the streams to irrigate. The
canyons and escarpments may provide major
obstacles to transportation and may even
make access to the uplands unduly difficult.
114
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
7 1 / 2 miles
GRAND CANYON
7500'n
LITTLE COLORADO
FIG. 5.19 Cross-section profiles of the Grand Canyon of the Colorado
River at Powell Memorial and of the canyon of the Little Colorado River
about 2 miles above its mouth. Vertical and horizontal scales are the same.
(From I/.S. Gtalogital Sun>ey topo^taphii \hffti Grand Canyon National Park, east half.)
Occurrence The principal tablelands of
North America, all in the western part of the
continent, differ in rock structure and in the
manner in which they have developed. The
several tablelands about the Colorado River
in Utah, Arizona, New Mexico, and Colorado
are strongly uplifted erosional plains on
gently warped rock strata. The climate is dry,
but a number of large streams emerging from
the more humid mountains round about have
cut profound canyons as they cross the up-
land (Fig. 5.22). In parts of eastern Washing-
ton and Oregon are tablelands carved out of
thick and extensive accumulations of highly
porous lava. Again the climate is dry, and
there is little local runoff. Canyon cutting is
accomplished by streams from the surround-
ing mountains. Just east of the Rocky Moun-
tains in the northern United States and
Canada is a broad area of tableland with
only modest relief. The smooth, grass-
covered upland is an ancient piedmont
F I G . 5 . 2 The ragged edge of an
escarpment that is being driven back by erosion.
Painted Desert, northeastern Arizona.
(Sfance Air Photo*.)
F I G . 5 . 2 1 A small mesa (right) and several
buttes in Monument Valley, Arizona. These are
detached outliers of a neighboring tableland.
(American Mu&tum of Natural History.)
Surfaces rougher than plains 1 15
FIG. 5.22 The Grand Canyon of the Colorado River in Arizona. I he
portion of the canyon visible here is cut in nearly horizontal sedimentary strata.
Resistant layers form the cliffs, weak strata the gentler slopes. (National Park Service.)
alluvial plain of unusual extent. More
recently it has been trenched by streams
coming out of the Rockies.
Other significant tablelands are the sand-
stone and lava-capped uplands of interior
southern Brazil, the low somewhat cuesta-
form plateaus of southern Russia, and the
broad, stony cuestas (hammadas) of the
northern Sahara.
PLAINS WITH HILLS
OR MOUNTAINS
Modes of development In contrast to
tablelands, plains with hills or mountains
owe their large relief to steeply sloping
features that rise above the more extensive
plain surface. As in tablelands, the high
relief requires that tectonic action shall have
raised at least part of the area above its sur-
roundings or above baselevel, while the
broad plains indicate active gradation.
There are at least two very different se-
quences of events that combine tectonics and
gradation in such a way that plains sur-
mounted by hills or mountains are produced.
In the first, a generally mountainous area
is reduced by erosion to the stage of early
old age, in which only a few mountain rem-
nants are left standing upon an erosional
116
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
FIG. 5.23 Near the western edge of the Appalachian Piedmont,
numerous small monadnocks remain standing upon an erosional plain
developed on crystalline rocks. This is Big Cobbler Mountain in northern
Virginia. (John L. Ruh. Courtesy of the "Geogiaphical Review, " American Geographical
Society of Neiv Yotk.)
plain. The second sequence produces spaced
mountains upon an original plain by the rais-
ing of fault blocks, the building of volcanic
cones, or other localized tectonic disturbances.
Erosional varieties Early in this chap-
ter it was stressed that erosion of a mountain-
ous area does not simply produce lower but
still continuous mountains. Instead, the
valleys widen at the expense of the moun-
tains between them, eventually becoming
very broad and merging with one another to
form a low-level plain of irregular outline.
For a long time, however, there may remain
standing upon such a plain numerous steep-
sided ridges, isolated peaks, and small groups
of hills.
Mountain-studded terrain of this origin
occurs in parts of the eastern fringe of
the Appalachians between Virginia and
Georgia (Fig. 5.23). Glaciated varieties are
found in parts of southern New England, the
western Adirondacks, and in many sections
of the extensive ancient-rock shield of cen-
tral and eastern Canada. Similar terrain
occurs in eastern Sweden and northern
Finland. The most extensive areas of
erosionally developed hill-studded plains are
found on the hard-rock uplands of central
Africa, where the remnants are often quite
small and isolated. Much the same develop-
ment has occurred in northeastern Brazil and
in the eastern Guiana Highlands north of the
Amazon plains (Fig. 5.24).
Tectonically produced varieties Sur-
Surfaces rougher than plains 117
faces that have followed the second sequence
of development are more widespread. In the
majority of examples the mountains have
been produced by faulting or folding, though
there are a number of limited areas of plains
dotted with volcanic cones.
As already suggested, the mountains,
whatever their tectonic origin, are attacked
by erosion as soon as they begin to appear,
so that they are rarely purely tectonic forms.
In many instances it is clear that the moun-
tains that now exist have been greatly re-
duced from their original size. Thus the
plain has expanded at the expense of the
mountains, and the surface represents the re-
sult of a combination of both of the
sequences of development.
One of the world's more extensive plain-
and-mountain areas is the Basin and Range
province, which occupies much of the south-
western United States and nearly all of
northern Mexico. From southern Oregon to
Mexico City this landscape of dry plains and
rather small but rugged mountain ranges ex-
tends without a break and with only a mod-
erate degree of internal variation. Most of
the ranges are believed to have originated as
raised and tilted fault blocks. Some of these,
especially near the northwestern corner of
the region, are fairly fresh and undissected.
Most, however, are strongly eroded and re-
duced in size, much of the debris of their
destruction having been deposited on the
plains between them (Fig. 5.25). Some show
evidence of more than one major period of
tectonic disturbance. A few volcanic moun-
tains exist, but they are numerous only at
the southern end of the region, in central
Mexico. In that section are hundreds of
cones, ranging from cinder hillocks to the
majestic and snow-capped Popocatepetl.
It cannot now be determined how smooth
o :jw
JFK*
ti
., "' ' ' . -' [ "' *"-&
'**" ""*" ft " ' '
: !
FIG. 5.24 Remnant hills left standing upon
an erosional plain developed on crystalline rocks in
British Guiana. (D. Holdtid&. Courtew oftht
"Gro^iaphiuil Review," American Geographical Society of
Jfew York.)
a plain existed in the Basin and Range
province before the fault blocks and cones
were formed. It is clear, however, that nearly
all of the individual plains there have recently
been extended and smoothed by erosion of
the mountains on their margins and by
deposition of alluvium on their floors. Many
are enclosed depressions, in the lowest parts
of which are salt-covered flats or temporary
saline lakes (Fig. 5.26).
Much of the Middle East, especially Iran
and Afghanistan, closely resembles the Basin
and Range province in topography and de-
velopment. In central Asia, Tibet, and the
central Andes of South America are surfaces
displaying features of the same kinds but on
a grander scale. The latter two areas are
among the world's highest: even the basin
floors stand at elevations of 10,000 to 16,000
ft, and the summits of the ranges often
exceed 20,000 ft.
Because of the discontinuity and wide
spacing of their peaks and ranges, plains
with hills or mountains rarely offer serious
hindrance to through transportation. Where
conditions of soil, water, and climate are
118
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
^-u^.''^^''.' 1 ' '"
FIG. 5.25 Deeply eroded ranges alternate with smooth basin floors in
the Mojave Desert of southeastern California. (Spence Air Photos.)
favorable, these plains also furnish agricul-
turally valuable land. Unfortunately, large
areas of this kind of terrain, such as the
Basin and Range province aad the Middle
FIG. 5.26 Extensive salt flat on floor of
enclosed basin in Basin and Range province.
Clayton Valley, near Silver Peak, Nevada.
(C. I). Walcott, U.S. Gfologiial Survey.)
East, are excessively dry. Yet the alluvial
fans, basin fillings, and occasional flood-
plains in these areas of dryness sometimes
provide possibilities for irrigation. Water
from the adjacent mountains may be con-
ducted to the alluvial surfaces by means of
ditches, or it may soak into the porous
alluvium where it can be reached by shallow
wells.
Surfaces rougher than plains 119
SELECTED REFERENCES
Atwood, W. W.: The Physiographic Provinces of North America, Ginn & Company, Boston, 1940.
Cotton, C. A.: Volcanoes as Landscape Forms, John Wiley & Sons, Inc., New York, 1952.
Fenneman, N. M.: Physiography of Eastern United States, McGraw-Hill Book Company, Inc.,
New York, 1938.
: Physiography of Western United States, McGraw-Hill Book Company, Inc., New York,
1931.
Finch, V. C., G. T. Trewartha, A. H. Robinson, and E. H. Hammond: Physical Elements of
Geography, 4th ed., McGraw-Hill Book Company, Inc., New York, 1957.
Flint, R. F.: Glacial and Pleistocene Geology, John Wiley & Sons, Inc., New York, 1957.
Lobeck, A. K.: Geomvrphology: An Introduction to tfte Study of Landscapes, McGraw-Hill Book
Company, Inc., New York, 1939.
Thornbury, W. D., Principles of Geomorphology, John Wiley & Sons, Inc., New York, 1954.
Wooldridge, S. W., and R. S. Morgan: An Outline of Geomorphology: The Physical Basis of
Geography, 2d ed., Longmans, Green & Co., Ltd, London, 1959.
CHAPTER 6
The margins
of the lands
+j . i .
FEATURES OF THE COASTAL ZONES
That the continents end and the oceans
begin at the shore line is obvious. But if the
statement is changed to "the continental plat-
forms end and the ocean basins begin at the
shore line," it is no longer true. Along most
continental shores the sea bottom does not
drop off immediately to great depths. Instead
it usually falls away gradually until it reaches
depths of 400 to 600 ft and then plunges
steeply to the floor of the deep-sea basin
(Fig. 6.1). The shallower, gently sloping
zone is a part of the continental mass both
in form and geologic structure, and is called
the continental shelf. Its steep outer margin,
120
the continental slope, is the true edge of the
continent. Widths of the continental shelf
vary from less than a mile to as much as 400
miles. The widest shelves are commonly
found adjacent to low-lying plains on the
continents (Fig. 6.2). Mountainous coasts,
on the other hand, are often bordered by
narrow shelves or by none at all.
As will be seen below, the shore lines of
the world have, in late geologic time, re-
peatedly shifted their positions across the
continental shelves and the lower margins of
the present land areas. The existing shore
lines have only recently been established.
The margins of the lands 121
Depth at edge of shelf
400 to 600 ft
FIG. 6.1 Relation of continental shelf and continental slope to shore line and ocean floor.
ISO 120 90 60 30 30 60 90 120 ISO 180
FIG. 6.2 World map of continental shelves.
CHANGES OF SEA LEVEL AND DEVELOPMENT
OF SHORE LINES
Erosion and deposition by waves and cur- their mark upon the features of the coastal
rents, deposition by streams and glaciers of zones. More important, however, have been
the adjacent lands, organic accumulations, large changes in the relative levels of land
and various tectonic happenings have all left and sea, for it is these changes that have
122
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
actively moved the shore lines about and
thus greatly influenced their pattern.
A change in the relative levels of land and
sea can be produced in two quite different
ways. First, the land itself can be raised or
lowered by crustal movement. Second, the
sea surface can be raised or lowered, either
by a change in the amount of water in the
oceans or by a tectonic alteration of the total
volume of the basins in which the oceans
rest. These things have all happened fre-
quently and on a large scale in relatively re-
cent time, producing two kinds of changes
in the position of the shore line, (a) If the
land rose or the sea level sank, the shore
line migrated seaward across the continental
shelf, (b) If the land sank or the sea level
rose, the shore line moved landward.
The changes in sea level that accompa-
nied the formation and disappearance of the
great Pleistocene ice sheets were especially
FIG. 6.3 Map of the Red Sea and its
surroundings, showing the relation of its outline to
major fault lines. (After Machatschck.)
significant for the earth as a whole. Since the
water that formed the glaciers originally
came from the sea and was returned to the
sea when the ice melted, the sea level sank
as each ice sheet grew, and rose again as the
ice melted. When the last glaciation was at
its maximum, the sea level is believed to
have been nearly 400 ft lower than it is now,
thus exposing as dry land vast areas of what
is now continental shelf. Thereafter the water
surface rose irregularly, probably reaching
its present level no more than a few thousand
years ago. Tectonic raising and lowering of
some coastal lands, especially in the Far
North and around the Pacific rim, have oc-
curred even more recendy.
As a result of these events, the present
position of the shore line has been only re-
cently attained; indeed, in some places the
shore is still shifting because of change of
level of the land. The effects of these move-
ments of the shore line may be clearly seen
in the present outlines of certain coasts and
in various features repeatedly encountered in
the coastal zones.
Bays The majority of bays and gulfs are
the result of a rising of sea level. In some in-
stances a section of the continent has been
carried below the level of the sea by warp-
ing, folding, or downfaulting. The Red Sea,
Persian Gulf, and Gulf of California are
major arms of the sea that occupy such struc-
tural depressions (Fig. 6.3). Some broader
bordering seas, such as Hudson Bay and part
of the Gulf of Mexico, may have resulted at
least in part from gentler downwarping of
sections of the continental margin. But most
of the innumerable indentations of the coasts
have been formed by the drowning of ero-
sional topography by a rising sea level. It is
not surprising to find such features so widely
The margins of the lands 123
distributed, since the large rise of sea level
during the melting of the last continental
glacier was world-wide.
A general rise of sea level or a broad tec-
tonic depression of the land permits the
establishment of a new shore line upon what
was previously the land surface. This new
shore line assumes the position of a contour
line upon the former land surface. Its out-
line, when first established, follows all of the
wanderings of that contour line and thus re-
flects the form of the drowned surface.
If the submerged surface was a smooth al-
luvial slope, its contours were regular, and
the new shore line is similarly regular. But
if the surface was an erosional surface, as is
more often true, its contours were probably
highly irregular, and the shore line resulting
from its drowning is also irregular. The sea
penetrates the valleys, forming bays, while
the higher divides remain above water as
peninsulas or headlands. Individual embay-
ments resulting from submergence are called
drowned valleys. Those at the mouths of
rivers are called estuaries.
Drowned valleys; estuaries If the gra-
dients of the valleys drowned are very
gentle, the resulting embayments reach far
into the land, while the drowning of steeply
pitching valleys produces only relatively
short indentations. The form and pattern of
the bays follow the form and pattern of the
valleys that have been drowned. Thus some
bays are dendritic, others are simple and
parallel, and still others are highly irregular.
An especially fine example of an estuarine
shore line is the Atlantic Coast of the United
States in Delaware, Maryland, Virginia, and
North Carolina (Fig. 6.4). There, through
the drowning of a dendritic system of broad
river valleys having particularly gentle gra-
F I G . 6 . 4 The middle Atlantic Coast of the
United States exhibits a remarkably fine
development of estuaries.
dients, estuaries of unusual size and length
were produced.
Other strikingly estuarine coasts occur in
northwestern Spain, Greece and western
Turkey, western Ireland and Scotland,
southern China, and southern Japan. The
coasts of New England, Nova Scotia, and
Newfoundland show an interesting variation
on the usual estuarine pattern, an unusual
complexity of outline resulting from the
drowning of an irregular glaciated surface
rather than a stream-eroded land (Fig. 6.5).
The extensive stretches of the world's
coast lines that are not impressively estuarine
are still rarely, if ever, completely free of
existing or former estuaries. Most of these
coasts are characterized by relatively steep-
gradient valleys that yielded only small
estuaries. And even these have been filled in
124
A
glaciated
3
FIG. 6.5 The drowned glaciated shore line of
Casco Bay, Maine, has many islands, rocky
peninsulas, and narrow inlets.
many instances by the deposits of the swift-
flowing streams. Thus, for example, the
Pacific Coast of the United States, though it
has but few large estuaries, abounds in sedi-
ment-filled valley mouths that were formerly
small bays formed by the rise of sea level.
Fiords Several mountainous coasts of
the world have large numbers of narrow,
deep, and spectacularly steep-walled bays,
some of which penetrate unusually far into
the land. These magnificent estuaries, known
by the originally Norwegian name fiord, are
ice-scoured mountain valleys drowned since
their formation (Fig. 6.6). The extreme depths
found in many fiords suggest that there must
F I G . 6 . 6 A characteristic fiord landscape on
the Norwegian coast. (Kirk H. si<w.)
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
have been glacial erosion below even the
lowered sea levels of glacial times. The prin-
cipal regions of fiords are the coast of Nor-
way, Greenland, northern Labrador and
the eastern Arctic Islands, British Columbia
and southern Alaska, southern Chile, and the
west coast of South Island, New Zealand
(Fig. 6.7). All of these are in the higher
middle latitudes, where, at least during the
glacial period, many valley glaciers descended
to the sea.
Cliffs and terraces Coastal cliffs and
terraces are largely the product of waves.
Wherever the water on exposed coasts
deepens rapidly to seaward, waves break
directly against the shore, sometimes with
F 1 G . 6 . 7 The f iorded coasts of Norway,
southern Chile, and British Columbia and southern
Alaska are similar in pattern.
_____
1
OF
TO
ALASKA
THE
FIORDEO
COASTS
OF THE
WORLD
AT
COMPARABLE
MAP
The margins of the lands 125
FIG. 6.8 Profile of the features on a shore line that is being subjected to wave erosion.
great force. Through this attack the shore is
undercut and driven back, producing a sea
cliff. Then, as the cliff is worn back, it leaves
behind at its base an erosional shelf sloping
gently seaward just below the sea surface.
The material eroded from the cliff face is
dragged out across this shelf by the return
flow from the breaking waves and is deposited
in deeper water at its outer edge. As the shelf
is widened by erosion at its inner edge it is
also extended seaward by deposition at the
outer margin. In this way a marine terrace is
cut and built by the waves (Fig. 6.8). Because
of local inequalities in the rate of erosion,
there are often many pinnacles and rocky
islets left behind on the widening erosional
terrace as the cliff retreats (Fig. 6.9).
In many places sea cliffs and marine ter-
races have been either submerged or left
high and dry by changes of relative levels of
land and sea. Submerged terraces are com-
monly hard to identify because they have
been obscured by later deposition. Cliffs and
terraces that have been exposed above sea
level, on the other hand, are common and
often prominent features of the world's coastal
belts. Some coasts display a whole series of
such terraces, rising like steps from the pres-
ent shore. Most elevated terraces are eroded
to some extent, and some have been warped
by tectonic action. There is reason to believe
that the world-wide changes of sea level dur-
ing the Pleistocene sometimes brought the
sea surface well above its present level, and
that many of the lower terraces, at elevations
of less than 300 ft, were cut at those times.
Beaches and bars On protected sec-
tions of the coast, even on exposed coasts
F I G . 6 . 9 A wave-cut cliff and detached rocky
islets on the exposed coast of Anacapa Island,
Channel Islands National Monument, California.
(Rogrt 'Ml, Motional Paik.Senwr.)
'^i^^*^]'
126
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
that are not too frequently swept by destruc-
tive storm waves, gentle wave action tends to
move sand or gravel onto the shore, forming
beaches. Along a smooth and low coast, the
beaches may form a continuous strip many
miles in length. On irregular coasts, however,
the sediment is concentrated in the bays, the
projecting points often being swept clear and
subjected to active erosion. Through this
erosion of headlands and the accumulation
of beaches in the coastal indentations, an
irregular shore line is gradually straightened
by wave action (Fig. 6.10).
Debris that is moved parallel to the shore
by obliquely striking waves and the currents
they generate continues to shift until it comes
to an angle in the shore line, to the sheltered
or deep waters of a bay, or to a protected
position between a close-in island and the
shore. At such a place it is dropped, making
a ridge or embankment upon the bottom.
With the aid of waves that occasionally strike
them from seaward, these deposits may in
time be built up to or above the water sur-
face, forming a bar or projecting spit (Fig.
6.11).
Along coasts where shallow water causes
the waves to break far from shore, long strips
of sand, called offshore bars, are formed just
inside the line of breakers. Such bars may
F I G . 6 . 1 A sandy beach has been deposited in this sheltered bay,
while the headland beyond has been swept clear by wave action. Cape
Sebastian, Oregon (On-^ini State Highway Commission.)
spit
Land-
tied
island
Predominant
of
strong
winds
FIG. 6.11 Characteristic types and locations
of bars, spits, and beaches.
touch the land at projecting points, but else-
where they are separated from it by shallow
lagoons. Occasional breaks through the bars
are kept open by the scouring of tidal cur-
rents. The lagoon behind a bar is gradually
filled by stream deposition and marsh growth,
so that the offshore bar may eventually be
joined to the mainland.
Offshore bars are unusually well developed
along the south Atlantic and Gulf Coasts of
the United States, where they are not far
from being continuous. Many of the famous
beach resorts, such as Atlantic City, Palm
Beach, Miami Beach, and Galveston, are built
The margins of the lands 127
on these bars and are reached from the main-
land by bridges or causeways. In North
Carolina broad estuarine lagoons (sounds)
are enclosed by an especially far-flung
cordon of offshore bars, the outermost point
of which is Cape Hatteras, famous for the
number of ships that have been driven
aground on its sandy shoals (Fig. 6.12).
Coral reefs Several types of organisms
that thrive in shallow waters are able to
change shore lines, largely through depositing
remnants of their own structures. Most of
these are only locally important because their
growth is restricted to small areas of pro-
tected waters. Only the minute marine ani-
mals called corals, which can survive even
on exposed coasts, achieve great significance.
The shallow waters of many tropical coasts
are characterized by reefs of limestone com-
prised principally of the crumbled skeletal
structures of colonies of corals. Most coral
FIG. 6.12 Offshore bars and sounds along the
coast of North Carolina.
128
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
FIG. 6.13 Coral reefs and sandy islets enclose a quiet lagoon in Wake
Islands, a small atoll in the central Pacific. Shallow reefs show as light-colored
submerged areas ringed with white strip of heavy surf, best seen in right
foreground. (OJfirial U.S. Navy photograph.)
reefs form as narrow fringes along the coasts.
Fringing reefs grow with such rapidity in
some clear, shallow, warm waters that they
build a shore line seaward in spite of wave
and current erosion. Under certain conditions
corals grow abundantly in shallow waters
some distance from shore, and their deposits
form a barrier reef which is separated from
the mainland by a broad lagoon (Fig. 6.13).
Of this nature is the great reef that for 1 ,000
miles parallels the northeast coast of Aus-
tralia.
Some small reef-encircled islands, mostly
volcanic, seem to have undergone slow sub-
mergence while the coral fringe about them
has continued to grow. Such encircling reefs
now appear at the surface as low, more or
less complete coral rings, called atolls, which
enclose shallow lagoons (Fig. 6.14).
Coasts and harbors There are many
significant relationships between coastal char-
acteristics and human activities, chiefly those
involving navigation and the development of
harbors. Clearly the configuration of the
shore line and of the bottom close to shore
are factors that must be taken into account in
locating or improving ports or channels.
It must be kept in mind, however, that a
port is a result of human need and work,
rather than a feature of physical geography.
The value of a harbor depends upon its own
physical characteristics but even more upon
whether it is located where a harbor is needed.
Of course where the need for a port exists,
the character of the coast may go far to deter-
mine the amount of work and money that
must be expended in order to develop the
necessary shelter, depth of channel, and dock-
ing facilities.
A deep and well-protected bay in a place
that is well connected with a productive or
populous hinterland is a resource of incal-
culable value. It is only natural that many
significant ports have developed on estuaries
and fiords or in waters sheltered by reefs, bars,
or offshore islands. However, some of the
most commodious and sheltered bays are of
almost no value as harbors because the land
behind them is unproductive, sparsely popu-
lated, or inaccessible. One example is the
magnificent fiords of southern Chile, almost
unused because they are backejd by a wild,
storm-swept, mountainous, and nearly un-
inhabited land.
On the other hand, some of the world's
busiest ports have been developed where no
natural harbor existed, because the hinterland
required a shipping and receiving facility for
its products and imports. The harbor of Los
Angeles and to an even greater degree that of
Callao, the port for Lima, Peru, are largely
man-made; at both places long breakwaters
have been built to protect an otherwise ex-
posed section of coast. At London, Rotter-
dam, Bremen, and Hamburg, shallow estu-
aries have been heavily dredged to provide
sufficient entrance depth, and basins that can
be closed off by lock gates have been exca-
vated to provide docking space unaffected by
the excessive rise and fall of the tides.
Islands The existence of isolated masses
and bits of land surrounded by the seas, oc-
curring sometimes in groups or strings and
sometimes quite alone in midocean, has never
failed to stir man's interest and curiosity. It
is hard to avoid feeling that such peculiar
The margins of the lands 129
features, strangely rising from the open sea,
must have some unique and startling mode
of origin. Yet this is a false notion, for the
processes that produce islands are not differ-
ent from those that produce high-standing
features on the continental masses themselves.
Some islands, such as the British Isles, the
islands of eastern Denmark, Ceylon, the Arc-
tic Islands of Canada, Vancouver Island, and
such islands off eastern North America as
Prince Edward, Cape Breton, Nantucket, and
Long Islands, appear to be no more than
portions of the continental masses that have
been isolated from the mainland by the
drowning of erosional channels. The channels
may well have been cut during low stands of
FIG. 6.14 How atolls are believed to develop:
(a) fringing coral reefs about mountainous islands;
(b) growing coral deposits keeping pace with
submergence, forming encircling reef; (c)
mountainous islands submerged, with only rings of
coral remaining, (V. c. Finch.)
130
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
sea level in glacial times and then drowned
during the postglacial rise of sea level.
Other islands, however, have come into
being as the mountain peaks and ranges on
the continents have, that is, by some kind of
tectonic disturbance. Some are raised or
tilted fault blocks. Thus originated Santa
Catalina and neighboring islands off southern
California, the several islands of the Gulf of
California, and, in a more complex manner,
the islands of Tasmania and Madagascar. But
by far the greatest number of islands have
originated as volcanic cones built up from
the sea floor, as complex folded, faulted, and
eroded masses similar to most of the con-
tinental mountain systems, or as combinations
of the two. Associated as they thus are with
significant tectonic activity, it is hardly sur-
prising that so many islands are rugged and
spectacular.
Isolated volcanic islands are very common,
especially in the western Pacific. There the
ocean floor is dotted with dozens of great
cones, some of which rise above the sea sur-
face, some of which are entirely submerged.
These include not only the currently active
cones, such as those of the Hawaiian Islands,
but also the many more inactive, eroded
FIG. 6.15 The Marshall Islands are coral atolls which have formed about
the summits of thickly clustered volcanic cones that rise more than 15,000 ft
from the ocean floor in the west central Pacific. (From "Depth Curve Chart of the
Adjacent Seas of Japan." Maritime Safety Agency, Tokyo, 1952.)
THE
MARSHALL
ISLANDS
Depth contours at
intervals of 500 fathoms
OCEAN
0106,000 it
0r 6.000ft
FIG. 6.16 Map of depths in the Atlantic Ocean. The Mid-Atlantic Ridge
and several lesser ridges may be clearly distinguished, along with their
associated islands.
132
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
cones, such as those of Tahiti, Samoa, Pon-
ape, and Truk. As previously mentioned,
some of the cones have slowly become sub-
merged, leaving on the surface only low-lying
islands or atolls that have been maintained
by the continuing growth of a cap of coral
atop the sinking peaks. Midway, Wake,
Bikini, and Eniwetok are of this type (Fig.
6.15). Other isolated volcanic islands occur
in the Indian and Atlantic Oceans. In the
Atlantic several of these, including the
Azores, Ascension, and Tristan da Cunha,
are cones built along the great Mid- Atlantic
Ridge, the complex, faulted, structural swell,
FIG. 6.17 The Kuril Islands, stretching northeastward from Japan, are
a typical island arc, with an associated trough, or deep. (From '"Depth d
Chart of the Adjacent Seas of Japan," Maritime Safety A^emy, Tokyo, 1952.)
THE
KURIL ISLANDS
Depth contours
(in fathoms)
similar in size to a continental cordilleran
belt, that runs almost squarely down the
middle of the Atlantic Ocean Basin from
north to south (Fig. 6.16).
Complex folded and faulted islands most
commonly occur off the coasts of similarly
structured mountainous parts of the conti-
nents. Among these are most of the islands
of the Mediterranean, including Corsica,
Sardinia, Sicily, and Crete, and many others,
such as Borneo, Newfoundland, and Kodiak.
Many curving strings of islands, known as
island arcs, festoon the borders of the conti-
nents, especially in the western Pacific. These
include the lesser West Indies, the Aleutians,
the Kurils, the Ryukyus, the Marianas, and
The margins of the lands 133
such larger masses as Japan, the southern
islands of Indonesia, and the eastern Philip-
pines (Fig. 6.17). These island chains are in
part volcanic and in part complex folded
structures. In most instances they are bor-
dered on their seaward side by ocean-bottom
trenches of tremendous depth. They are
known to be among the most active tectonic
zones on earth, and appear to represent sites
where deformation is proceeding actively be-
cause of lateral compression of the crust. In
many respects they correspond closely to
certain of the great curving continental moun-
tain systems, such as the Himalayas, the Alps-
Carpathians, and the northern Andes.
SELECTED REFERENCES
Finch, V. C., G. T. Trewartha, A. H. Robinson, and E. H. Hammond: Physical Elements of
Geography, 4th ed., McGraw-Hill Book Company, Inc., New York, 1957.
Johnson, D. W.: Shore Processes and Shoreline Development, John Wiley &. Sons, Inc., New
York, 1919.
Kuenen, P. H.: Marine Geology , John Wiley & Sons, Inc., New York, 1950.
Shepard, F. P.: Submarine Geology, Harper 8c Brothers, New York, 1948.
Steers, J. A.: Tlie Coastline oj England and Wales, Cambridge University Press, New York,
1946.
Thornbury, W. D.: Principles of GeonwrpJwlogy, John Wiley 8c Sons, Inc., New York, 1954.
Wooldridge, S. W., and R. S. Morgan: An Outline of Geornorphology: The Physical Basis of
Geography, 2d ed., Longmans, Green & Co., Ltd., London, 1959.
CHAPTER 7
Introduction
to climate;
air temperature
and solar energy
Man permanently occupies only the solid,
or land, part of the earth's surface, not the
liquid, or sea, part. Yet in a sense he does carry
on his activities at the bottom of a sea a sea
of air hundreds of miles deep which surrounds
the solid-liquid earth. This envelope of
atmosphere is not to be thought of as some-
thing above or beyond the earth proper; it is
just as integral a part of the planet as the
land and water, and just as important a part
of the total human habitat. Man and all other
land animals, as well as the plant life from
which most animals draw their sustenance,
134
are greatly influenced by their atmospheric
environment, which varies greatly from one
part of the earth to another.
The atmosphere is very largely a mixture
of two gases, nitrogen and oxygen, but other
minor gases in it chief among them water
vapor are the greatest influences on the
atmospheric conditions experienced by man.
Thus water vapor, which on very hot, humid
days may comprise as much as 4 per cent of
the volume of surface air, is the source of
moisture for clouds and all forms of precipi-
tation. This same gas is the principal absorber
Introduction to
of solar energy and of radiated earth energy
as well, and so it greatly influences temper-
ature distribution over the earth.
The conditions of the atmosphere are ex-
pressed by the terms weather and climate.
Weather refers to the atmospheric condition
over a brief period of time, such as an indi-
vidual day or week. In contrast, climate is a
composite of the varieties of day-to-day
climate; air temperature and solar energy 135
weather over a considerable number of years.
Among the other constituents of the earth's
natural equipment, such as terrain, native
vegetation, soils, water, and minerals, climate
ranks high as a cause of regional variations in
productive capacity. In addition climate
directly affects the character of such other
natural features as vegetation cover, soil,
drainage, and to a lesser degree terrain.
ELEMENTS OF CLIMATE
Weather and climate are described in terms
of several familiar elements, chief of which, in
their effects on the earth's living things, are
the above-mentioned temperature and precipi-
tation. A third element, winds, while by no
means equal in importance to temperature
and precipitation, has risen .in rank since air
transport has become common. The three are
the principal ingredients which comprise
climate. It is their combination in varying in-
tensities and amounts which causes climates
to differ so greatly over the earth and thus
leads to great differences in regional produc-
tivity and in man's use of land.
CONTROLS OF CLIMATE
But what causes the climatic elements to
vary so much from one part of the earth to
another and from one season to another? The
answer is to be found in the operation of the
climatic controls. Each of the climatic elements
temperature, precipitation, and winds also
functions as a control over the other elements;
indeed, winds are actually far more important
as a control than as an element of climate.
Other climatic controls are solar energy, air
masses and fronts, altitude, mountain barriers,
the great semipermanent centers, or cells, of
high and low pressure, ocean currents, and
atmospheric disturbances of various kinds. It
is these controls, themselves acting in differ-
ent combinations and with variable intensities,
that give rise to the areal and seasonal differ-
ences in temperature and precipitation which
in turn result in the great variety of climates
characterizing this planet.
Although the goal of the discussion of
climates is to make clear the world pattern of
climates, it is believed that a description of
types of climates and their distribution over
the earth will be more intelligible if it is ap-
proached through preliminary analysis of the
individual climatic elements and the more
important climatic controls. Such is the con-
tent of this and the three chapters which
follow.
136
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
SOLAR ENERGY
DEFINITION AND
DISTRIBUTION
Solar energy, which expresses its influence
most directly upon temperature distribution,
is the prime control of climate. So there ap-
pears to be good reason for beginning a dis-
cussion of climates with a description of solar
energy its distribution, how it heats the
atmosphere, and the way it functions to pro-
duce the great variety of temperatures which
characterize the different latitudes and regions
of the earth.
Earlier it was pointed out that temperature
acts both as an element and as a control of
climate. As a control it affects the density and
hence the weight of the air, resulting in differ-
ences in atmospheric'pressure. These pressure
differences in turji determine the rapidity and
direction of airflow. Temperature differences,
therefore, are a ^ main cause of the earth's
wind systems. Temperature of the air is also
a major determinant both of the atmosphere's
capacity for moisture and of its buoyancy,
and hence is closely related to the processes
of condensation and precipitation.
As an element of climate temperature is at
least coequal in rank with precipitation, and
its distribution over the earth is a feature of
the highest geographical importance. But be-
fore temperature distribution can be under-
stood and appreciated it is necessary to
understand the distribution of solar energy
and how this energy is converted into atmo-
spheric heat.
THE SUN AS SOURCE OF ATMOSPHERIC HEAT
The sun is the single important source of
heat for the earth's atmosphere. From this
star, whose surface is estimated to have a
temperature of about 10,000 Fahrenheit (F),
energy streams outward in all directions into
space. 1 The earth, over 90 million miles dis-
tant, intercepts less than one two-billionth of
the sun's energy output, yet this small frac-
tional part is responsible for maintaining the
atmospheric processes. Energy from the sun,
called solar radiation or insolation, is trans-
mitted to the earth in the form of very short
waves, a part of the energy being visible as
light. But there are some wavelengths of solar
radiation which are too short, and others that
are too long, to be seen by the human eye.
FACTORS DETERMINING THE
DISTRIBUTION OF SOLAR RADIATION
Disregarding for the moment the effects of
an atmosphere and its clouds, the amount of
solar energy, in other words, climatic energy,
that any latitude on the earth's surface receives
depends largely upon two factors: (a) the in-
tensity of solar radiation, or the angle at which
the rays of sunlight reach the various parts
of the earth's spherical surface, and (b) the
duration of solar radiation, or length of day-
light.
Because an oblique solar ray is spread out
over a larger surface than a vertical one, it
delivers less energy per unit area. An oblique
ray is weaker also because it has passed
through a thicker layer of scattering, absorb-
ing, and reflecting air (Fig. 7.1). Outside the
tropics, therefore, winter sunlight is much
weaker than that of summer. For example, in
1 Throughout the book the Fahrenheit (F) scale commonly
used in the United States and England will be employed to
express temperature degrees, except where it is noted spe-
cifically that degrees centigrade (C) are being used.
Introduction to
Sun's rays
F I G . 7 . 1 An oblique ray, (a), delivers less
energy at the earth's surface than a vertical ray,
(b), because an oblique ray'S energy passes !
through a thicker layer of absorbing and reflecting
atmosphere and then Is spread-over a larger
surface.
late December at Madison, Wisconsin, located
at 43 N, the noon sun is only 23V6 above
the horizon, whereas in late June 'it has an
elevation there of 70^, and thus Madison is
colder in winter. For the same reasons, the
daylight period is characterized by a sun
much more intense at noon than in the early
morning or late afternoon hours*.
As regards duration of solar radiation, it
would seem to be enough to say that the
longer the sun shines, i.e.', the longer the day,
the greater the amount of solar energy re-
ceived, all other conditions being equal (fol-
lowing table; Fig. 7.2). Thus it is quite under-
standable that in the middle latitudes sum-
mer temperatures are much higher than those
of winter; it is not only because in summer
the sun's rays are less oblique but also be-
cause days are much longer in summer.
climate; air temperature and solar energy 137
Since on any one day both the length of
day and the angle of the sun's rays are equal
all along any parallel of latitude, all parts of a
parallel (allowing for differences in the trans-
parency of the atmosphere) receive identical
amounts of solar energy in a whole year as well
as on any day. Similarly, different parallels re-
ceive unlike amounts of solar radiation, the an-
nual amount decreasing from equator to poles.
Thus if solar energy were the only control of
weather and climate, all places in the same
latitude would have identical climates.
Although all such places certainly are not
identical in climate, the strong temperature
resemblances within latitude belts testify to
the dominant, even though not exclusive,
rank of sun control.
Earth and sun relations The rotation
and revolution of the earth and the inclina-
tion and parallelism of its axis were discussed
in Chapter 1. It remains to be analyzed how
these earth motions and axis positions act to
produce the changing lengths of day and
varying angles of the sun's rays, which in
turn are the causes of the seasons.
The equinoxes: spring and fall Twice
during the yearly period of revolution, on
March 21 and September 23, the sun's noon
rays are directly overhead, or vertical, at the
equator (Fig. 7.2). At these times, therefore,
the circle of illumination, marking the posi-
tion of the sun's tangent rays, passes through
both poles and consequently cuts all the
earth's parallels exactly in half. One-half of
each parallel (180) consequently is in light
Extremes in Length of Day for Different Latitudes (in hours and minutes)
Latitude (degrees)
10
20
30
40
50
60
66 ! /2
Longest day
Shortest day
12:00
12:00
12:35
11:25
13:13
10:47
13:56
10:04
14:51
9:09
16:09
7:51
18:30
5:30
24:00
0:00
138
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
DEC. 22 WINTER SOLSTICE
(N. HEM.)
6 Months
Night
LENGTH OF DAY AT EVERY
10 LATITUDE IS STATED
IN HOURS AND MINUTES
JUNE 22 SUMMER SOLSTICE
(N. HEM.)
6 Months
SUN
r /o wionii
~*~ **% * Day
. ^fc\o\ ^SiB^pi
6 Months
Night
SEPT. 23 AUTUMNAL EQUINOX
MARCH 21 VERNAL EQUINOX
SUN
F I G . 7 . 2 At the equinoxes, when the sun's vertical rays are at the
equator, the circle of illumination cuts all the parallels in half, and days and
nights are equal in length over the entire earth. At this time insolation
decreases regularly from equator to poles (Fig. 7.4b). At the times of the
solstices the sun's vertical rays have reached their greatest poleward
migration. The circle of illumination cuts all the parallels (except the
equator) unequally, so that days and nights are unequal in length except at
latitude (Fig. 7.4c, d).
and the other half in darkness at these times.
Since the path described by any point on the
earth's surface during the period of rotation
is coincident with its parallel of latitude, days
and nights are equal (12 hr each) over the
entire earth. Because of this fact the two
dates March 21 and September 23 are called
the equinoxes. (Equinox is derived from Latin
words meaning equal night.) At these times
the maximum solar energy is received in
Introduction to
equatorial latitudes; the solar energy received
diminishes regularly toward either pole,
where it becomes zero. /
The solstices; summer and winter On
June 22 the earth is approximately midway in
its orbit between the equinoctial positions,
and the North Pole is inclined 23^ toward
the sun (Fig. 7.2). As a result of this axial
inclination, the sun's rays are shifted north-
ward the same number of degrees, so that the
noon rays are vertical at the Tropic of Cancer
(23 ^N), and the tangent rays in the Northern
Hemisphere pass over the pole and reach the
earth 23% of latitude beyond it, at the
Arctic Circle (66!/2N.). In the Southern
Hemisphere the tangent rays do not reach the
pole but terminate at the Antarctic Circle,
23^ short of it. Thus while all parts of the
earth north of the Arctic Circle are experienc-
ing constant daylight, similar latitudes in the
Southern Hemisphere (poleward from the
Antarctic Circle) are entirely without sunlight.
On June 22 all parallels, except the equator,
are cut unequally by the circle of illumina-
tion, those in the Northern Hemisphere hav-
ing the larger segments of their circumferences
toward the sun so that days are longer than
nights. These longer days, plus a greater
angle of the sun's rays, make for a maximum
receipt of solar energy in the Northern Hem-
isphere at this time. Summer, with its associ-
ated high temperatures, is the result, and
north of the equator June 22 is known as the
summer solstice. In the Southern Hemisphere
at this same time, all of these conditions are
reversed, nights being longer than days and
the sun's rays relatively oblique, so that solar
radiation is at a minimum and winter condi-
tions prevail.
On December 22, when the earth is in the
opposite position in its orbit from that of
June 22, it is the South Pole that is inclined
climate; air temperature and solar energy 139
23% toward the sun. The latter's noon rays
are then vertical over the Tropic of Capricorn
(23% S), and the tangent rays pass over the
South Pole to the Antarctic Circle 23^
beyond (66% S). Consequently, south of
66% S there is constant light, while north of
66% N there is no sunlight. All parallels of
the earth except the equator are cut un-
equally by the circle of illumination, with
days longer and sun's rays more nearly ver-
tical in the Southern Hemisphere. This,
therefore, is summer south of the equator
but winter in the Northern Hemisphere
(winter solstice), where opposite conditions
prevail.
Effects of the atmosphere upon incom-
ing solar energy The total effect of the at-
mosphere upon a beam of sunlight passing
through it is to reduce its intensity by amounts
varying with latitude, the seasons, and cloudi-
ness (Fig. 7.3). The atmosphere weakens solar
FIG. 7.3 Only about one-half (51 per cent) of
the incoming solar radiation passes through the
atmosphere and heats the earth's surface. Another
14 per cent is absorbed by the atmosphere. Some
35 per cent of the solar energy is scattered and
reflected back to space, and thus has no effect on
heating either the earth's surface or its atmosphere.
SHORT WAVE SOLAR
RADIATION
Reflected to space
by (a) clouds
-29 (6) earth
s//////////?/^^^
Absorbed by
earth
Absorbed by
earth
140
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
energy through (a) selective scattering,
chiefly of the short waves of blue light, by
very small obscuring particles, (b) diffuse re-
flection of all wavelenjgths by larger particles,
such as cloud droplets, and (c) absorption of
selected wavelengths, chiefly by water vapor
concentrated in the lower strata of the atmos-
phere. The scattering and reflecting processes
operate to send a part o/ the solar radiation
they effect back to space, but some of it
reaches the earth's surface as diffuse daylight.
Quantitatively, it is estimated that about
35 per cent of the solar radiation reaching
the outer limits of the air layer is returned to
space by scattering and reflection from clouds,
small dust particles, and molecules of air, and
by direct reflection from the earth's surface.
This has no part in heating either the earth or
its atmosphere. Fourteen per cent of the solar
radiation is absorbed directly by the atmos-
phere, most of it by the water vapor. The
remaining 51 per cent reaches the earth's sur-
face either as direct sunlight or as diffuse
daylight, is absorbed by it, heats it, and
eventually heats the atmosphere as well.
Thus only some 65 per cent of the solar
radiation (14 per cent absorbed by the atmos-
phere directly, and 51 per cent absorbed by
the earth's surface) is available for heating the
atmosphere. Also, and equally significant,
the atmosphere receives several times as
much energy from the heated earth's sur-
face as it does from direct absorption of solar
radiation.
DISTRIBUTION OF SOLAR RADIATION
OVER THE EARTH'S SURFACE
As the previous discussion has indicated,
the belt of maximum solar radiation swings
back and forth across the equator during the
course of a year, following the shifting rays
of the sun, with the two variables angle
of sun's rays and length of day largely determ-
ining the amount of solar energy received at
any time or place.
Distribution from pole to pole Assum-
ing a cloudless sky, solar radiation for the
year as a u'lwle is highest at the equator and
diminishes with regularity toward the poles;
and the Northern and Southern Hemispheres
share equally in the annual amounts of solar
energy received (Fig. 7.4). This distribution
has important climatic consequences. Chief
of these is that average air temperatures for
the year are highest in the tropics, or low
latitudes, and decrease toward the poles.
At the time of the equinoxes, disregarding
again the effects of clouds, the latitudinal dis-
tribution of solar radiation is similar to that
for the year as a whole. There is a maximum
of solar radiation at the equator and a mini-
mum at each pole. This fact also has impor-
tant climatic implications. For it is in the
equinoctial seasons of spring arid fall, when
the Northern and Southern Hemispheres are
receiving approximately equal amounts of
solar energy, that temperature conditions in
the two hemispheres are most nearly alike.
Similarly, pressure, wind, and precipitation
conditions, and as a result the over-all weather
situation, are more in balance to the north
and south of the equator than at other times.
Finally, world temperature, pressure, wind,
and precipitation-distribution patterns for
spring and fall bear close resemblances to
those for the year as a whole.
At the time of the two solstices when the
sun's noon rays are vertical 23V& poleward
from the equator and the length of day increases
toward one pole, the latitudinal distribution
of solar radiation is very asymmetrically de-
veloped. The summer hemisphere receives
two to three times as much as the winter
Introduction to climate; air temperature and solar energy 141
hemisphere (Fig. 7.4). 2 Latitudinal distribu-
tion of solar radiation at the surface of the
earth shows a broad maximum in the belt of
latitude that extends from about 30 to about
40, while latitude 60 receives as much as or
more than the equator. It is not surprising,
therefore, that the highest surface-air temper-
atures in summer occur over the land masses
of the lower middle latitudes (30 -40) and
not at the equator.
During the course of a year the zone of
maximum solar radiation shows a total lati-
tudinal displacement of more than 60 (Fig.
7.4<:, </), which must have important effects
upon seasonal temperatures, rainfall, pres-
sure, and winds. It is significant also that the
latitudinal solar-radiation gradient (the rate of
change in solar radiation) is much steeper
in the winter hemisphere than in the summer
hemisphere.
The characteristics of solar-radiation dis-
tribution at the times of the solstices, which
are the extreme seasons of summer and winter,
provide the basic explanations for many of
the earth's larger features of weather and cli-
mate, (a) Marked north-south migration of
the temperature, wind, and precipitation belts
follows a similar migration of solar-radiation
2 By summer hemisphere is meant the hemisphere that has
summer. Thus the Northern Hemisphere would be the
summer hemisphere in July, and the Southern Hemisphere
would be the summer hemisphere in January.
FIG. 7.4 Latitudinal distribution of the
maximum values of solar energy at the earth's
surface. For the year as a whole and at the two
equinoxes, solar energy is symmetrically
distributed in the Northern and Southern
Hemispheres. There is a maximum in equatorial
latitudes and there are minima at the North and
South Poles. At the solstices solar energy is very
unequally distributed, with the summer
hemisphere receiving two to three times the
amount of the winter hemisphere.
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110,000
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10,000
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300
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Latitude, degrees
142
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
belts, (b) Warm-to-hot summers occur in the
lower middle latitudes where solar radiation
reaches a near maximum for the summer
hemisphere, (r) There are much steeper
temperature gradients in the winter hemi-
sphere than in the summer hemisphere, the
temperature gradients paralleling solar-radia-
tion gradients, (d) Greater storminess and
weather variability occur in the winter
hemisphere.
Annual distribution of solar radia-
tion for representative latitudes The
yearly solar-radiation patterns (curves) for
the different latitudes can be arranged in three
general groups, those for low, middle, and
high latitudes (Fig. 7.5). (a) In the low, or
tropical, latitudes between the Tropics of
Cancer and Capricorn, solar radiation is high
and varies little throughout the year. This
accounts for the constant year-round heat of
the tropics. Since during the course of a year
all regions between the two tropics are passed
over twice by the vertical rays of the sun, the
annual solar-radiation curve for low latitudes
contains two weak maxima and two slight
minima, (b) The middle-latitude curve, on the
other hand, has a single maximum, but as in
the tropics, solar radiation at no time declines
FIG. 7.5 In the very low latitudes close to the
equator the amount of solar energy received is
large and varies little throughout the year. In the
middle and higher latitudes there are large
seasonal differences in the receipt of solar energy.
Jan | Feb | Mar | Apr | May | June [ July | Aug | Sept | Oct | Nov | Dec
to zero. The great seasonal contrasts in solar
radiation are reflected in similar large seasonal
contrasts in temperature, (c) The regions
poleward from the Arctic and Antarctic
Circles also have but one maximum and one
minimum period of solar radiation. But un-
like the other latitudes there is a portion of
the year when direct sunlight is completely
absent. For this reason, the high-latitude, or
polar, solar-radiation curve does decline to
zero. Also, seasonal contrasts in solar radia-
tion are marked, and temperature contrasts
are strong.
HOW SOLAR RADIATION
HEATS THE EARTH'S
SURFACE AND ATMOSPHERE
HEATING AND COOLING LAND
AND WATER SURFACES
Thus far the discussion has been con-
cerned largely with the latitudinal distribu-
tion of solar energy, the single important
source of atmospheric heat. Now the com-
plexities of how the heating of the atmosphere
occurs will be explored.
Sun energy is in the form of such short
wavelengths that only relatively small amounts
(around 14 per cent, as we have seen) can be
absorbed directly by the earth's atmosphere,
chiefly by the water vapor in it. Three to
four times as much gets through to the earth's
surface and is absorbed by it and heats it be-
cause the solid-liquid surface is capable of
absorbing the short-wave solar energy more
readily than the atmosphere is. Thus the
heated earth's surface becomes a radiator of
energy.
Because the earth's temperature is lower
than that of the sun, earth radiation is com-
posed of longer wavelengths and so is much
more readily absorbed by the atmosphere
Introduction to
than short-wave solar radiation. As a con-
sequence the atmosphere receives most of its
heat indirectly from the sun and directly from
the earth's surface, which has served to con-
vert the solar energy into more readily ab-
sorbable earth radiation. It is obviously
necessary, therefore, to understand how dif-
ferent kinds of earth surfaces react to solar
energy and the contrasting temperatures that
they acquire as a result, before specific dis-
cussion of heating and cooling the atmos-
phere is begun.
The greatest contrasts in temperature are
between land and water surfaces, although
land surfaces of different shades (snow, light
sand, green fields, and forests) also will differ
in temperature by reason of their unlike re-
flection and absorption of solar energy.
Land and water contrasts For a num-
ber of reasons a land surface without snow
heats (and cools) more rapidly than a water
surface even when both receive similar
amounts of solar energy. Most importantly,
the fluid character of water causes vertical
and horizontal currents, tides, and waves to
distribute the energy received from the sun
throughout a large mass. Similarly, when a
water surface begins to cool, the surface
water becomes heavier and sinks, to be re-
placed by warmer, lighter water from below.
Both kinds of distribution make for slower
temperature change in water than on the
solid land surface.
A supplementary, although less important,
factor is that water is more transparent than
land. The sun's rays are able to penetrate a
water body to considerable depths, and in
this way also to distribute energy throughout
a somewhat larger mass. On the other hand,
the opaqueness of land concentrates the sun
energy close to the surface, which results in
comparatively rapid and intense heating. This
climate; air temperature and solar energy 143
concentration likewise permits the land area
to cool more rapidly than a deeply wanned
water body.
Also of some significance is the fact that
the specific heat of water is higher than that
of land. In other words, it requires only one-
third to one-half as much energy to raise a
given volume of dry earth by one degree as
it does an equal volume of water.
Thus it becomes evident that land-
controlled, or continental, climates should be
characterized by large daily and seasonal ex-
tremes of temperature, becoming alternately
hot and cold, whereas ocean-controlled, or
marine, climates should be more moderate,
with only small seasonal and daily temper-
ature changes.
HEATING AND COOLING THE ATMOSPHERE
Now that there has been discussion of the
distribution of solar energy over the earth,
the contrasting reactions of land and water
surfaces to this solar energy, and the fact that
the air receives most of its energy directly
from the earth's surface and only indirectly
from the sun, it is appropriate to proceed
with an analysis of the processes involved in
heating and cooling the atmosphere.
Absorption of solar radiation As in-
dicated previously, the earth's atmosphere is
capable of absorbing only about 14 per cent
of the short-wave solar energy that enters it.
Moreover, while most of the absorption oc-
curs in the more humid lower atmosphere, it
takes place well above the immediate surface
layer. As a consequence this absorption is
not very effective in heating the air very close
to the ground. This is evident from the fact
that on a clear winter day the air may re-
main bitterly cold in spite of a bright sun.
Conduction The solid earth (without a
snow cover), being a much better absorber of
144
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
solar radiation than air, attains a higher tem-
perature during the daylight hours. But when
two bodies of unequal temperature are in
contact with one another, energy in the form
of heat passes from the warmer to the colder
object until they both attain the same tem-
perature, a process called conduction. By con-
duction, therefore, the layer of air resting
upon the warmer earth becomes heated. Yet
air is a poor conductor, so that heat from the
warmed layer in contact with the earth's sur-
face is transferred very slowly to those above,
unless there is some air movement.
Heating of the air by conduction is pri-
marily a daytime and a summer process. An
opposite process, cooling by conduction,
occurs under opposite conditions. That is,
just as a warm earth on a summer day heats
the air layer next to it by conduction, so a
cold land surface on a winter night, chilled
through energy losses to space by earth
radiation, cools the air by conduction. But in
general, conduction is a relatively unimpor-
tant factor in heating and cooling the whole
atmosphere.
Earth radiation The surface of the earth
without a snow cover readily absorbs short-
wave solar energy and is heated by it. As a
consequence the heated earth becomes a
FIG. 7.6 The greenhouse effect of the earth's
atmosphere. The glass in the roof and sides of the
greenhouse, like the atmosphere, is relatively
transparent to the short-wave solar energy but
relatively opaque to the long- wave earth radiation.
radiating body giving off* heat in the form of
long-wave earth radiation. In this the atmos-
phere is importantly involved, for while it is
able to absorb only about 14 per cent of the
short-wave solar radiation, it can absorb up
to 90 per cent of the long-wave earth radia-
tion. The atmosphere thus acts like a pane of
glass in a greenhouse or automobile, letting
through much of the incoming short-wave
solar energy but greatly retarding the escape
of the heat, or long-wave earth radiation.
This is called the greenhouse effect of the at-
mosphere (Fig. 7.6). Its total influence on
climate is to maintain surface-air tempera-
tures considerably higher than they otherwise
would be, and to prevent great extremes in
temperature between day and night. The fact
that it is chiefly the atmosphere's water vapor
which is effective in absorbing earth radiation
is illustrated by the rapid night cooling in
deserts, where the dry air and cloudless skies
permit a rapid escape to space of heat radiated
from the earth.
Radiation of heat from the earth's surface
upward through the atmosphere toward space
is a continuous process. During the daylight
hours and up to about midafternoon, how-
ever, more energy is received from the sun
than is radiated from the earth, with the re-
sult that surface-air temperatures usually
continue to rise. But during the night, when
receipts of solar energy cease, a continued
loss of energy through earth radiation results
in a cooling of the earth's surface and a con-
sequent drop in air temperature.
Being a better radiator than air, the ground
indeed becomes cooler than the air above it
during the night. When this happens, the
lower layers of atmosphere lose heat by radia-
tion to. the colder ground as well as upward
toward space. This process is particularly
important during the long nights of winter
Introduction to
when, if the skies are clear and the air is dry,
the loss of heat is both rapid and long-
continued.
If a snow cover mantles the ground, cool-
ing is even more pronounced, for most of the
incoming solar radiation during the short day
is reflected by the snow and thus does not
heat the earth's surface. At night the snow,
which is a poor conductor of heat, allows
little energy to come up from the soil layer
below to replenish that lost by radiation from
the top of the snow surface. As a result, the
snow surface becomes extremely cold, and
then so does the air layer resting upon it.
Water, like land, is a good radiator, but as
has been indicated, the cooled surface waters
keep constantly sinking to be replaced by the
warmer water from below. Extremely low air
temperatures over water bodies are therefore
impossible until they are frozen over, after
which they act like a land surface.
Humid air or a cloudy sky tends to pre-
vent rapid earth radiation, so that air temper-
atures remain higher and frosts are less likely
on humid nights, especially when a cloud
cover prevails. In contrast, there are authentic
cases in the dry air and under the cloudless
skies of Sahara, of day temperatures of 90
having been followed by night temperatures
slightly below freezing. When clouds cover
the sky, all the earth radiation is completely
absorbed at the base of the cloud sheet,
which reradiates a part of it back to the earth
so that cooling of the earth is retarded. Water
vapor likewise absorbs and reradiates out-
going terrestrial energy, but not so effectively
as liquid or solid cloud particles.
Warming the atmosphere by heat of
condensation A large amount of the solar
energy which reaches the earth's surface is
consumed in evaporating water (changing it
into a gas). This converted solar energy con-
climate; air temperature and solar emrgy 145
sequently is contained in the atmosphere's
water vapor in latent or potential form. When
condensation occurs and the water vapor is
returned to the liquid or solid state this latent
energy is again released into the atmosphere
and heats it. This heat of condensation is a
principal source of heat energy for the atmos-
phere.
TRANSFER OF HEAT BY CURRENTS
IN THE ATMOSPHERE
The heat acquired by the atmosphere by
absorption of solar energy, conduction and
radiation processes, and condensation is
transferred from one part of the atmosphere
to another by vertical and horizontal
currents.
Vertical transfer Vertical transfer of
heat results from convectional (vertical) cur-
rents, mechanical turbulence, and eddy
motions in the atmosphere. When surface air
is heated, it expands in volume and conse-
quently becomes less dense. Hence it is
forced upward by the surrounding colder,
denser air which at the surface flows toward
the warm source. Such circulation is called a
convectional system. Warm surface air, ex-
panded and therefore less dense, is like a
cork that is held under water, i.e., unstable
and inclined to rise. Vertical circulation,
whether by thermal convection, turbulence in-
duced by rough terrain, or eddy currents, is
the most important method of distributing
the heat acquired by the surface air through
the higher layers of the atmosphere.
Horizontal transfer Horizontal transfer
of heat, which is accomplished by winds, is
called advection. For the earth as a whole
this is the most important means of heat
transfer. Advection by winds causes many of
the earth's day-to-day weather changes, as
well as the storminess of winter climates in
146
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
the middle latitudes. For instance, in the
Northern Hemisphere middle latitudes a
south wind usually means unseasonably high
temperatures, as almost everyone who lives
there knows. In this case the wind acts as the
conveyer of heat from lower latitudes where
solar radiation is greater and higher temper-
atures are normal. Such an importation of
southerly warmth in winter results in mild
weather, with melting snow and slushy streets.
In summer, several days of south wind may
result in a heat wave, with maximum temper-
atures over 90.
In a similar way, north winds from colder,
higher latitudes, or from the cold interiors of
continents in winter, bring lower temper-
atures. These cold importations are particu-
larly effective where there are no mountain
barriers to block the wind movement. In
central eastern North America where low-
lands prevail, great masses of cold polar air
pour down over the Mississippi River Valley
at irregular intervals, occasionally carrying
severe freezing temperatures even to the
margins of the Gulf of Mexico. Large-scale
horizontal transfer of temperature conditions
likewise may result from winds moving on-
shore from large bodies of water.
HEAT BALANCE IN THE ATMOSPHERE
Since the mean temperature of the earth as
a whole gets neither colder nor warmer, it is
clear that the heat lost by the earth through
radiation to space is identical in amount with
the energy received from the sun. But this
balance does not hold for individual latitudes.
In the low latitudes, equatorward from about
37, the incoming solar energy exceeds the
outgoing earth energy, whereas poleward
from latitude 37 exactly the reverse is true.
This means there would be a constant in-
crease in the temperatures of low latitudes
and a constant decrease in the temperatures
of the middle and higher latitudes if there
were not a continuous transfer of energy from
low to high latitudes of the earth. But this
transfer is accomplished by the winds and
the ocean currents. In fact, in the unequal
latitudinal distribution of solar and terrestrial
radiation is to be found the ultimate cause
for the earth's atmospheric circulation and
for much of its weather.
AIR TEMPERATURE
TEMPORAL DISTRIBUTION:
MARCH OF TEMPERATURE
The average temperature of any month,
season, year, or even long period of years, is
determined by using as a basic unit the mean
daily temperature, which is the average of the
highest and the lowest temperatures recorded
during the 24-hr period.
The daily march, or cycle, of temperature,
obtained by plotting the temperature for each
hour of the day, chiefly reflects the balance
between incoming solar radiation and out-
going earth radiation (Fig. 7.7). From about
sunrise until 2 : 00 to 4 : 00 P.M., when energy
is being supplied by incoming solar radiation
faster than it is being lost by earth radiation,
the daily temperature curve usually rises
(Figs. 7.7, 7.8). Conversely, from about
3 : 00 P.M. to sunrise, when loss by terrestrial
radiation exceeds receipt of solar energy, the
temperature curve usually falls.
Introduction to climate; air temperature and solar energy 147
Max. temperature
Mm. temperature
8 10 Mt.
FIG. 7.7 The march of incoming solar
radiation and of outgoing earth radiation for the
daily 24-hr period at about the time of an equinox,
and their combined effects upon the time of daily
maximum and minimum temperatures.
In marine locations the daily temperature
curve is relatively flat in appearance, there
being only a modest difference between day
and night. By contrast, in continental, or
land-controlled, climates the amplitude of the
daily temperature curve is much greater. Im-
portant modifications of the symmetrical
daily temperature curve are frequently im-
posed by the presence of a cloud cover
which obstructs both incoming and outgoing
radiation, or by the importation of tempera-
ture by winds.
The annual march, or cycle, of temperature,
obtained by plotting the mean temperature
for each month, reflects the increase in insola-
tion (and hence heat accumulated in the air
and ground) from midwinter to midsummer
and the corresponding decrease from mid-
summer to midwinter. Usually the reaching
of temperature maxima (and minima) lags 30
to 40 days behind the periods of maximum
(and minimum) insolation. This seasonal tem-
perature lag may be even greater over oceans
and along windward coasts in middle lati-
tudes, where August may be the warmest
month and February the coldest. Normally,
marine locations have annual temperature
curves, just as they have daily curves, with a
much smaller amplitude or range than those
of continental locations.
10
in
Dec. 22, 1924
Sunset 4-21
pm >
^
^J
TEMPEI
(ATURE
Daily march of insolation
at Madison, Wis , on clear
days in summer and
winter Expressed in
calories per cm
per mm
6 p.m.
FIG. 7.8 Daily march of temperature (a, b)
and of insolation (c) on clear days in winter and
summer at Madison, Wisconsin. The total solar
energy recorded was 3'/4 times as great on June 23
as on December 22. Note that temperature lags
behind insolation. South winds prevented normal
night cooling on December 22.
GEOGRAPHICAL DISTRIBUTION
OF TEMPERATURE
Vertical decrease with altitude Num-
erous observations made during mountain,
balloon, and airplane ascents have shown that
under normal conditions there is a general
decrease in temperature with increasing eleva-
tion. The rate of decrease is not uniform,
varying with time of day, season, and loca-
tion, but the average is approximately 3.6
148
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
11,700
9,900
900
VERTICAL DISTRIBUTION
OF TEMPERATURE
B- Average conditions
C-Ona summer day
Decrease in
temperature more
rapid close to
warm earth than
aloft
10
FIG. 7.9
20 30 40 50
Temperature, F
60 70
for each 1,000-ft rise (Fig. 7.9). The fact that
air temperature generally is highest at low
elevations close to the earth's surface and de-
creases with altitude is a clear reflection of
the fact that most of the atmospheric heat is
received directly from the earth's surface and
only indirectly from the sun.
Inversions Temperature inversions are
said to exist when the temperature increases
with distance from the earth instead of decreas-
ing. Such reversed temperature conditions can
exist in the lowest layer of the atmosphere
next to the earth's surface, or they can be
found at altitudes of several thousand feet
above the surface.
Surface inversions Surface temperature in-
versions, one of the commonest and most
readily observed kinds, originate from cool-
ing of the lowest layers of air at night when,
as described earlier, the land surface cools
more rapidly than the air. The lowest air
layer is chilled by radiation and by conduc-
tion of the heat of the air to the adjacent
cold ground. This air layer thereby becomes
colder than the air farther removed from the
earth's surface (Fig. 7.9).
Local, diurnal, surface (ground) inversions
of a few score or hundred feet in depth are
well-known nighttime phenomena of the
cooler seasons. Ideal conditions for these in-
versions are (a) long winter nights, which
have a relatively long period when outgoing
earth radiation exceeds incoming solar radia-
tion, (b) a clear sky, under which loss of heat
by terrestrial radiation is rapid and not re-
tarded by a cloud cover, (c) cold, dry air that
absorbs little earth radiation, (d) calm air to
keep much mixing from taking place and thus
to help the surface stratum become very cold
by conduction and radiation, and (e) a snow-
covered surface, which, owing to reflection
of solar energy, heats little by day and, being
a poor conductor, retards the upward flow of
heat from the ground below the snow cover.
Some of the deepest, most extensive, and
most persistent surface inversions are those
which prevail over the snow-covered northern
parts of North America and Eurasia in winter.
A very close relationship also exists between
surface temperature inversions and frost and
fog, since the same conditions are favorable
for all.
During a surface temperature inversion,
when the coldest, densest air is at the surface,
the air is stable or nonbuoyant, that is, not
inclined to rise. Such a condition, therefore,
is opposed to the formation of precipitation.
Above-surface inversions Inversions also
occur in the free atmosphere well above the
earth's surface. They are usually a result of
the settling and warming of air in a large
semistationary anticyclone (high-air-pressure
system). Such above-surface inversions act to
hinder the upward movement of air currents,
and consequently they are opposed to the
formation of rainfall. Some of the most ex-
tensive and well-developed above-surface
inversions are found in the eastern and cen-
tral parts of the great subtropical anticyclones
and their trade-wind circulations, and these
Introduction to climate; air temperature and solar energy 149
regions characteristically have dry climates.
(These regions will be considered in detail in
the next chapter.)
Surface inversions and air drainage
Although surface temperature inversions are
common over flattish land surfaces, they are
most perfectly developed in low spots or
topographic depressions. This results from
the fact that the cold surface air, because of
its greater density, moves downslope into the
low areas where it collects in the form of
pools (Fig. 7.10). This downslope movement
of cold air is known as air drainage. It is
well known that the first frosts in fall and the
last in spring occur in bottomlands, and that
on a clear, cold night valleys and depressions
experience the lowest temperatures.
Frost and its distribution Frost is
simply a temperature of 32 (freezing tem-
perature) or below. The period between the
last killing frost in spring and the first in fall
is known as the growing season. Thus
throughout the middle latitudes frost is most
serious as a menace to crops in spring and
fall, or at the beginning and end of the
growing season; in subtropical areas such as
Florida and California midwinter frosts are
critical because of the active growth of sensi-
tive crops during their normally mild winters
(Fig. 7.11).
Ideal conditions for killing frost are those
which favor rapid and prolonged surface
cooling, namely, importation of a mass of
dry, cool, polar air, followed by a calm, clear
night during which the temperature of the
surface air is brought below the freezing
point through radiational cooling. With the
air temperature already low but still above
freezing, the following loss of heat by earth
radiation is all that is required to reduce the
temperature of the surface air below freezing.
If the late afternoon temperature of the cool,
FIG. 7.10 Cold surface air, because it is
denser, tends to settle in lower places. This
drainage of the cold, dense air into depressions
makes frost and fog more common in low places
than on adjacent slopes.
northerly air is not much over 40, and skies
are clear and air calm, killing frost is likely
during the following night. But even when
conditions are generally favorable for a kill-
ing frost over an extensive area, the destruc-
tive effects upon plant life are usually local
and patchy in distribution. This is chiefly a
matter of terrain irregularities and air drain-
age, with most frost damage restricted to the
lower lands. This is why sensitive crops such
as fruit and vegetables are commonly planted
on slopes rather than in valley bottoms.
Protection against killing frost For small-
scale vegetable gardeners or fruit growers the
simplest and most effective means of protec-
tion against frost is to spread over the crop
some nonmetallic covering such as straw,
paper, or cloth. Such a covering tends to re-
tard the heat loss by radiation from the ground
and the plants. The function of the cover is
not to keep the cold out but the heat in. But
this method is not suitable for protecting ex-
tensive orchards, so in such places as the
valuable citrus groves of California and
Florida small oil heaters are commonly used
to prevent a bad freeze. Such heaters, well
distributed among the fruit trees, are kept
burning for several hours during those night
>s
If
15
"Cf V)
2 ' ~
IJ
<y flj
dj <JJ
w)
^21
rc O
2 & I
*= ^
Introduction to climate; air temperature and solar energy 151
FIG. 7.12
AVERAGE SEA-LEVEL TEMPERATURES
(After Shaw. Brunt and Others)
JANUARY
hours when the lowest temperatures are to be perature distribution over the earth is repre-
expected. sented on maps by isotherms, lines connect-
Horizontal distribution of temperature ing places with the same temperature (Figs.
over the earth; isothermal maps Tern- 7.12, 7.13). All points on the earth's surface
FIG. 7.13
AVERAGE SEA- LEVEL TEMPERATURES
(After Show, Brunt and Others)
JULY
152
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
through which any one isotherm passes have
identical average temperatures. In Figs. 7.12
and 7.13 temperatures of all locations have
been reduced to the temperatures that would
prevail if the locations were at sea level,
by use of the formula which indicates an
average vertical change of about 3.6 in
temperature for each 1,000-ft change in alti-
tude. If the temperatures recorded by widely
distributed stations in a great variety of loca-
tions were not so reduced to eliminate the
effects of altitude, the complications in the
isotherms caused by hills and mountains
would make the maps so confusing that the
general world patterns of temperature distri-
bution would be difficult to observe.
One very conspicuous feature of the tem-
perature maps is that the isotherms have a
strong east- west alignment, roughly follow-
ing the parallels of latitude. This is not sur-
prising, given the fact that, except for dif-
ferences in the transparency of the atmos-
phere, all places in the same latitude receive
identical amounts of solar energy. This east-
west course of the isotherms simply illustrates
that solar energy is the single most important
control of the broadscale temperature distri-
bution over the earth.
WORLD-WIDE FEATURES
OF ANNUAL TEMPERATURE
DISTRIBUTION
As already indicated, for the year as a whole
the highest average temperatures are in the low
latitudes where the largest amounts of solar
energy are received; the lowest average tem-
peratures are in the vicinity of the poles, the
regions of least solar energy. Within a broad
belt some 40 wide in the tropics, or low
latitudes, the temperature differences in a
north-south direction are small and the
thermal conditions are relatively uniform. It
is chiefly in those latitudes poleward from
20 -25 N and S, chiefly the middle and
higher latitudes, that the average annual tem-
peratures decrease rapidly toward either pole.
Isotherms tend to be straighter and also
more widely spaced in the Southern Hemis-
phere where oceans predominate. The
greatest deviations from east-west courses are
where the isotherms pass from continents to
oceans. This is caused by the contrasting
heating and cooling properties of land and
water surfaces and by the effects of ocean
currents. Next to solar energy, land and
water distribution is the most important con-
trol of temperature distribution. Cool ocean
currents off the coasts of Peru and northern
Chile, southern California, and southwestern
Africa cause equatorward bending of iso-
therms. Similarly, warm currents in higher
latitudes cause isotherms to bend poleward, a
condition most marked off the coast of north-
western Europe.
January and July average tempera-
tures For the earth in general, January and
July are the months of seasonal extremes of
temperature, and the temperature maps for
these two months (Figs 7.12, 7.13) illustrate
significant features of seasonal temperature
distribution.
(a) From a comparison of the two maps it
is obvious that there is a marked north-south
shifting of the isotherms, and thus temper-
ature belts, between July and January, follow-
ing the north-south migration of sun's rays
and the belt of maximum solar energy, (b)
This shifting is much greater over continents
than over oceans because of the greater
seasonal extremes of temperature over land
masses, (c) The highest temperatures in both
January and July are over land areas, and
much the lowest temperatures in January are
Introduction to
over Asia and North America, the largest
land masses in the middle and higher lati-
tudes, (d) In the Northern Hemisphere the
January isotherms bend equatorward over
the colder continents and poleward over
the warmer oceans, whereas in July exactly the
opposite condition prevails, (e) No such
seasonal temperature contrasts between land
and water are to be found in the Southern
Hemisphere, for there large land masses are
absent in the higher middle latitudes. (/)
The lowest temperatures in January are over
northeastern Asia, the leeward side of the
largest land mass in higher middle latitudes.
The next lowest temperatures are over Green-
land and North America, (g) North-south
temperature gradients (rates of horizontal
temperature change), like solar-energy grad-
ients (Fig. 7.4), are steeper in winter than in
summer. Steep gradients, represented by
close spacing of the isotherms, are particularly
conspicuous over the Northern Hemisphere
continents in January.
Annual range The difference between
the average temperatures of the warmest and
climate; air temperature and solar energy 153
coldest months is the annual range of tem-
perature. The largest annual ranges are over
the Northern Hemisphere continents, which
become alternately hot in summer and cold
in winter (Fig. 7.14). Ranges are never large
(a) near the equator, where insolation varies
little, or (b) over large water bodies, which
explains why ranges are everywhere small in
the middle latitudes of the Southern Hemis-
phere. In general, they increase toward the
higher latitudes, with the increase much
more marked over the continents than over
the oceans.
Air temperature and sensible temper-
ature Correct air temperature can be
obtained only by an accurate thermometer
properly exposed. The instrument must not
be in the sun; otherwise it receives energy not
only from the surrounding air but also from
the absorption of solar energy. It also should
be protected against direct radiation from
the ground and adjacent buildings.
Sensible temperature is the sensation of
temperature that the human body feels, as
distinguished from actual air temperature re-
F I G . 7.14 Average annual ranges of temperature are smallest in low
latitudes and over oceans. They are largest over continents in the middle
and higher latitudes.
154
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
corded by a properly exposed thermometer.
Unlike a thermometer, which has no temper-
ature of its own, the human body is a heat
engine, generating energy at a relatively fixed
rate when at rest. Anything, therefore, that
affects the rate of loss of heat from the body
affects physical comfort. Air temperature, of
course, is one important factor, but so also
are wind, humidity, and sunlight. Thus a hot
day that is humid is more uncomfortable
than an equally hot day that is dry, since
loss of heat by evaporation is retarded more
when the air is humid. A hot day with a
good breeze feels less oppressive than a still
hot day because of increased evaporation. A
windy cold day feels uncomfortable because
the loss of heat is speeded up by greater
evaporation. A sunny day in winter feels less
cold than it actually is, owing to the body's
absorption of solar energy. Cold air contain-
ing moisture particles is particularly penetrat-
ing because the skin becomes moist and
evaporation results, and further loss of heat
results from contact with the cold water. Be-
cause of all this sensitiveness to factors other
than air temperature, the human body is not
a very accurate thermometer. This should
always be remembered in considering world,
regional, and local temperatures and their
effects on human beings.
CHAPTER 8
The circulation
of the
atmosphere: ^
winds
and pressure
CLIMATIC IMPORTANCE OF WINDS AND
ATMOSPHERIC PRESSURE
The film of air which envelops the solid- direction essentially horizontal to the earth's
liquid earth is in motion almost everywhere surface, is known as wind. Air is set in motion
at all times. Such movement, when it is in a by differences in its density which cause
155
156
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
variations in the weight or pressure of the
atmosphere at the same altitude.
As climatic elements affecting living things
on the earth's surface, atmospheric pressure
and winds are of small importance compared
with temperature and precipitation. This is
especially true of pressure. Winds do affect
sensible temperatures and rates of evapora-
tion, and winds of high speed may be gen-
uinely destructive. Nevertheless, the funda-
mental reason why winds and pressure are
considered in this book is that they are es-
sential keys to an understanding of tempera-
ture and precipitation distribution over the
earth. The basic importance of atmospheric
pressure is that pressure differences generate
winds. And the fundamental and very impor-
tant climatic functions of winds are (a) the
maintenance of a heat balance between the
high and low latitudes, in spite of a constant
low-latitude excess and high-latitude defi-
ciency of radiant energy, and (b) the transport
of water vapor from the oceans to the lands
where the water vapor may condense and fall
as rain.
PRESSURE DIFFERENCES AND THEIR ORIGINS
The downward pressure (weight) of the air
is measured by a barometer. This weight at
sea level is balanced against the weight of a
column of mercury 29.92 in., or 762 mm
(millimeters), in length having the same cross-
sectional area. The more air pressure, the
higher the mercury rises in the tube. Until
about 1914 it was customary to report pres-
sure in units of length (inches, millimeters,
or centimeters of mercury). Since then a new
measuring unit, called the millibar (mb), has
come into general use by the weather services
of the world. Since Vio in. of mercury is
equivalent to 3.4 mb, sea-level atmospheric
pressure may be expressed as 29.92 in., 760
mm, or 1013.2 mb.
Any map representing atmospheric pres-
sure at sea level shows at once that pressure
is not uniform over the earth but varies with
latitude and also from one region or locality
to another. The variations can be classified
in two general types of pressure systems: (a)
high-pressure systems, called anticyclones or
highs, and (b) low-pressure systems, called
cyclones, depressions, or lows.
At present there is no simple and adequate
explanation for the average annual or seasonal
arrangement of high- and low-pressure sys-
tems over the earth, nor for the moving cells
of high and low pressure that appear on the
daily weather map. Some highs and lows
appear to have their origin in temperature
characteristics of the air. Thus, cold air
being denser and heavier than warm air, it is
not surprising for low air temperatures some-
times to be the cause of high pressure and
for high temperatures to result in low pres-
sure. Examples of high-pressure systems pro-
duced by low surface temperatures are the
extensive highs over central and eastern
Eurasia and northern North America in
winter. Similarly, heat-induced lows are to
be found over such superheated locales as
the Pakistan-northwestern India area and the
Conversion scale
948 956 964
Millibars I . t . I . i . I . i
Inches I ' r "'l 1 ' 1 ! * f n'l'l
972
980
. I .
988 996
M+W*
1004 1012 1020
i I . l . I i I . I .
1028 1036
.l.i.i.
1044
28.0 8.2 8.4 8.6 8.8 29.0
9.2 9.4
46
9.6 9.8
1 T j r
30.0
0.2
^T
0.4
1 r i i
0.6 0.8
31.0
The circulation of the atmosphere: winds and pressure 157
southwestern United States in summer appears to be associated instead with mechan-
(Figs. 8.3, 8.4).
More numerous and widespread, however,
are pressure systems, both high and low,
whose immediate and direct cause is not
surface- temperature differences. Their origin
ical processes involving such factors as cen-
trifugal force, surface friction, and the
blocking effects of highlands which lie
athwart the paths of extensive and deep air
streams.
DISTRIBUTION OF ATMOSPHERIC PRESSURE
Vertical distribution Since air is very
compressible, there is a rapid decrease in air
weight or pressure with increasing altitude.
The lower layers of the atmosphere are most
compressed, or densest, because the weight
of all the layers above rests upon them. For
the first few thousand feet above sea level the
rate of pressure decrease is about 1 in., or 34
mb, of pressure for each 900 to 1,000 ft. As
higher altitudes are reached, the air rapidly
becomes much thinner and lighter, so that at
an elevation of 18,000 ft. one-half the atmos-
phere by weight is below the observer, al-
though a rarefied atmosphere extends to a
height of several hundred miles.
Horizontal distribution at sea level
Just as temperature distribution is repre-
sented by isotherms, so atmospheric pressure
distribution is represented by isobars, that is,
lines connecting places having the same at-
mospheric pressure at a given elevation. On
the isobaric charts here shown (Figs. 8.3,
8.4), all pressure readings have been reduced
to sea level to eliminate the effects of altitude
on pressure. Most pressure-distribution charts
represent sea-level pressures, although the
need for understanding upper-air flow has in
recent years caused the development of pres-
sure charts for higher levels, usually for about
10,000 ft (750 mb) and 18,000 ft (500 mb).
Where isobars are closely spaced, there is
a rapid horizontal change in pressure in a
direction at right angles to the isobars. The
rate and direction of the change is called the
pressure gradient. Where isobars are widely
spaced, the pressure gradient is weak.
The arrangement of average sea-level pres-
sures is reasonably well generalized and por-
trayed by both the idealized isobaric chart
(Fig. 8.1) and the meridional profile of pres-
F I G . 8.1 Arrangement of zonal sea-level
pressure. Except in the higher latitudes of the
Southern Hemisphere, the pressure zones, or
belts, are in the nature of cells of low or high
pressure arranged in latitudinal bands.
POLAR HIGH
S U B 1>R P I (XA L HJ G H
90
60
-30
EQUATORIAL LOW
SUBPOLAR LOW
POLAR HIGH
30
-60
90
158
Seasonal profiles of
sea-level pressure
from 60 N to60S.
in January and July
_L ( after Mmtz and Dean)
Eq-
Latitude
FIG. 8.2 Profiles of sea-level pressure from
60N to 60S averaged for all longitudes, at the
time of the extreme seasons. Equatorial low,
subtropical highs, and subpolar lows are
conspicuous features. Note the seasonal
north-south movements of the pressure belts,
following the sun.
sure (Fig. 8.2). Both figures suggest that
pressure, averaged for all longitudes, is
arranged in zones resembling belts. The so-
called belts are more accurately described as
centers, or cells, of pressure, arranged in
latitudinal bands. The belted arrangement is
more conspicuous in the relatively homo-
geneous Southern Hemisphere, where oceans
prevail. This suggests that two features of the
FIG. 8.3
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
great Northern Hemisphere continents their
highlands which obstruct the free flow of the
atmosphere and their great seasonal temper-
ature changes have much to do with the
origin and arrangement of pressure cells.
The most noteworthy features of the gen-
eralized world pattern of sea-level pressure
can be derived from Figs. 8.1 and 8.2. (a)
The dominant and key element is the series
of high-pressure cells which form irregular
belts of high pressure at about 25 -30 N
and S. These are the subtropical highs. Their
origin is not fully understood, but certainly it
is mechanical or dynamic, not thermal, (b)
Between the belts of subtropical high pres-
sure is the equatorial trough of low pressure.
(c) Poleward from the subtropical highs,
pressure decreases toward either pole, with
minima being reached in the vicinity of
65 N and S. At these latitudes are the sub-
polar centers, or troughs, of low pressure.
Their origin is not so clear, but it is due
more to mechanical than to thermal causes.
(d) Poleward from about latitude 65 data are
so scanty that the pattern of pressure distribu-
AVERAGE SEA- LEVEL PRESSURES AND WINDS
JANUARY
The circulation of the atmosphere: winds and pressure 159
FIG. 8.4
AVERAGE SEA-LEVEL PRESSURES AND WINDS
JULY
tion is not well known. It is generally assumed
that fairly shallow surface highs of thermal
origin occupy the inner polar areas.
If one were to inspect a weather map of
the earth for a single day, the above-described
arrangement of zonal surface pressure would
not be so evident. This would suggest that
the generalized cells and zones of surface
pressure are, partly at least, in the nature of
statistical averages of complicated day-to-day
systems of moving highs and lows.
Sea-level pressure distribution in January
and July, the extreme seasons Figures 8.3
and 8.4 illustrate some of the features of
seasonal pressure distribution which are of
the greatest significance climatically, (a) Pres-
sure belts and cells, like those of temperature,
migrate northward with the sun's rays in July
and southward in January. This is most
readily observed in Fig. 8.2, which shows
the meridional profiles of pressure. In gen-
eral, pressure is higher in the winter, or cold,
hemisphere, (b) The oceanic subtropical
highs are best developed over the eastern sides
of the oceans and tend to be weaker toward
the western sides, (c) The subpolar low in the
Southern Hemisphere is very deep, i.e., char-
acterized by very low pressure, and forms a
continuous circumpolar trough in both Janu-
ary and July, but the subpolar low in the
Northern Hemisphere consists of individual
pressure cells which are much more seasonal
in character, being strongest in winter, (d) In
January a strong thermal cell of high pressure
develops over the cold continent of Eurasia, a
weaker one over smaller and less frigid North
America. In July these same continents, now
warm, develop weaker thermal lows.
RELATION OF WINDS TO PRESSURE
Large-scale vertical and horizontal move-
ments of air are required to correct the un-
balanced distribution of energy which results
from latitudinal inequalities in the amount of
incoming solar, and outgoing earth, radiation.
Wind and the pressure gradient As
160
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
29.9 30.0 30.1 In. however, the actual flow of air from high to
low pressure is very oblique, or indirect.
Deflection by earth rotation On a
nonrotating earth, air set in motion by pres-
sure differences would simply flow along the
pressure gradient at right angles to the isobars.
But on a rotating earth where meridians and
parallels are constantly changing direction,
winds have an apparent deflection from the
gradient direction so that they cross the
isobars at an oblique angle. Over land sur-
faces, where friction is relatively great, the
surface winds make an angle with the isobars
of 20 to 40. Over the oceans, where friction
is much less, the angle may be as low as 10,
and in the free atmosphere several thousand
feet above the earth's surface, winds nearly
parallel the isobars, the angle being as low as
1 to 3.
In the Northern Hemisphere earth rotation
causes all winds to have an apparent deflec-
tion to the right of the gradient direction; in
the Southern Hemisphere the apparent de-
flection is always to the left (Fig. 8.6). This
rule will not appear to be true when tested,
however, unless it is kept in mind that one
must always face in the direction the wind is
blowing in order to observe the proper de-
flection. Deflective force of earth rotation in-
creases with increasing latitude, i.e., toward
either pole; only at the equator is it absent.
Wind direction Winds are always
named by the direction from which they come.
Thus a wind from the south, blowing toward
the north, is called a south wind. The wind
vane points toward the source of the wind.
Windward refers to the direction from which
a wind comes, Leeward refers to that toward
which it blows. Thus a windward coast is
one along which the air is moving onshore,
while on a leeward coast winds move off-
shore. When a wind blows more frequently
1013- 1016- 1019+ Mb
FIG. 8.5 Pressure gradient is the rate and
direction of pressure change. Gradient is
represented by a line drawn at right angles to the
isobars.
noted previously, air is set in motion by differ-
ences in air density which result in horizontal
differences in air pressure. Wind, therefore,
represents nature's attempt to correct pressure
inequalities. The rate and direction of pressure
change, or pressure gradient, largely deter-
mines the speed and general direction of the
wind. There are two fundamental principles
which control the relationship between pres-
sure and winds:
1. The rate of airflow, or speed of the
wind, depends upon the steepness of the
pressure gradient (rate of pressure change).
When the gradient is steep, with isobars
closely spaced, airflow is rapid. When the
gradient is weak, with isobars widely spaced,
the wind is likewise weak. Calms prevail
when pressure differences over extensive
areas are almost, or quite, nil.
2. The direction of airflow is from high to
low pressure, or down the gradient (Fig. 8.5).
This is just as natural as the well-known fact
that water, following the law of gravitation,
runs downhill. Because of earth rotation,
SUBPOLAR LOW
WESTERLIES
SUBTROPICAL HIGH
TRADES
V
EQUATORIAL LOW
30
TRADES
SUBTROPICAL HIGH
WESTERLIES
The circulation of the atmosphere: winds and pressure 161
60 bars with the lowest pressure at the center is
a cyclone. When the highest pressure is at
the center, such a system is an anticyclone.
In a cyclone, or low-pressure system, the air-
flow is from the margins toward the center. It
is a converging-wind system. The deflective
force of earth rotation causes the converging
air to move anticlockwise north of the
equator and clockwise south of it. In an anti-
cyclone air flows from the center toward the
margins, so that it is a diverging-wind system,
clockwise in the Northern Hemisphere and
anticlockwise in the Southern (Fig. 8.7).
F I G . 8 . 7 A much idealized representation of the
earth's surface winds. Airflow is from the east in the
low latitudes and from the west in the middle latitudes.
SUBPOLAR LOW
30
60
FIG. 8.6 Apparent deflection of the planetary
winds on a rotating earth. Double-line arrows
indicate wind direction as it would be developed
from pressure gradient alone. Solid-line arrows
indicate the direction of deflected winds.
from one direction than from any other, it is
a prevailing wind. On the daily weather map
wind arrows fly with the wind.
Cyclonic and anticyclonic circulations
As indicated earlier, sea-level-pressure
patterns are commonly cellular in character
and appear on an isobaric chart as systems of
closed isobars. Such a system of closed iso-
Polar
easterlies?
Westerlies
Horse latitudes
Trop. easterlies
(trades)
I.T.C. and
doldrums
Tropical
easterlies
(trades)
Horse latitudes
Westerlies
Polar
easterlies?
LOW
60
30
30
60
THE EARTH'S SURFACE WINDS 1
Zonal pattern From the meridional
profile of pressure (Fig. 8.2) or from the ideal-
ized sketch showing arrangement of pressure
belts (Fig. 8.1), the principal elements of the
earth's zonal surface-wind system can be
readily visualized (also Fig. 8.7). From the
1 The term surface wind refers to the lower few thousand feet
of the atmosphere.
162
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
subtropical highs at about 25 -30 N and S
surface winds flow both from north and south
toward the low-pressure trough near the equa-
tor. Earth rotation deflects these two air
streams into oblique easterly winds appropri-
ately designated as the tropical easterlies. They
are also known as the trade winds: northeast
trades north of the equator and southeast
trades to the south of it.
Poleward from the subtropical high-
pressure ridge in each hemisphere, winds
flow downgradient toward the subpolar lows.
They are turned by earth rotation so that
they have a general west-to-east movement.
These are the middle-latitude westerlies:
southwest in the Northern Hemisphere and
northwest south of the equator.
Poleward from about 65, where weather
observations are few, the nature of the sur-
face-wind system is uncertain. It seems prob-
able, however, that easterly winds prevail.
To summarize zonal surface winds, there is
a predominance of easterly winds in the low
latitudes, or tropics, and a prevalence of
westerly winds in the middle latitudes.
Between the converging trades, in the
vicinity of the equatorial trough of low pres-
sure, is a zone of variable and weak winds.
This transition belt between the two trades
has various names: intertropical convergence
zone (ITC), doldrums, and equatorial belt of
variable winds. Here, in some seasons and
over extensive longitudes, equatorial westerly
winds are in evidence. At other times and
places here, easterly flow prevails.
In the intermediate area between the diverg-
ing trades and middle-latitude westerlies,
fairly coincident with the crests of the sub-
tropical highs located at about 25 -30 in
each hemisphere, is still another belt of weak
and variable winds, the horse latitudes.
Seasonal contrasts The concept of a
simple, zonal arrangement of winds as pre-
viously described is a satisfactory introduc-
tion to surface winds. It is not, however,
adequate for portraying a number of features
of the atmospheric circulation which have
important climatic significance, such as
seasonal surface winds. For instance, a some-
what more realistic representation of die sur-
face winds in the extreme seasons, winter
and summer, is shown by Figs. 8.3 and 8.4.
Here a zonal belted arrangement of winds is
not so conspicuous. More prominent are the
spiraling cyclonic and anticyclonic circula-
tions around individual cells of low and high
pressure. Most striking are the large systems
of divergent anticyclonic circulation around
the subtropical high-pressure cells. These
systems dominate in both January and July.
The equatorward branches of anticyclonic
circulation are the tropical easterlies, or trade
winds; the poleward branches are the middle-
latitude westerlies. Close observation of Figs.
8.3 and 8.4 reveals a north-south shifting of
the wind systems, comparable to such shift-
ing of temperature belts, following the
seasonal course of the sun.
Notable in January are the well-developed
converging cyclonic circulations around the
expanded and deepened subpolar cells of low
pressure over the North Atlantic and North
Pacific Oceans. Equally important winter
climate features are the anticyclonic circula-
tions around the seasonal cells of high pres-
sure over the cold land masses of Eurasia and
North America. These diverging-wind sys-
tems are the winter monsoons. A well-
developed center of low pressure with con-
verging cyclonic circulation is conspicuous
over the heated continent of Australia, south
of the equator.
In July the cyclonic circulations over the
North Atlantic and North Pacific are weaker,
The circulation of the atmosphere: winds and pressure 163
but over heated Asia and North America
there are extensive cyclonic circulations with
converging winds developed around low-
pressure cells. These are the well-known
summer monsoons.
Zones and areas of average horizontal
divergence and convergence From the
preceding analysis of the surface winds, it
becomes clear that there are certain zones
and areas where the surface winds converge,
or tend to meet along a line or at a center.
There are others where the winds diverge, or
move away from a common line or center of
origin. Where surface winds converge, there
has to be an escape of the air through up-
ward movement, a condition which favors
condensation, the development of storms, and
associated clouds and precipitation. By con-
trast, where winds diverge, there must be a
downward movement of air from aloft (sub-
sidence) to compensate for the divergent sur-
face flow. Since such subsidence heats and
dries the air, divergence and subsidence are
opposed to the development of storms and to
the formation of clouds and precipitation
(Fig. 8.8).
Divergence and subsidence Some prom-
inent and best developed of all the zones of
subsidence and divergence are the zones of
divergence associated with the subtropical
anticyclones located at about 25 -30 north
and south of the equator. These zones are
not continuous around the earth, for the
oceanic cells of high pressure are strongest
toward the eastern sides of the oceans. There
subsidence is best developed and deserts are
conspicuous features of the adjacent lands.
Toward the western sides of the oceans
where the subtropical anticyclones, and thus
divergence and subsidence, are weaker, humid
climates are likely to prevail. Surface diver-
gence is likewise strong in the winter anti-
Convergence and
ascent
Divergence and
settling
FIG. 8.8 Where surface winds flow toward
each other, or converge, there is a resulting upward
movement of air. Where surface winds move apart,
or diverge, there must be a settling or subsidence of
the air.
cyclones over northern and eastern Asia and
over northern North America.
Convergence and ascent Most prominent
of all the extended lines of wind convergence
is that located between the Northern Hemis-
phere trades and the Southern Hemisphere
trades in the general vicinity of the geo-
graphic equator, the previously mentioned
intertropical convergence zone (ITC).
There are other zones of convergence as-
sociated with the subpolar troughs, or cells,
of low pressure where the westerlies flowing
poleward from subtropical latitudes meet air of
polar origin. These zones appear to be less
continuous both areally and temporally than
the ITC. Much of the convergence appears to
occur in the individual moving cyclonic storms
which are numerous in these latitudes, espe-
cially over the oceans. Seasonal areas of conver-
gence, associated with thermally induced
lows, are to be observed over southern and
eastern Asia and interior North America
in July.
SPECIFIC SURFACE
WINDS AND
THEIR CHARACTERISTICS
Tropical easterlies, or trade winds
The easterly winds which move obliquely
downgradient from the subtropical anti-
cyclones toward the equatorial trough of low
164
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
pressure, roughly between latitudes 20 -25
and 5-10 in each hemisphere over the
oceans, probably got their other name, trade
winds, from their prevailingly fair weather
and steadiness of flow (Fig. 8.7). Yet, the
trades are now known to be neither as uni-
formly steady in direction and speed nor as
full of fair weather as was formerly thought.
Over the eastern parts of the oceanic trades,
where the subtropical anticyclone and its
subsidence are strong, storms are few, winds
steady, cloud and rainfall meager, and fair
weather prevails. But in the western parts,
where the anticyclone and the accompany-
ing subsidence are weaker, atmospheric dis-
turbances are more numerous, the trades less
constant, and cloud and rainfall more prev-
alent (Fig. 8.9).
There are likewise important weather con-
trasts within the trade winds in a north-south
direction. Toward their poleward margins,
which are close to the centers of the sub-
tropical highs where subsidence is strong, the
trades are characterized by dry weather and
much sunshine (Fig. 8.9). Here the air is said
to be stable, for it lacks buoyancy, that is, it
is disinclined to rise, and is opposed to the
FIG. 8.9 The circulation pattern around a
subtropical anticyclone with general areas of
stability and instability shown. The eastern end of
the cell is much more stable than the western. The
poleward parts of the trades are more stable than
the equatorward parts.
r '
Subsidence*
weak. V \
Air neutral
or % "
unstable - B ^ /
(wet) ^(TRADES)
.^Airtinstable x
-* (wet) ^
formation of rainbringing storms. But very
gradually as the trades move equatorward
and westward from their source in the sub-
tropical highs, they are importantly modified
in ways which create an atmospheric environ-
ment more favorable for the development of
clouds and rainfall. One such modification
occurs as the winds, in passing over great ex-
panses of tropical ocean, take on large addi-
tions of moisture through evaporation. A
second modification of great climatic signifi-
cance is a consequence of the trade winds
having left a region of strong divergence and
subsidence in the subtropical anticyclones and
gradually approaching the equatorial trough
of low pressure where horizontal convergence
and lifting, and so instability of the air, are
characteristic. Here atmospheric disturbances
are more common, convectional overturning
of the air easier, and clouds and rainfall more
abundant. Thus it is that the trades are likely
to be dry, fair-weather winds in their pole-
ward and eastern oceanic parts, while their
western and equatorward sections are char-
acterized by more weather disturbances,
cloud, and precipitation.
But even in these rainier parts of the
tropical easterlies or trades the air ordinarily
is not so buoyant and unstable as it is in
equatorial latitudes close to the ITC, or
where equatorial westerlies prevail. As a con-
sequence, in the absence of highlands, the
total annual rainfall is not excessive.
Winds of the intertropical convergence
zone Winds are very complex in the vicinity
of the equatorial trough of low pressure,
which in the mean is located close to the
geographic equator but in the extreme sea-
sons may shift 10 to 15 to the north and
south over the continents (Fig. 8.7). Air move-
ment is usually light, and it is variable both
in speed and direction. Calms are frequent.
The circulation
At some times and places airflow is from the
west (equatorial westerlies), at other times and
places from the east. As a rule this region
between the trade winds is characterized by
much horizontal convergence and associated
rising air, so that the atmosphere is buoyant
and unstable. Only slight lifting is required
to trigger off strong vertical air currents, re-
sulting in cumulonimbus clouds of great
height, capable of producing heavy, showery
rains. Convergence and ascent of air cannot
be continuous, however, for all days are not
rainy. Probably, therefore, the convergence
and lifting are concentrated in the numerous
weak disturbances which infest these areas
close to the ITC.
Winds of the subtropics The latitudes
from 25 to 30 are close to the centers of
great anticyclonic circulations around zonally
arranged cells of high pressure. The poleward
branch of this anticyclonic circulation is the
middle-latitude westerlies; the equatorward
branch is the tropical easterlies or trades.
Thus these latitudes the subtropics, or horse
latitudes are the transition area between the
diverging surface trades and westerlies. As
noted earlier, such divergence must be accom-
panied by a slow settling of the air, or sub-
sidence, and both divergence and subsidence
are opposed to the formation of clouds and
precipitation. So the centers of the subtropical
anticyclones are characterized by much fair
weather and meager rainfall.
Like the equatorial trough of low pressure,
the centers of the subtropical anticyclones
are characterized by light winds coming from
a variety of directions, and by frequent calms.
However, the two regions are quite unlike in
their general weather conditions, the former
being a region of horizontal convergence,
numerous atmospheric disturbances, and
much showery rainfall, the latter an area of
of the atjnosphere: winds and pressure 165
horizontal divergence, few disturbances, and
generally fair weather.
In reality the horse latitudes are not alto-
gether similar in weather throughout (Fig.
8.9). Toward the western margins of each
oceanic anticyclone there is less atmospheric
subsidence than in the center and eastern
parts, so that while the latter are character-
istically dry, the western parts have a moder-
ate amount of cloud and precipitation. This
is how it develops that the eastern parts of
the subtropical oceans and adjacent (western)
parts of continents have dry and subhumid
climates, while the western parts of sub-
tropical oceans and their bordering land
areas have humid climates.
The variable westerlies of middle lati-
tudes The stormy westerlies of the Northern
and Southern Hemispheres move obliquely
downgradient from the centers of subtropical
high pressure to the subpolar lows. They are
located from roughly 35 -40 to 60 -65
(Fig. 8.7). Here the airflow is highly variable
in speed and direction. At times, especially in
the winter, the westerlies blow with gale force;
sometimes mild breezes prevail. Although
the winds are designated as westerlies, west-
erly being the direction of most frequent and
strongest winds, they do blow from all points
of the compass.
The variability of these winds in both
direction and strength is largely the result of
the procession of storms cyclones and anti-
cyclones which travels from west to east in
these latitudes. The storms, with their local
systems of converging and diverging winds,
tend to disrupt and modify the general west-
erly air currents. Moreover, on the eastern
sides of Asia, and to a lesser degree North
America, the continental wind systems called
the monsoons tend to modify the westerlies,
especially in summer. It is in the Southern
166
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
Hemisphere, where in latitudes 40 -65
land masses are largely absent, that the stormy
westerlies can be observed at their maximum
and least interrupted development. Over these
great expanses of ocean, winds of gale strength
are common in summer as well as winter. The
westerlies of the Northern Hemisphere, where
the great land masses with their seasonal
pressure reversals cause the wind systems to
be much more complex, are considerably less
boisterous in summer than in winter.
The poleward margins of the westerlies
near the subpolar troughs of low pressure are
particularly subject to great surges of cold
polar air in the winter season. A sinuous line
of discontinuity known as the polar front
separates the cold, dry, polar air from the
warmer and more humid mass coming from
the subtropics as westerlies and is the zone
of origin for a great many middle-latitude
cyclones and anticyclones. It follows, there-
fore, that the poleward margins of the west-
erlies are more subject to stormy, variable
weather than are the subtropical margins.
The polar front and the accompanying belt
of storms migrate with the sun's rays, retreat-
ing poleward in summer and advancing
equatorward in winter, so aperiodic storm
control of weather in the middle latitudes
should be most pronounced in the winter
season.
Winds of the polar regions As pre-
viously indicated, the few and poorly dis-
tributed aerological observations in the higher
latitudes beyond about 65 make an adequate
description of the polar wind systems impos-
sible at this time, but would appear to indicate
that wind from an easterly direction generally
prevails.
THE GENERAL CIRCULATION
OF THE ATMOSPHERE
Up to this point atmospheric circulation in
the lower atmosphere has been emphasized.
But some features of climatic distribution,
especially as it is related to precipitation, are
affected by the airflow at higher elevations as
well. The topic of general circulation, includ-
ing both high-level and low-level winds, will
be briefly considered at this point so that
some of the larger distribution features of
world weather and climate may be better un-
derstood. It must be borne in mind, however,
that important elements of the general circu-
lation remain controversial.
The necessity for a general circulation of
the atmosphere to compensate for the un-
F I G . 8.10 Schematic representation of the general circulation. In both
the low and the high latitudes the atmospheric circulations resemble that of
a convectional system, with the surface flow moving toward lower latitudes;
the opposite prevails aloft. In the middle latitudes the circulation is more
complex, and its features are not so well understood. In general, this
intermediate region appears to be a meeting place of contrasting air streams
from high and low latitudes.
EASTERLY
3O
"LATITUDE
;. 60* '
90*
The circulation of the atmosphere: winds and pressure 167
equal distribution of solar energy over the
earth between poles and equator has been
described earlier. It has also been said that
winds and ocean currents are the means by
which the excess of energy received in the
tropics is carried to the deficit regions farther
poleward. One might, then, readily conceive
of the atmospheric circulation as being in the
form of a gigantic convectional system with a
direct meridional flow, aloft from warm
equator to cold poles, at the surface from cold
poles to warm equator. But on the rotating
earth, with its surface composed of continents
and oceans with contrasting frictional and
heating properties, such a simple direct con-
vectional circulation between equator and
poles could not exist. The one that actually
prevails is much more complex, so much so
that no acceptable unified theory of the gen-
eral circulation now exists.
In its simplest form, the atmospheric circu-
lation between equator and poles is usually
represented as broken down into three smaller
meridional circulations, tropical, middle-lati-
tude, and polar (Fig. 8.10). In the low lati-
tudes equatorward from about 30, and also
in the high latitudes poleward from about
65, the evidence suggests circulations which
resemble that of a convectional system. In
both there is, on the average, an oblique
movement of air equatorward at the surface
and poleward aloft. Deflective force of earth
rotation causes the surface winds to be
easterly (trades and polar easterlies) and
those at higher levels to be westerly. Such a
circulation requires an upward movement of
air in equatorial latitudes and likewise in the
vicinity of latitude 60. Similarly, there is a
settling of the upper air both in subtropical
latitudes and near the poles.
It is principally in the middle latitudes
(3 5 -60) that the nature of the circulation
pattern is most complex and controversial.
WINTER
SUMMER
X
X
^\
E \
\ W
$\ \ > /Z
90
60
30
30
60
90
F I G . 8 . 1 1 A pole-to- pole cross section of the
planetary winds up to about 8 or 9 miles above the
earth's surface. E = tropical easterlies, or trades;
W = westerlies; x = average location of the jet
stream; w = the somewhat doubtful belt of
equatorial westerlies; e = polar easterlies.
(From Flohn.)
Here, apparently, much of the necessary
north-south heat exchange occurs in the form
of irregular invasions of cold polar air moving
equatorward and of warm surges of tropical
air moving poleward. In these latitudes, both
at the surface and aloft, the air movement is
prevailingly from the west, but wind and
weather are both highly variable, a result of
the frequent, alternating passage of cyclones
and anticyclones, accompanied by shifts in
wind direction and marked changes in tem-
perature. These cyclones and anticyclones
are associated with the alternating thrusts of
cold and warm air, which in turn accomplish
the required heat exchange. It is significant,
also, that only in the middle-latitude cell of
meridional circulation do the surface winds
flow poleward, and hence in an opposite
direction from those of a convectional circu-
latory system, in contrast to those of the
polar and tropical cells on either side. Be-
cause of this, there is a zone of divergence on
the equatorial side of the middle-latitude
westerlies, as well as a zone of frequent con-
vergence in their other parts, a fact that has
important consequences in weather and
climate.
The preceding analysis and Fig. 8.11 show
168
FIG. 8.12 Jet-stream map for January in the
Northern Hemisphere, showing average positions
and speeds in miles per hour at an elevation of
about 35,000 ft. (From Naming)
that a west-to-east circulation prevails through-
out most of the earth's atmosphere. This
westerly flow is often obscured at the earth's
surface, of course, by the frictional effects of
terrain irregularities and by numerous atmos-
pheric disturbances in the form of storms.
Also, there are at least two important excep-
tions to this general west-to-east movement
of the earth's atmosphere. The first and prin-
cipal one is the east-to-west flow in most of
the low latitudes. The tropical easterlies, or
trades, of these latitudes may reach heights
of about 6 miles near the equator. Poleward
from the equator they decline rapidly in
depth until they cease to exist at about 30 N
andSLat(Fig.8.11).
The jet stream, upper-air waves and
surface weather A relatively spectacular
feature of the atmospheric circulation, whose
existence has been made known only recently,
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
is the jet stream (or streams). The jet resembles
a meandering fast-flowing river. It travels at
speeds of 100 to 150 miles per hour, moving
from west to east at elevations of 20,000 to
40,000 ft (Fig. 8.12). Its position shifts north
and south with the seasons, but its average
location is between 30 and 40. Even
though the jet stream and the high-level
westerly winds of which it is a part are essen-
tially zonal (east-west) in character, they do
develop north-south oscillations, or waves, of
enormous length.
It is known that the jet stream and these
waves are closely associated with surface
weather conditions. For example, the waves
on the jet are directly related to the horizon-
tal expulsion of great masses of polar and
tropical air. When the jet stream is unusually
sinuous, these north-south thrusts of air
masses are strong and frequent. During such
periods storms are numerous and weather is
very changeable in the middle latitudes. Less
variable and stormy conditions prevail when
the jet is characterized by few and weaker
oscillations. Well-developed middle-latitude
cyclones extend upward into the jet-stream
waves, and may have their origin in these
waves. At least such storms intensify when
they are positioned underneath the jet, and
the jet stream appears to have the effect of
steering the cyclonic disturbances across the
earth's surface. It seems reasonable, therefore,
that cyclonic rainfall should be concentrated
underneath the jet stream.
TERRESTRIAL
MODIFICATIONS OF
THE SURFACE WINDS
Some effects of the earth's surface upon
surface winds which have been mentioned
incidentally in the previous discussion can
now be usefully amplified.
The circulation of the atmosphere: winds and pressure 169
Latitudinal shifting of the wind belts
Because of the parallelism and inclination of
the earth's axis, the sun's vertical noon rays
shift during half the annual period of revolu-
tion from 23^ N (summer solstice) to
231/2 S (winter solstice), a total of 47, as
noted previously. But the belt of maximum
solar energy actually undergoes a latitudinal
shift of 60 to 70. Following this north-
south migration of solar energy comes a
similar shift in temperature belts, largely sun-
controlled, and in pressure and wind belts,
in part thermally induced. The north-south
shifting of wind belts is by no means so sim-
ple a thing as it may appear to be from this
description, for it varies in amount and
rapidity from one part of the earth to another
because of the variations in the earth's surface.
Over the oceans and along coasts where the
latitudinal shift of winds is more readily
observable, the total migration is not great,
usually not much over 10 to 15. Over con-
tinents, on the other hand, where seasonal
temperature changes are greater, the total
latitudinal shift of winds, as well as pressure,
is also greater. In general, there is a lag of a
month or more behind the sun. But the time
lag is considerably less over land than over
oceans.
latitudes affected by more than one wind
belt The north-south shifting of the wind
belts is especially significant in those latitudes
lying between two wind systems having un-
like weather conditions, as for example,
between a converging and a diverging system.
Such latitudes are encroached upon by one of
the contrasting wind systems and its weather
conditions at one season, by the other wind
system and its weather at the opposite season.
Two such latitudes will be noted (Fig. 9.12).
1. Latitudes 5 to 15 north and south of
the equator lie between the equatorial con-
vergence zone (ITC) with its unstable air,
numerous atmospheric disturbances, and
abundant rainfall, and the subtropical zone
of divergence and subsidence where stable
air and few disturbances result in meager
rainfall. These latitudes experience weather
associated with the ITC and its disturbances
at the times of high sun (summer); but in the
season of low sun (winter) the fair, dry
weather of the divergence zone of the sub-
tropical anticyclones and their trade winds
prevails. A wet summer and a dry winter are
the results. This seasonal variation in weather
is not so conspicuous along the eastern side
of a continent where the anticyclone is weaker
and the air less stable.
2. Latitudes 30 to 40 are located between
the dry subtropical anticyclones and the
middle-latitude westerlies with their numer-
ous rainbringing cyclones. Drought associated
with anticyclonic subsidence and divergence
is characteristic of summer, while in winter
there is adequate precipitation from cyclonic
storms in the westerlies. This seasonal rain-
fall variation in latitudes 30 -40 is confined
largely to the eastern side of oceans and the
adjacent western side of continents where the
subtropical anticyclone is well developed and
the air stable.
Monsoon winds Winds which reverse
their direction of flow during the course of a
year and prevailingly blow from land to sea
in winter and from sea to land in summer are
called monsoon winds. Commonly this seasonal
reversal of wind direction in a monsoon is at-
tributed to the unequal heating of land and
water surfaces.
In winter, for example, a large continent
in middle latitudes is colder than the sur-
rounding sea surface, so that the air over the
land is colder and denser and the atmospheric
pressure higher (Fig. 8.3, eastern Asia). As a
consequence there is a flow of surface air
from land to sea. Because this winter mon-
170
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
soon originates over a cold land mass, it is
dry, cold, and stable and therefore resists
upward movement which might result in
cloud and precipitation.
In summer, by contrast, the land air is
warmer and less dense than that over the sea,
with atmospheric pressure lower over the
land. As a consequence surface air flows
from sea to land (Fig. 8.4, eastern Asia). This
summer monsoon originates over the sea, and
usually over relatively warm waters. Thus its
humidity content is high, with the conse-
quence that it provides an atmospheric
environment favorable for the development of
atmospheric disturbances capable of produc-
ing cloud and rainfall. The following illus-
trates the above-described causation sequence
of monsoon systems:
Winter land cold with high pressure
surface winds from land to sea.
Summer land warm with low pressure
surface winds from sea to land.
Monsoon winds are climatically significant
chiefly because of their effects upon temper-
ature and precipitation conditions of those
parts of continents where they prevail.
Ideally, monsoons should produce a climate
characterized by seasonal extremes of both
temperature and precipitation, with winters
that are cold and dry and summers that are
warm and wet. Actually, the effects, as well
as the origin and nature, of monsoon winds
have been greatly simplified in the preceding
description. For example, much of the
seasonal wind reversal to be observed in
tropical lands well exemplified in southern
Asia, northern Australia, and tropical Africa
is more a consequence of the latitudinal
shifting of wind belts following the sun
than it is the unequal heating of land and
water (Figs. 8.3, 8.4). It is chiefly the mon-
soons of middle latitudes, such as those in
eastern Asia and in eastern North America,
that owe their origin to land-water thermal
contrasts.
Moreover, neither the winter nor the
summer monsoons consist of a steady, un-
interrupted flow of air. Both are infested with
a variety of atmospheric disturbances which
interrupt the onshore and offshore flow of air
and at times even reverse its direction. These
are the same atmospheric disturbances which
produce the abundant summer, and the more
modest winter, precipitation in the monsoon
currents. Spells of weather are characteristic
of both monsoons.
Land and sea breezes Like monsoons,
land and sea breezes are wind reversals that
have their origins in the unequal heating of
land and water surfaces. But these wind re-
versals have a daily periodicity, not the
seasonal one of the monsoons. At night the
land's greater coolness results in an offshore
breeze; by day the heated land causes the
wind to flow onshore. Universally the effects
of the sea breeze are felt for only a few miles
inland, and this only in the warmer seasons
of middle latitudes. Along tropical coasts the
daily sea breeze is not limited to any season.
Modest temperature and rainfall effects are
produced by the sea breeze. Along coasts
high daytime temperatures may be appreciably
reduced by the cooler sea or lake air moving
onshore. It is believed, also, that the sea
breeze may have some generating effect on
thunderstorm activity, thereby increasing the
total precipitation along littorals (coastal
regions).
CHAPTER 9
Precipitation
WATER VAPOR, OR HUMIDITY
Only in the invisible, or gas, form is water
an integral part of the atmosphere; it is then
referred to as water vapor or as humidity.
The water vapor in the atmosphere varies in
quantity from place to place and also from
time to time, hut always comprises only a
small part of the total atmosphere. If all of
the water vapor in the air were condensed to
the liquid form and evenly distributed over
the earth as rain, it would form a layer only
about 1 in. deep. Nevertheless, water vapor
is by far the most important gas in the atmos-
phere as far as weather and climatic phenom-
ena are concerned, for several reasons, (a)
The amount of moisture in the air is directly
related to precipitation possibilities, (b) The
more water vapor in the atmosphere, the
more stored-up energy available for the growth
of atmospheric disturbances which produce
rainfall, (c) Water vapor is the chief absorber
of both solar radiation and energy radiated
from the earth, and therefore regulates tem-
perature, (d) The relative amount of water
vapor affects the human body's rate of cooling
and hence its feeling of heat and cold, i.e.,
the sensible temperature.
Evaporation-condensation cycle and
sources of water vapor The water vapor
in the air is derived from water in the liquid
or solid form through the process of evapora-
tion. In reverse, condensation occurs when
water vapor is changed to the liquid or solid
state to form clouds. The condensed water
or ice may then fall on the earth's surface as
171
172
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
precipitation. Among the several gases of the
air, only water vapor condenses to form
a liquid (water) or a solid (ice) within the
range of atmospheric temperatures.
The principal source of atmospheric
humidity is the oceans which cover nearly 71
per cent of the earth's surface. By winds the
water vapor evaporated from the oceans is
carried in over the continents. Indeed, most
of the continental precipitation is derived
from moisture from over the oceans. How-
ever, a portion of the atmosphere's humidity
is derived from evaporation which occurs
over the continents evaporation from the
moist soils, the vegetation cover, and from
inland bodies of water.
The evaporation-condensation-precipita-
tion cycle is constantly in progress as water
vapor is added to the atmosphere by evapora-
tion and removed by condensation-precipita-
tion (Fig. 16.3). Over the oceans evaporation
exceeds precipitation; over the continents
precipitation is in excess of evaporation. But
these differences are equalized by (a) the dis-
charge of rivers and glaciers into the oceans,
and, to a much greater extent, by (b) the
transfer of land-evaporated moisture seaward
in the great streams of continental air.
Latent energy in water vapor Energy
in the form of heat is required to change ice
(solid) into water (liquid), and water into
water vapor (gas). The unit of heat energy,
the calorie (cal), is the amount of heat re-
quired to raise the temperature of a gram (g)
of water one degree centigrade (c). It takes
79 cal to convert 1 g of ice into 1 g of water
at freezing temperature. A much greater
quantity of heat, 607 cal, is required to evap-
orate 1 g of water (change it to water vapor)
at freezing temperature. Thus, water vapor
must contain more potential energy than
water or ice. This energy stored up in water
vapor is called latent heat. For the most
part it is transformed solar energy which has
been used in the process of evaporation.
If energy is consumed in the process of
evaporation, then, conversely, energy in the
form of heat must be released into the atmos-
phere v/hen water vapor is condensed into
the liquid or solid state to form clouds. This
latent heat of condensation furnishes one of
the principal ways in which the atmosphere
is heated. On a cloudy night when conden-
sation is in progress, the liberated heat acts
to retard the normal night cooling. The heat
of condensation is likewise a principal source
of energy for the growth and development of
storms, especially thunderstorms, hurricanes,
and other tropical disturbances. Thus it plays
an important role in causing precipitation
and in determining its distribution.
Evaporation heat consumed
Solid Liquid
(ice) (water)
Condensation heat released
Gas
(water vapor)
Atmospheric humidity The amount of
water vapor that air can hold depends almost
wholly upon air temperature. Air that is
warm is able to contain much more water
vapor than air that is cold. Moreover, capacity,
the maximum amount of water vapor that a
given volume of air can hold at a given tem-
perature, increases at an increasing rate as
the temperature rises. This is made clear by
the following table and by Fig. 9.1. Thus,
if the temperature of 1 cu ft of air is in-
creased by 10F, from 30 to 40, the mois-
ture capacity is advanced only 1 grain, while
a similar 10 increase from 90 to 100 in-
creases the capacity 5 grains. So it is evident
that the warm air of the tropics has a far
greater moisture capacity than has the cold
air of the subarctic and polar regions. Like-
Precipitation 173
wise, warm summer air is able to contain
much more water vapor than cold winter air
is. These facts have important climatic impli-
cations, for they help to explain the meager
precipitation of the polar regions compared
with the abundant rainfall of equatorial lati-
tudes, as well as the greater precipitation in
summer than in winter over the middle-
latitude continents.
Distribution of humidity The water- vapor
content of the air (expressed as specific
humidity, absolute humidity, or vapor pres-
sure) normally is highest near the earth's sur-
face and decreases rapidly upward. Such a
vertical distribution is to be expected since
the earth's surface is the source of the atmos-
phere's humidity and temperatures normally
are higher at low elevations. Half the water
vapor in the air lies below an altitude of
6,500 ft.
In a north-south direction, or along a
meridian, water-vapor content is highest in
the low latitudes near the equator and de-
creases poleward (Fig. 9.2). This also is as-
sociated with temperature distribution. As a
general rule, winds arriving from tropical
latitudes, especially from oceanic sources,
contain an abundance of water vapor, and so
20
D 10 20 30 40 50 60 70 80 90 100
Temperature F
FIG. 9.1 The capacity of air to contain water
vapor not only increases as the temperature of the
air rises, it increases at an increasing rate.
are conducive to large-scale condensation
and precipitation. By contrast, air derived
from cold polar sources and some conti-
nental areas is low in water-vapor content
and hence too dry to yield much precipitation.
Relative humidity Relative humidity
refers to the amount of water vapor in the air
(absolute or specific humidity) compared
with the greatest amount that the air could
contain at the same temperature (its capacity).
Relative humidity is always expressed in the
form of a fraction, ratio, or percentage. For
example, air at 70 has the capacity of con-
taining 8 grains of water vapor per cu ft. If it
Maximum Water-vapor Capacity of 1 Cu Ft of Air at
Varying Temperatures
Temperature,
degrees Fahrenheit
Water vapor,
grains
Difference between
successive 10
intervals, grains
30
40
50
60
70
80
90
100
1.9
2.9
4.1
5.7
8.0
10.9
14.7
19.7
1.0
1.2
1.6
2.3
2.9
3.8
5.0
174
FUNDAM
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FUNDAMENTALS OF PHYSICAL GEOGRAPHY
SATURATED CONDITIONS
(relative humidities IQQ%)
North South
Latitude, degrees
FIG. 9.2 Zonal distribution of the water-vapor
content of the air. Specific humidity is highest in
the vicinity of the equator and decreases toward
the poles. There is a northward displacement in
July and a southward displacement in January
because of the shift of temperature belts. Specific
humidity at each latitude is higher in summer than
in Winter. (From Haurwitz and Austin.)
actually does contain 6 grains, then it is only
three-fourths saturated, and its relative humid-
ity is 75 per cent. When the relative humidity
reaches 100 per cent the air is said to be
saturated.
The relative humidity of an air mass can
be altered in two ways, (a) by a change in
the amount of the water vapor in the air, or
(b) by a change in the temperature of the air
and hence its capacity. The following table
shows how air which is saturated (relative
humidity 100 per cent) at 40 acquires suc-
cessively lower relative humidities simply by
leu ft
3 grains
water
vapor
40 F
(1 grain 0.002 ounce)
(a)
UNSATURATED CONDITIONS
80 F
/*'
/
Relative humidityv
17^ 91%
1 X
/ leu ft
/
/
/ 1 cu ft \
\
\
leu ft \
% grain
water
vapor
1/2 grain
water
vapor
10 grains
water
vapor
0F
40 F
(b)
80 F
FIG. 9.3 Absolute and relative humidity.
an increase of its temperature, the water-
vapor content remaining unchanged. Various
humidity relationships are also illustrated by
Fig. 9.3<z and b. Figure 9.3a shows the
changing capacity for water vapor of a cubic
foot of air under three different temperature
conditions. Figure 9.36 shows the same
cubic-foot samples as Fig. 9.3a ? but with
varying relative humidities.
Since relative humidity is an important
Temperature,
degrees Fahrenheit
40
50
60
70
80
90
Absolute humidity,
grains
2.9
2.9
2.9
2.9
2.9
2.9
Relative humidity,
per cent saturated
100
71
51
36
27
20
Precipitation 175
60
90 70 60 50
Latitude, degrees
30
South
40 50 60 7090
FIG. 9.4 Zonal distribution of relative humidity. Note that the
north-south distribution of relative humidity is quite different from that of
specific humidity (Fig. 9.2).
determinant of the amount and rate of evap-
oration, it is critical in determining the rate
of moisture and heat loss by plants and ani-
mals, including human beings. Consequently
it importantly affects the sensible temperature
and therefore human comfort. Relative
humidity is also closely related to the devel-
opment of clouds and precipitation. Air that
is close to the saturation stage requires only
a minimum amount of cooling to bring about
condensation and the formation of clouds.
On the other hand, air with low relative
humidity requires a large amount of cooling
in order to form clouds and cause rainfall. In
desert regions the relative humidity is usu-
ally so low that only rarely is the air cooled
enough for rain clouds to form.
Meridional distribution There is a strong
maximum of relative humidity near the
equator, from which there is a decline pole-
ward to minima located at about 25 N and S
(Fig. 9.4). Subsidence of deep air masses at
these latitudes of the subtropical anticyclones
is the cause of the reduced relative humidity.
Here are located some of the earth's most ex-
tensive deserts. Poleward from the subtropics,
the relative humidity again increases as the
temperature declines, and a second pair of
maxima are located in the higher middle
latitudes (about 60 N and S). During the
daily period of 24 hr, relative humidity of air
near the ground is everywhere usually highest
in the cool early-morning hours and lowest
in the warm midafternoon.
CONDENSATION
Origin If nonsaturated air is subjected to
progressive cooling and its capacity for
moisture thereby reduced, a temperature is
eventually reached at which the air is sat-
urated with moisture (relative humidity 100
per cent), even though the total amount of
water vapor has remained unchanged. This
critical temperature is called the dew point.
If the air is then cooled below the dew point,
the excess water vapor, over and above what
the air can contain at this lower temperature,
forms as minute globules of water (if the tern-
176
TYPES OF FOG
1. Ground-inversion fog
ild air FOG Temperature inver
'///////////////////// S
2 Advection fog
F I G . 9.5 Common types of fog and ways in
which they form.
perature is above 32) and possibly tiny ice
crystals (if below 32). When this has
happened, condensation has taken place. As
an example, air with a temperature of 80
and containing 8 grains of water vapor per
cu ft has a relative humidity of 73 per cent
(table, p. 173). If this air is gradually cooled
and its capacity tor water vapor thus lowered,
it eventually reaches its dew point, 70, the
temperature at which it is saturated. Further
cooling results in condensation and the re-
lease of latent heat, the varying amount of
condensation that occurs at the different tem-
peratures reached reflecting the changing
water-vapor capacity of the air. It bears re-
peating that an equivalent cooling of warm
air and of cold air does not result in the
same amount of condensation (table, p. 173).
Most large-scale condensation, including
the formation of all precipitation, is a conse-
quence of the reduction of air temperature
below the dew point. When the relative
humidity of any mass of air is high, only a
slight amount of this cooling is required be-
fore the dew point is reached and condensa-
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
tion begins. Condensation, therefore, depends
upon two variables: (a) the amount of cool-
ing, and (b) the relative humidity of the air.
Surface condensation The temperature
of shallow layers of surface air may be lowered
below the dew point, resulting in condensa-
tion, by direct cooling through conduction
arid radiation of heat to a cold earth surface
or by the mixing of two air masses having
unlike temperatures and humidities. Ap-
preciable rainfall probably never results from
these cooling processes; rather, such forms of
condensation as dew, white frost, and fog are
the consequences. In a description of world
climates such surface condensation is not of
great importance.
Fog Fog, by far the most climatically im-
portant of the several forms of surface con-
densation, can develop in a variety of ways.
One of the commonest types of land fog,
known as radiation or ground-inversion fog,
results from the cooling, by radiation and
conduction processes, of shallow layers of
quiet air overlying a chilled land surface
(Fig. 9.5). Clear nights with little wind favor
the development of such fog. Also, it is most
prevalent and becomes deepest in valleys and
depressions where, as a result of air drainage,
the colder, heavier air collects. Being char-
acteristic of the cooler night hours, radiation
fog is usually short-lived, for it tends to dis-
sipate with sun heating during the day. But
in the vicinity of large industrial cities where
sulfurous condensation nuclei are numerous,
it is likely to be more persistent, as well as
denser. Such a combination of smoke, auto-
mobile exhaust fumes, and fog is known as
smog.
Another very common kind of fog is the
advection fog which develops in moving, rather
than quiet, air. Fogs of this kind are formed in
mild, humid air as it moves over a colder
surface and is chilled by radiation and con-
duction (Fig. 9.5). They are very common
over oceans, especially in summer, along sea-
coasts and the shores of large inland lakes,
and over middle-latitude land surfaces in
winter. They are particularly prevalent in the
vicinity of cool ocean currents. In the in-
teriors of continents advection fogs are com-
monly associated with a poleward flow of
mild, humid air from low latitudes over a cold
and snow-covered surface. In general, advec-
tion fogs are less local in development than
the simple radiation type, and they tend to
persist for longer periods of time. Thus days
as well as nights may remain shrouded by them.
Distribution of fog. Generalization about
fog distribution is not easy. Yet without much
doubt fog is more common over oceans than
over continents, and it is more frequent over
oceans in middle and higher latitudes than
over those in the tropics. On the continents
the coastal areas have the greatest number of
days with fog. In the United States fog days
are most frequent along the Pacific Coast
and the North Atlantic seaboard and over
the Appalachian Highlands. The least foggy
area is the dry interior of the western countiy.
Condensation in the free atmosphere:
clouds and associated precipitation Of
incomparably greater climatic importance
than surface condensation in the form of dew,
white frost, and fog is cloud condensation
occurring well above the earth's surface, for
all of the earth's precipitation originates in
clouds. Clouds of great vertical thickness,
capable of yielding moderate or abundant
precipitation, are the product of one atmos-
pheric process almost exclusively, viz., cool-
ing as a result of expansion in upward-
moving, thick air masses.
When air rises, no matter what the cause,
it expands because there is less weight of air
Precipitation 177
upon it at the higher altitudes. For example,
if a mass of dry air at sea level rises to an
altitude of about 18,000 ft, the pressure upon
it is reduced by one-half, and consequently
its volume is doubled. Thus 1 cu ft of sea-
level air would occupy 2 cu ft if carried to
that altitude. To make room for itself as it
ascends and gradually expands, this air has
to displace other air. The work of displacing
the surrounding air requires energy, and this
necessary energy is taken out of the rising air
mass in the form of heat, resulting in a lower-
ing of its temperature. Conversely, when air
descends from higher altitudes, it is com-
pressed by the denser air at lower levels. Work
is done upon it, and its temperature conse-
quently is raised. It is obvious, therefore, that
rising air cools, while descending air is
warmed. This is spoken of as adiabatic tem-
perature change.
The rate of cooling resulting from the
ascent of dry or nonsaturated air the dry
adiabatic rate is constant and is approxi-
mately 5.5 per 1,000-ft change in altitude.
This rate of cooling of ascending air is con-
siderably greater than the normal rate of tem-
perature decrease with increasing elevation
(about 3.6 per 1,000 ft) called the lapse
rate. These two rates, the adiabatic rate and
the lapse rate, should be clearly distinguished
as being very different things, for one repre-
sents the cooling of a rising and therefore
moving mass of air, while the other represents
the change in air temperature that would be
recorded by a thermometer carried up through
the atmosphere by a balloon or airplane.
It bears repeating that this process of cool-
ing, by expansion within rising air currents,
is the only one capable of reducing the tem-
perature of thick and extensive masses of air
below the dew point. It is the only process,
therefore, which is capable of producing con-
178
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
densation on such a large scale that abundant
precipitation results. There is no doubt that
nearly all the earth's precipitation is the result
of expansion and cooling in rising air cur-
rents. The direct result of cooling due to
ascent is clouds.
Conditions affecting the buoyancy of
air Since nearly all of the earth's precipita-
tion originates in thick clouds that are a
consequence of cooling in ascending air, the
conditions which promote or hinder such
upward movement are of prime importance.
Stability As previously suggested, when
air resists vertical movement and tends to
remain in its original position it is said to be
stable. Normally an air mass is most stable
when dry and colder air underlies warmer air.
The denser air below the lighter air makes
upward movement difficult. Thus, in highly
stable air abundant precipitation is less likely
to occur. However, even stable, nonbuoyant
air may be forced to rise, cool, and produce
cloud and rainfall, as when an air stream is
obstructed by mountains or hills or when two
air streams converge and come into conflict.
Atmospheric stability is promoted in at
least two ways. If an air mass is chilled at its
base through loss of heat by radiation and
conduction to a cold underlying surface, the
density of the lower air is increased, and so
the stability is also increased (Fig. 9.6). A
surface temperature inversion, therefore, is an
instance of stability. Another way a mass of
air can develop stability is by subsiding and
spreading laterally (horizontal divergence).
This process of stabilization occurs in high-
pressure anticyclonic systems.
Instability When air does not resist up-
ward vertical displacement but, on the con-
trary, has a tendency to move upward from
its original position, a condition of instability
prevails. In such buoyant air upward vertical
movement is made easy, and clouds and pre-
cipitation are likely. Instability is character-
istic of warm, humid air in which there is a
rapid vertical decrease in temperature, i.e.,
a steep lapse rate, and humidity.
Instability is developed in an air mass when
it is warmed and humidified in its lower
layers by moving over a warm earth's surface
FIG. 9.6 Atmospheric stability and instability. When the lapse rate
exceeds the abiabatic rate, instability prevails. When the reverse is true, the
air is stable.
OU
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2000
1500
1000
500
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20 30 40 50 60 70 80 9
85'
Temperature, F.
Precipitation 179
(Fig. 9.6). Instability is likewise promoted in
a thick air mass when it is forced to rise.
Hence the air in any converging-wind sys-
tem, such as a cyclonic storm, is likely to
become unstable. Or again, when humid air
that is originally mildly stable is forced to
rise over mountain barriers or over colder
wedges of air, the resulting condensation may
add so much heat to the ascending air that it
becomes buoyant and unstable. Then it con-
tinues to rise with accompanying heavy pre-
cipitation. Precipitation occurring in unstable,
buoyant air is likely to be heavy and showery
in character, and usually is associated with
thick cumulus clouds (Fig. 9.7). Unstable air
will continue to rise until it reaches air layers
having temperature and density similar to
its own.
STABLE AIR
UNSTABLE AIR
FIG. 9.7 Cloud formation in the forced ascent
of air which is stable in one case and unstable in
the other. In stable air, which is nonbuoyant, the
clouds are not so thick in a vertical dimension, and
the resulting precipitation will probably be lighter.
PRECIPITATION
FORMS
Although all precipitation originates in
clouds, by no means do all clouds yield pre-
cipitation. This is because the condensed
water or ice particles which form clouds are
too small to fall to the earth's surface. To
produce precipitation the myriad of tiny cloud
droplets must be forced to combine to form
drops of sufficient size and weight to reach
the surface.
Rain, which is much the commonest and
most widespread form of precipitation, results
from cloud condensation in ascending air at
temperatures either above or below freezing
(Fig. 9.8). Some of the earth's rain certainly
originates as ice and snow particles formed
at temperatures below 32, which subse-
quently melt as they fall through the wanner
atmosphere closer to the earth's surface.
The most common form of solid precipita-
tion is snow. Its fundamental form is the
intricately branched, flat, six-sided crystal
which occurs in an almost infinite variety of
patterns. Numbers of these crystals matted
together comprise a snowflake. Snow must
develop from condensation taking place at
temperatures below freezing, and on the
average it takes about 1 ft of snow to yield as
much water as I in. of rain.
Data on the amount and distribution of
snowfall are very scant for much of the earth.
Snow occasionally falls near sea level in
subtropical latitudes, but it does not remain
on the ground; farther equatonvard it is not
recorded at low elevations. A winter snow
cover durable enough to last for a month or
more does exist at low elevations in the
interior and eastern parts of Eurasia and
North America poleward from about 40.
ISsb^*-
. * . t ,_
^7'r*mfc,,, r , '-^w',. ^v <o , "^vfcr = fiK-_ >^^ " f# "*' f*^* * '-XT*,
'^^^^t^^^J^S
- ; ': ; ; ^-':'t---^^i- '.;,:>-::; .?#
"'-. ;;,;, ..,. '. ^Cv'^;;-^ ----. ^V.r^-;-..
, . ,
FIG. 9.8 Very generalized representation of the forms and elevations of
the principal cloud types.
In low and middle latitudes a permanent
snow cover is characteristic only of elevated
areas, with the height of the snow line, or
limit of perpetual snow, declining poleward.
Thus in the deep tropics permanent snow is
.usually found only at elevations over 15,000
ft, but at 60 N in Norway snow remains on
the ground throughout the year at an eleva-
tion of about 3,500 ft.
Sleet and hail are other forms of solid pre-
cipitation. They occur only very occasionally
and are restricted in their distribution; thus
their total climatic significance is minor. Sleet
is frozen raindrops. Hail, which falls almost
exclusively in the violent thunderstorms to
be described in the next chapter, is ice lumps
which are larger than sleet.
PRECIPITATION
CLASSIFIED BY CAUSES
OF AIR ASCENT
Since almost all precipitation originates in
ascending air which is cooled by expansion, it
is essential, for an understanding of precipita-
tion distribution over the earth, to be familiar
with the causes for the ascent of thick and
often extensive masses of air. Therefore three
principal kinds of atmospheric lifting and
their associated precipitation will be noted.
But it should be emphasized that none of
these three ordinarily exists in pure form:
most precipitation is a consequence of the
combined effects of more than one type of
atmospheric lifting.
Orographic precipitation Even a hasty
observation of a precipitation map of the
earth shows that many areas of above-average
precipitation are coincident with highlands
(Plate 1). This is at least partly the result of
the forced ascent of air currents whose course
is obstructed by hills or mountains. In addi-
Precipitation 181
don, convectional updrafts caused by strong
solar heating along some mountain slopes
increase precipitation in highlands. Since
water vapor is largely concentrated in the
lower layers of the atmosphere, even the
modest upthrust of air masses caused by a
highland of moderate elevation may be suffi-
cient to induce important rainfall effects.
Rainfall produced this way is called oro-
graphic precipitation. Noteworthily, it is con-
centrated on the windward side of a highland
where the lifting effect on the approaching
winds is concentrated; the lee side where the
air is descending and warming is much drier
(Fig. 9.9).
Convectional precipitation Convec-
tional preclpiation is associated with strong
vertical updrafts of air and with the towering
cumulonimbus thunderstorm clouds which
the updrafts produce. Such rapidly ascending
air currents are likely to develop when
humid air containing much latent heat is
subjected to strong surface heating. Such a
condition often prevails on a hot summer
afternoon when the earth's surface has be-
come unusually warm through solar heating.
The heated surface air expands, becomes less
dense, and, like an inflated toy balloon, is
inclined to move upward.
Strongly heated surface air creates a general
environment favoring such convectional over-
turning of the atmosphere, so it is not sur-
prising that this type of precipitation is most
FIG. 9.9 Precipitation conditions on windward
and leeward slopes of highlands.
Prevailing drift
Leeward slopes
/ "ram shadow"
i arid to semiand
182
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
common in tropical latitudes, and that there,
as well as in the middle latitudes, it is con-
centrated in the warmer months of the year
and the warmer hours of the day. Yet it has
been observed that much convectional rain-
fall does not have the distribution which
would be expected if widespread surface
heating were the single cause. Instead, the
convectional activity is organized in character
and appears to develop only in certain areas.
One favored area is the vicinity of highlands,
where orographic effects supplement those of
surface heating. More common is the moving
area of organized convectional activity associ-
ated with an extensive atmospheric disturb-
ance in which a general horizontal convergence
of air streams lifts the air and increases its
instability.
Convectional overturning has a cellular
pattern of local ascending and descending air
currents which are small in horizontal dimen-
sions. Consequently the cumulonimbus clouds
marking the tops of the ascending warm
currents, while very thick in a vertical direc-
tion, are not areally extensive, and are fre-
quently isolated and distinct, with patches of
clear sky between. Therefore convectional
rainfall is usually in the form of locally heavy
showers which do not last long. Long-con-
tinued, steady rains falling from a uniformly
gray overcast are not characteristic.
Horizontal convergence in extensive
atmospheric disturbances In any zone or
area of horizontal wind convergence an up-
ward movement of air must occur, with con-
sequent cooling. In the heart of the tropics,
where the converging air streams usually are
similar in temperature and density, the up-
ward movement of air is commonly in the
form of vertical convective currents which
result in showery rainfall. But outside the
tropics horizontal wind convergence usually
produces a conflict between air masses of
contrasting temperature and density, with the
cooler, denser air providing an obstacle over
which the warmer, lighter air is forced to
ascend (Fig. 9.10). The boundary between
such unlike air masses is called a front.
Some of the most commonly occurring
convergence areas of the earth are those
associated with the great variety of extensive,
moving atmospheric disturbances, one such
disturbance being the cyclonic storm of
middle latitudes. In parts of the earth atmos-
pheric disturbances are so numerous that for
a month or a year their paths can appear on
mean-pressure and mean-wind charts as
average lines of horizontal wind convergence.
FIG. 9.10 The origin of precipitation along a front. Here the warmer
and less dense air cools because of expansion as it ascends over a wedge of
cooler, denser air.
Earth's surface
The zone of convergence between the trades
(ITC) and the zones of convergence in the
North Atlantic and North Pacific lows in
winter may be partly of such origin.
Since in middle-latitude cyclones and
along their fronts the air commonly rises
obliquely over mildly inclined surfaces of
colder air, the cooling of the rising air is less
rapid than in vertical convectional currents.
As a result, precipitation in cyclones is char-
acteristically less showery and heavy than
convectional rainfall, and is inclined to be
steadier and longer-continued. The dull, gray,
overcast skies and prolonged precipitation
which result in some of the most unpleasant
weather in the cooler months in middle
latitudes are usually associated with cyclonic
storms; and most of the long-lasting, mild
winter precipitation of lowlands in the middle
latitudes is cyclonic or frontal in origin. But
by no means is all the precipitation in a
convergent system of the mild and prolonged
kind, for not infrequently there is enough
initial upthrust of air along a front to make
it so unstable that intermittent showery rain
may result.
IMPORTANT
CHARACTERISTICS
OF PRECIPITATION
Even an incomplete description of the pre-
cipitation of a region requires taking note of
at least three features: (a) the annual amount
of precipitation, (b) the seasonal distribution
of the annual total, and (c) its dependability,
or conversely, its variability.
Amount It is estimated that if the total
annual rainfall were spread evenly over the
earth's surface, it would form a layer about
39 in. deep. Actually precipitation is distrib-
uted very unevenly, for there are extensive
Precipitation 183
areas that receive less than 5 in. and there are
a few spots that receive over 400 in. (Plate 1).
Seasonal distribution The seasonal
distribution of precipitation is as important as
amount. The fact that Omaha, Nebraska,
receives 30 in. of rainfall annually is no more
significant than the fact that 17 in. (58 per
cent of the annual total) falls during the warm
months from May to August and only 3 in.
(11 per cent) falls during the cold period
from November to February. Seasonal distri-
bution of precipitation is of greatest impor-
tance in the middle latitudes where the
winter is a dormant season for plant growth
imposed by low temperatures. In the tropics
where frost is practically unknown except at
higher elevations, rainfall is effective for plant
growth no matter what time of year it falls.
In the middle latitudes, however, only the
part of the annual precipitation which falls
during the frost-free season is effective, so
that in severe climates it is desirable to have
a strong concentration of rainfall in the
warmer months when plants can use it.
Variability Data on the dependability
or reliability of the annual or seasonal precipi-
tation express its variability as well, and are
scarcely less important than those concerned
with amount and seasonal distribution (Fig.
12.1).
~~ Variability of precipitation may be defined
as the deviation from the mean computed
from 35 years or more of observations. In
humid climates the annual variability is
usually not greater than 50 per cent on either
side of the mean; i.e. the driest year can be
expected to have about 50 per cent of the
normal value, the wettest year 150 per cent.
In dry climates these values vary between
about 30 and 250 per cent. Thus it is a
general rule that variability increases as the
amount of rainfall decreases. Variability of
184
1500
1000
500
N 80
60 40 20
N. Latitude
20 40 60
S Latitude
80 S
FIG. 9.11 Zonal distribution of precipitation.
The amount for any latitude represents the average
for all longitudes. (From Brwh and Hunt.)
precipitation must be taken into consideration
when agricultural plans are made, for it must
be expected that there will be many years
when the precipitation is less than the average.
In semiarid and subhumid climates where crop
raising normally depends on a small margin
of safety from failure, rainfall variability is of
utmost concern. Moreover, the agriculturist
in such regions must bear in mind that nega-
tive deviations from the mean are more
frequent than positive ones; that is, a greater
number of dry years are compensated for by
a few excessively wet ones. In dry climates
and other climates as well, variability of
seasonal and monthly rainfall amounts is even
greater than that for annual values.
DISTRIBUTION
OF PRECIPITATION
Average annual precipitation amounts
A glance at Plate 1 makes it obvious that the
distribution of annual precipitation is very
complicated. No simple explanation for this
will suffice, but fundamentally two things
are involved. They are (1) the nature of the
air itself, especially its varying humidity and
stability, and (2) the distribution over the
earth of influences on vertical movement of
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
the air, which include (a) the principal zones
of horizontal convergence and divergence,
(b) atmospheric disturbances, (c) thermally
induced convectional overturning, and (d)
highland barriers. The nature of the air as
regards its moisture content is chiefly deter-
mined by its place of origin: maritime or
continental, as well as tropical or high-lati-
tude. Some of the controls, for example the
zones of horizontal convergence arid diver-
gence, are relatively zonal (having an east-
west alignment) in their influence. Others, like
the distribution of land and water and of
highlands, operate to modify zonal controls.
Zonal features of annual rainfall dis-
tribution Some of the most fundamental
facts about world rainfall distribution may be
presented in the form of a meridional profile
of the precipitation means for the different
parallels. Such a profile is shown in Fig. 9.1 1.
It suggests the existence of a strong primary
maximum of rainfall amounts in the vicinity
of the ITC. Belts of lower rainfall are char-
acteristic of subtropical latitudes, where anti-
cyclonic diverging-wind systems and vertical
subsidence are relatively strong. Poleward
from the subtropics rainfall increases again
so that secondary, or lower, maxima are
indicated for latitudes 40 -50 N and S.
These are the middle-latitude convergences
with their numerous cyclonic storms. Pole-
ward from about 50 -55 precipitation de-
clines sharply; minima of 10 in. and less
characterize the very high latitudes where
low temperatures, low moisture content, and
subsidence are characteristic (Fig. 9.12).
Nonzonal features of annual rainfall
distribution An analysis of Plate 1 and of
Fig. 9.13, which attempts to generalize the
features of precipitation on a hypothetical
continent, reveals that precipitation amounts
Precipitation 185
8
7
6
5
4
3
2
1
2
3
4
5
6
7
8
c
c
0>
/>
b
V)
ro
(/>
/)
to
ft
oJ
o g
|
c c
ilJ
_
ES
c
.S
^
o
c
C
|I|
<^e^
0.*
ii|l
CL (/) Q)
ol o
"c g
'i
=
(0
ght summe
Summer re
Vmter dryn
(O (/)
c
||
n
U)
ra
s
ight winter
Winter ra
jmmer dry
r 8 !
^ S
"
^
t/>
CO
s
90
c
FIG. 9.12 Schematic cross section through the atmosphere along a
meridian, showing the main zones of horizontal convergence and ascent, and
of divergence and subsidence, together with associated precipitation
characteristics: (a) during the Northern Hemisphere summer; (b) during the
Northern Hemisphere winter; (c) zones of precipitation. It must be
emphasized that many nonzonal features of precipitation distribution cannot
be adequately represented on this type of diagram. (Fn>m -SWr/r /v//m.rw,
Introduction to Mefeorohgy, 2d ed., McGraw-Hill Book Company, Inc., New York, 1958.)
vary not only in a latitudinal direction, or
zonally, but also longitudinally. Thus the
typical areas with below-average precipitation
(arid, semiarid, and subhumid) are asym-
metrically developed in the tropics and sub-
tropics, for such areas are concentrated in the
western and central parts of the large land
masses. These are the regions of strong hori-
zontal wind divergence and vertical sub-
sidence associated with the stable eastern
end of the subtropical anticyclones. Cool
ocean currents along the tropical and sub-
tropical west coasts serve to intensify the
aridity.
In the middle latitudes the dry and sub-
humid low-rainfall areas are located toward
186
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
80
60
40
20
20
40
- Precipitation
regions
Humid
Subhumid
QDry
60
FIG. 9.13 The distribution, on a hypothetical
continent, of the four great moisture regions,
based largely upon annual amounts of
precipitation. (From Ttwrnthwaite.)
the centers of the continents, which are
farthest removed from the oceanic sources of
moisture supply.
Abundant rainfall conditions extend across
the entire breadth of the continents in the
low latitudes close to the equator, with
somewhat heavier precipitation characterizing
the eastern and western oceanic margins.
This equatorial zone of abundant rainfall is
less wide in the western part of a continent,
where the subtropical anticyclone is strong,
than in the east, where the anticyclone is
weaker, the coastal waters are warmer, and
the tropical easterly winds are onshore. In
the large middle-latitude continents humid
conditions are to be found both to the east
and the west of the drier continental interiors.
Variations in seasonal distribution
Total precipitation in oceanic areas is not
only greater than over the lands but is also
less seasonal in concentration. Land masses,
with their tendency to strong summer heating,
associated thermal convection, and onshore
summer winds, are likely to have more of
their annual precipitation concentrated in
summer. Over large land masses in middle
and higher latitudes the cold-season anti-
cyclone with its diverging winds also makes
for dry winters.
In the vicinity of the equator where the
ITC prevails at all seasons, rainfall not only
is abundant but also falls throughout the
year; i.e., there is no dry season (Figs. 9.12,
9.14, 9.15). Farther away from the equator,
from about 5 -10 out to 15-20, rainfall
becomes more seasonal as it decreases in
amount, with a marked dry period in the
low-sun season, or winter. The high-sun
period, or summer, is wet (Figs. 9.12, 9.15).
This sequence of high-sun rainfall and low-
sun drought is associated with the latitudinal
shifting of the zones of convergence and
divergence following the sun (Figs. 9.12,
9.15). These latitudes are affected by the
ITC at the time of high sun, but feel the
effects of the subtropical anticyclone and
divergence at the time of low sun. This area
of winter drought does not extend to the
east side of the continent where the sub-
tropical anticyclone is weaker.
In the subtropical, or lower middle, lati-
tudes at about 30 -40 are areas, restricted
to the western side of the continents and
usually of limited extent, where summer is
the season of precipitation deficiency and
winter is wet (Fig. 9.15). Here, because of
latitudinal migration of pressure and wind
systems following the sun, the stable eastern
limb of a subtropical anticyclone controls
the weather in summer, and cyclones associ-
ated with the middle-latitude convergence
zone prevail in winter (Figs. 9.12, 9.15).
JFMAMJJASOND
Precipitation 187
Mogador-3130'N
16.0 in.
NewAntwerp-l36'N
65.1 in.
FIG. 9.14 The change in the features of the
annual rainfall profiles in tropical latitudes, from
the equator to about 30 N, in Africa. The stations
are arranged according to latitude, with
Nouvelle-Anvers (New Antwerp) closest to the
equator.
In the middle latitudes poleward from
about 40 there is usually no dry season,
some precipitation falling at all times of the
year (Fig. 9.15). Yet this is not to say that all
seasons have equal amounts. It is in the
interiors of the great continents that the
seasonal precipitation maximum and mini-
mum are most emphatic and most consistent.
Here summer, with its warmer air of higher
moisture content, is usually the wettest season.
As explained earlier, the drier winter is related
to the lower temperatures and the anticyclonic
wind system of that season.
FIG. 9.15 Seasonal rainfall distribution on <
hypothetical continent. (From Thornlhwaite.)
80
60
40
20
JFMAMJJASOND
20
40
60
In summer)
fawinttr)
INTRODUCTION
The climate in a locality or region is a
generalization of the day-to-day weather pre-
vailing there. And the weather of an area is
closely identified with the air masses which
prevail there and with the atmospheric dis-
turbances, known commonly as storms, which
develop there.
Those areas which experience almost ex-
clusively one type of air mass are likely to
CHAPTER 10
Atmospheric
disturbances;
air masses
and fronts
have relatively uniform weather. Such areas
are the central Sahara in summer and the
upper Amazon River Basin at all times of the
year. But much of the earth's surface is
affected by more than one air mass, and this
causes changeable weather. In some parts of
the earth the change from control by one air
mass to control by another is largely seasonal
in effect, while in others, especially the middle
188
Atmospheric disturbances; air masses and fronts 189
latitudes, rapidly shifting air masses with
striking temperature and humidity contrasts
may produce highly changeable weather even
within a short period of a few days.
The zone of contact between unlike air
masses, where air streams of contrasting
temperature and humidity converge, is called
a front, as stated earlier. These frontal zones
of converging air are the breeding area for
atmospheric disturbances of various kinds.
Consequently it is along air-mass boundaries,
or fronts, that weather changes are concen-
trated and much of the earth's precipitation
is developed.
Thus some understanding of air masses,
fronts, and atmospheric disturbances and the
relationship between them is essential to an
appreciation of the world pattern of climates
the discussion of which follows this
chapter.
AIR MASSES AND FRONTS
ORIGIN, DEVELOPMENT,
AND MOVEMENT
An air mass is defined as an extensive body
of air whose temperature and humidity char-
acteristics are relatively uniform in horizontal
directions. An air mass develops whenever
the atmosphere remains in contact with an
extensive and relatively uniform area of the
earth's surface for a sufficiently long period
for the properties of the air to become similar
to those of the surface. These areas where air
masses develop are called source regions.
The earth's principal source regions occur
where the surface is relatively uniform and
where, in addition, the wind system is a
divergent one. In regions of convergence un-
like temperatures are brought close together,
so that thermal contrasts are great. Anti-
cyclonic circulations, therefore, provide the
most ideal source-region conditions. The
snow-covered arctic plains of Canada and
Siberia in winter, large areas of tropical ocean,
and the hot, arid Sahara in summer are good
examples of source regions.
As a rule air masses do not remain in their
source regions but sooner or later move out
to invade other areas whose weather they in
turn affect. Moreover, the moving air itself is
affected by its new surface environment, so
that it slowly changes in character.
When air streams of unlike temperature
converge, as they do in low-pressure centers,
fronts are usually present. In the tropics,
however, where air temperatures are relatively
uniform, genuine density fronts are infrequent
even in the presence of wind convergence.
When air masses having different temper-
ature and humidity characteristics come
together, they do not mix freely with each
other. They tend, rather, to maintain a fairly
distinct sloping boundary surface between
them, the warmer and therefore less dense air
mass being forced aloft over the wedge of
colder air (Fig. 10.1). This sloping surface is
called either a front or a surface of discontinu-
F I G . 10.1 Three-dimensional representation
of an atmospheric front.
190
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
ity. Where a surface of discontinuity, or front,
in the free atmosphere intersects the earth's
surface, a surface front is formed.
Surface fronts are not lines but rather zones
varying from 3 to 50 miles in breadth, which
usually bring observably marked changes in
temperature and humidity as they pass. In-
deed, the location of fronts and the nature of
the contrasting air masses on either side of
them are of great importance in weather fore-
casting, for along fronts a great many storms
and associated weather changes originate.
It is unusual for a front to remain very
long in a stationary position. Usually one of
the unlike air masses it separates is more
active than the other and advances into the
latter's domain. As a consequence the posi-
tion of the front is shifted. When the warmer
air mass is more aggressive and advances
against the cold air, there is an active upward
movement of the lighter warmer air over the
wedge of colder denser air (Fig. 10.2). This is
a warm front. When the colder air is the
aggressor, it underruns the warmer air and
forces it upward. This is a cold front. How
the two differ in their effects on weather will
be considered in later discussion of middle-
latitude cyclones.
CLASSIFICATION OF
AIR MASSES
Any classification of air masses must be
based primarily upon the characteristics of
their source regions. For this reason air
masses are designated by the name or ab-
breviated name of the source region. The
source regions fall naturally into two great
groups: those of high latitudes, or polar
regions (T 3 ), and those of low latitudes, or
tropical regions (T). It is largely in the high
and the low latitudes that there are large
areas of relatively homogeneous surface con-
ditions and relatively light air movement.
F I G . 1 . 2 (a) A warm front; (b) a cold front.
(U.S. Weather Bureau.)
The middle latitudes are the scene of intense
interaction between the polar and tropical air
masses and generally lack the uniform con-
ditions essential to a source region (Figs.
10.3, 10.4).
The air-mass classification may be further
refined by dividing both the polar (F) and
the tropical (7") groups into continental (c)
and maritime (m) subgroups. This results in
four main types of air masses: polar conti-
nental (c/ 3 ), polar maritime (mF), tropical
continental (cT), and tropical maritime (mT).
The types correspond generally to moisture
and temperature differences. Thus polar air
masses are characteristically colder than
tropical air masses, while maritime air is usu-
ally more humid than continental air.
All of these four main air-mass types may
180 150 120 90 60 30
120 EAST 150 180
Front
- mb Isobar
AIR MASS SYMBOLS T- Tropical P- Polar m- Mantima c - Continental
W - Warmer than, K - Colder than, underlying surface t - Stable aloft u - Unstable aloft
FIG. 10.3 Air masses and fronts in January. (Ftom Haum<itz and Au\Hn.)
F I G . 1 . 4 Air masses and fronts in July. (From Haurwih and Austin.)
180 150 120 90 60 30
30 60 90 120 150 180
ISO 150 WEST 120 90 60 30
AIR MASS SYMBOLS T- Tropical P. Polar m Maritime c- Continental
W Warmer than, K Colder than, underlying turfece. t - Stable aloft u - Unstable aloft
192
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
be modified in important respects as they
move away from their source regions and
travel over parts of the earth's surface which
are unlike their source regions (Figs. 10.3,
10.4). The air mass will become actually or
potentially more buoyant and unstable, and
so inclined to rise and produce rain, (a) when
it is warmed at the base by passing over a
warm land or water surface (Figs. 10.3, 10.4,
symbol k), (b) when it is humidified from
below while passing over a water surface as
a consequence of large-scale evaporation
from the water, or (c) when it is forced to rise
as a result of wind convergence, as in a
cyclonic system (symbol u). Nonbuoyancy
and stability, on the other hand, result when
the air mass is (a) cooled from below as it
travels over a cooler surface (symbol w) or
(b) when it subsides or sinks as a conse-
quence of wind divergence (symbol s), as in
an anticyclone. These principles of air-mass
FIG. 10.5 North American air masses and
their source regions.
modification will be used later in describing
world climates and their temperature and
precipitation characteristics (Fig. 10.5).
ATMOSPHERIC DISTURBANCES
FUNCTION AS MODIFIERS
OF TEMPERATURE AND
GENERATORS OF
PRECIPITATION
An atmospheric disturbance may be thought
of as an extensive wave, eddy, or whirl of air
existing within any of the earth's great wind
systems. As observed on a daily weather map
of the earth through their isobar and wind
patterns, these disturbances are character-
istically so numerous and widespread that
they tend to obscure the lineaments of the
general atmospheric circulation. The circula-
tion is somewhat analagous to a river so full
of minor whirls and eddies that it is difficult
to distinguish the main current.
Atmospheric disturbances are of great im-
portance to weather and climate because
many of them are accompanied by cloud and
precipitation. In fact atmospheric disturbances
cause a great deal of the earth's precipitation
and have important temperature effects as
well. They travel in the general direction of
the wind system in which they exist, so that
in some latitudes they move from west to east
and others from east to west.
TRAVELING CYCLONES
AND ANTICYCLONES
OF MIDDLE LATITUDES
Of principal importance in producing the
frequent, erratic, day-to-day weather changes
characteristic of middle and high latitudes
are the moving cyclones and anticyclones
Atmospheric disturbances; air masses and fronts 193
which fill the westerly wind belts. In these
parts of the world the fickleness of the weather
is proverbial, so it is not surprising that
weather-forecasting services are most neces-
sary and, indeed, best developed here.
No two disturbances are exactly alike, and
storms differ from region to region, so that
the generalizations concerning cyclones and
anticyclones which follow must not be ex-
pected to fit any particular storm in all
respects.
Nature and size Cyclones, the low-
pressure disturbances commonly called lows
or depressions, and anticyclones (highs) are
chiefly characteristic of the regions of air-
mass conflict within the belts of westerly
winds, and consequently are best known in
latitudes 30 -70.
These disturbances are represented on sur-
face weather maps by series of closed con-
centric isobars, roughly circular or oval in
shape (Figs. 10.6, 10.7). In the cyclone the
lowest pressure is at the center, and pressure
increases toward the margins; in the anti-
F I G . 1 . 6 A model cyclone (Northern
Hemisphere) showing arrangement of isobars,
wind system, warm and cold air masses, and
surface fronts.
NW
quadrant
NE
quadrant
--Isobar
SW
quadrant
NW
quadrant
NE
quadrant
SW
quadrant
SE
quadrant
FIG. 10.7
Hemisphere).
A model anticyclone (Northern
cyclone the pressure is highest at the center
and decreases outward. No definite difference
in pressure distinguishes lows from highs;
pressure difference between them is entirely
a relative thing.
Normally there is a pressure difference of
10 to 20 mb, or several tenths of an inch,
between the center and the circumference of
a low. In highs the pressure difference be-
tween center and margins is likewise variable,
but commonly it is somewhat less than in
lows. It is a general rule that both cyclones
and anticyclones are less well developed,
have smaller internal differences in pressure
and weaker pressure gradients, and travel
more slowly in summer than in winter.
There are great variations in the size of
these storms, but on the whole they spread
over huge areas, sometimes as large as one-
third of the United States, or 1 million square
miles, although most of them are smaller.
Diameters of 500 to 1,000 miles are common.
Such storms are extensive rather than
intensive.
Direction and rate of movement Cy-
clones and anticyclones in middle latitudes
194
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
are carried along in a general west- to-east
direction by the system of westerly winds in
which they exist. That is not to say, however,
that storms always move due eastward. To be
sure, they follow different routes, just as they
vary in concentration from region to region.
But in spite of these vagaries in direction
the general eastward progress is maintained.
It is easy to understand, therefore, why a
weather forecaster in the middle latitudes
bases his prediction upon weather conditions
to the west, rather than on those to the east,
of his station. The storms to the east already
have passed; those to the west are
approaching.
Cyclones and anticyclones vary in rate of
movement both with the season and with in-
dividual storms; in general the highs are
somewhat slower than the lows. In the
United States cyclones move eastward across
the country at velocities averaging 20 miles
an hour in summer and 30 miles an hour in
winter; in winter a well-developed low char-
acteristically requires 3 to 5 days for the
transcontinental journey across the United
States. In summer, when the whole atmos-
pheric circulation is slowed down and storm
speeds are reduced, the contrasts between
cyclones and anticyclones are less pro-
nounced. As a consequence, warm-season
weather is less changeable, and atmospheric
disturbances are less vigorous.
Just as temperature, pressure, and wind
belts shift poleward in summer and equator-
ward in winter, following the seasonal move-
ments of the sun's rays, so also do the storm
tracks. This helps to explain why there are
fewer and weaker storms over the lower
middle latitudes in summer than in winter.
Origin It is very likely that middle-
latitude cyclones have more than one type of
origin. Many such storms appear to have
their beginnings as wavelike disturbances
along air-mass fronts, i.e., where winds of
contrasting temperature and density converge.
Consequently, regions of strong horizontal
temperature contrasts and of air-mass con-
vergence favor cyclonic development. Also
most, if not all, fully developed middle-
latitude cyclones appear to extend upward
from the surface and make connection with
an upper-level trough in the high westerlies
and the jet stream they include. This seems
to suggest that cyclone origin is associated
with disturbances on the jet stream, although
the precise nature of the connection is not
clear, as indicated in the earlier discussion of
the jet stream. As stated then, it is certain that
cyclones strengthen underneath the jet, and
their courses appear to be steered by it.
There appear to be at least two quite dif-
ferent types of anticyclones. The rapidly mov-
ing cold anticyclone is essentially a mass of
cold polar air (cP) which originates over a
cold surface in higher latitudes. This anti-
cyclone is the product of heat transfer by
conduction and radiation processes from the
air to the cold surface. Understandably such
anticyclones are chiefly a phenomenon of the
middle and higher latitudes and are most
common and intense over continents in the
winter season. The slowly moving warm
anticyclone is especially characteristic of the
subtropics and poleward margins of the
tropics, and of the lower middle latitudes in
summer (cTand mT). Its origin is not well
understood.
Structure of a model cyclone A cy-
clonic storm frequently begins as a wave or
indentation along a surface front (Fig. 106).
As the frontal wave deepens the storm grows
in size and intensity, and extensive cloud and
precipitation areas develop. The six stages
shown in Fig. 10.8 illustrate the life history
Atmospheric disturbances; air masses and fronts 195
C " :: " v Cold air n .- :1 :; 1L .
Front
Warm air
A
Cold air
Warm air
B
Cold
Warm air
Warm air
Warm air
F I G . 1 . 8 Six stages in the life cycle of a frontal-wave cyclone:
(b) shows the beginning of a small horizontal wave along the front. In (c) the
wave development has progressed to the point where there is a definite
cyclonic circulation with well-developed warm and cold fronts, (d) shows a
narrowed warm sector as the more rapidly advancing cold front approaches the
retreating warm front. In (e) the occlusion process is occurring, the cyclone has
reached its maximum development, and the warm sector is being rapidly
pinched off. In (f) the warm sector has been eliminated; the cyclone is in its
dying stages, and is represented by only a whirl of cold air. (V.S. Weather Butran.
of a cyclone. A prime feature of such a dis-
turbance is the fact that wind, temperature,
cloud, and precipitation are not the same
throughout it.
Normally a cyclone is made up of two
contrasting air masses. To the south and
southeast there is a poleward tongue of
warm, humid, southerly air which originated
in lower latitudes (Fig. 10.9). Enveloping
this warm tongue of tropical air on its
western, northern, and eastern sides are
colder, drier, polar air masses. The zone of
conflict between the tropical and polar air is
the front. Along most parts of this front the
less dense tropical air is ascending over a
mildly inclined wedge of polar air. This lift-
ing of the warm air is the cause of many of
the clouds and much of the precipitation in
the storm. That part of the extended front
lying ahead, or east, of the advancing tongue
of warm air is the warm front, while that to
the rear, or west, of the warm air is the cold
front. In the later stages of the cyclone the
cold front overtakes the warm front, result-
ing in a narrowing, and eventually a pinch-
ing out, of the tongue of warm air at the sur-
196
'////.'/ 'SS '//V/'/S. //'//////,'/////
VERTICAL SECTION OF STORM ALONG AB BELOW
(a)
COLD FRONT WARM FRONT
<0
VERTICAL SECTION OF STORM ALONG CD ABOVE
FIG. 10.9 Structure of a model cyclone:
ground plan (b) and vertical sections (a and c) of a
fully developed wave cyclone in the middle
latitudes of the Northern Hemisphere.
face. The cyclone then becomes a whirl of
cold air and the storm is said to be occluded
(Fig. 10.8).
Wind systems Unlike the violent and
destructive winds of tornadoes and hurri-
canes, the winds in middle-latitude cyclones
and anticyclones are usually only moderate
in speed. Of much greater importance than
the speed, anyway, is the horizontal direc-
tion and vertical nature of the air movement.
Wind system of the cyclone As previously
stated, the cyclone is a converging system of
surface winds with the lowest pressure at the
center, in which, because of the deflective
force of earth rotation, the winds cross the
isobars at an oblique angle, forming an in-
ward spiral which in the Northern Hemis-
phere has a counterclockwise rotation. (Figs.
10.6, 10.10).
To the converging movement of air much
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
of the weather in a cyclone can be attributed.
It is convergence, as previously indicated, that
leaves the air in the cyclone only one way
to escape, through upward movement, and
makes the storm a region of ascending air,
with all of the favorable implications this
has for the development of clouds and
precipitation.
The nature of the ascent itself is of great im-
portance because throughout much of the cy-
clone the upward movement of air is not the
rapid vertical ascent that occurs in a thunder-
storm. The ascent is rather a slower gliding of
warmer air up over a mildly inclined wedge
of colder, denser air. Thus the resulting
clouds are not likely to be the towering
cumulus type; instead extensive sheets of
flattish cloud forming a uniformly gray over-
cast are the rule.
Changes in wind direction with the passage
of a cyclone Because the cyclone's wind
system is a converging one, the winds to the
east, or front, of the storm's center must be
easterly, while to the rear, or west, of the
center the airflow must be from the west.
Easterly winds in middle latitudes, therefore,
often foretell the approach from the west of a
cyclonic storm with its accompanying cloud
and rain; westerly winds, by contrast, indi-
cate the retreat of the storm center and the
coming of clearing weather.
If the cyclone center passes to the north of
a weather station, placing the observer in the
southern parts of the storm, the wind shift
from easterlies on the front to westerlies on
the rear will be accompanied by a southerly
flow of air (Fig. 10.11). Under these condi-
tions weather will be relatively warm, the
cloud and precipitation may be less persistent,
and in winter rain is as likely as snow. But
when the storm center travels south of the
station northerly winds are characteristic,
Atmospheric disturbances; air masses and fronts 197
F I G . 1 . 1 A well-developed wave cyclone over eastern United States.
This weather-map representation should be compared with the ground plan
of the model cyclone in Fig. 10.9.
temperatures are relatively low, the cloud
cover (deck) more lasting, and snow is more
common.
Wind system of the anticyclone The anti-
cyclone's wind system is opposite in all re-
spects to that of the cyclone: pressure is
highest at the center, so surface winds flow
outward, or diverge, from the center, and
there must be a compensating downward, or
subsiding, movement of air to feed the sur-
face flow (Figs. 10.7, 10.12). (Earth rotation
causes this diverging flow to have something
of a clockwise whirl in the Northern Hemis-
phere.) This slow subsidence in an anti-
cyclone is as important as ascent of air is in
a cyclone. Subsidence causes anticyclonic air
to be stable and nonbuoyant, so that above-
surface temperature inversions are developed
and the whole environment becomes opposed
to precipitation.
Precipitation in cyclones and anti-
cyclones As the preceding discussion has
198
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
suggested, cyclones and anticyclones are as
unlike in precipitation as in their wind sys-
tems, cyclones being much more favorable
for it because with their upward movement
of air, cooling by expansion occurs and
cloud and rain result. By contrast, the diver-
gence and subsidence in an anticyclone
operate to make it an area of generally fair
weather. It should be remembered, to be
sure, that not all cyclones are rainy, for the
rising air in them may be too dry to permit
abundant condensation.
Most of the precipitation that falls on low-
lands in middle latitudes owes its origin
either directly or indirectly to cyclonic storms.
Although in the warmer months much of the
rainfall may be showery and convective in
nature, a large part of even this precipitation
is organized in character and occurs in as-
sociation with cyclonic systems.
Since in a cyclone, as previously stated,
warm and less dense air commonly glides up
over a mildly inclined wedge of cooler, denser
air, cooling of the lifted air is not very rapid,
with the consequence that much cyclonic
rainfall is moderate in its rate of fall. But be-
cause the cyclone is extensive, and the slow
FIG. 10.11 Veering and backing wind shifts
as a cyclonic storm approaches and passes an
observer.
N
BACKING WIND SHIFT
VEERING WIND SHIFT
2
lifting of the warm air is thereby widespread,
its precipitation is likely to be of relatively
long duration and to cover a wide area.
Fairly steady precipitation is as typical of
cyclonic weather as the dull, gray, uniformly
overcast skies from which the rain falls.
However, the likelihood of precipitation
and its nature and origin are not the same in
all parts of a cyclone. In general the eastern,
or front, half where convergence is greater is
cloudier and rainier than the western, or rear,
half. In winter lows, snow is more common
in the colder northern and northeastern parts
than in the warmer southern parts.
The most extensive area of general pre-
cipitation is usually found in association with
the above-surface warm front. As Fig. 10.9
shows, here, to the east, northeast, and north
of the storm center, warm southerly air flows
up over a gently inclined wedge of colder air
and widespread cloud and precipitation are
the result. Along the cold front, usually
positioned to the south and southwest of the
storm's center, is a second region of active
air ascent with accompanying precipitation.
Here cold northwesterly air underruns the
warm southerly air and lifts it. But here, be-
cause the sloping surface of the cold air is
steeper than along the warm front, the rise of
the warm air is more rapid. As a consequence
cold-front precipitation is likely to be some-
what more vigorous but of shorter duration
than precipitation along the warm front.
However, where the air to the rear, or west,
of the cold front is from maritime sources, as
it is along west coasts in middle latitudes,
showery rainfall in the cool, moist polar air
may extend for some distance to the rear of
the surface cold front. In the warmer months
thunderstorms may occur along both fronts,
but they are more common and likely to be
stronger along the cold front.
Atmospheric disturbances; air masses and fronts 199
4:30 A. M. PST
5: 30A.M. MST
6:30A.M.CST
7:30 A. M. EST
-1017
o Clear Cloudy
o Partly cloudy F=Fog
R = Ram S=Snow
Arrows fly with the wind
Miami
66'
January 18, 1940
F I G . 1 . 1 2 A well -developed cold anticyclone in winter. This cold
anticyclone moved into the United States from the arctic plains of Canada as
a mass of cold, stable, polar continental (cP) air. St. Joseph, Missouri,
experienced a minimum temperature of 21 below zero, Galveston, Texas,
on the Gulf of Mexico a minimum of 15 above zero. Such southward
thrusts of polar air are essential elements of the moisture-balancing and of
the heat-balancing mechanisms of the earth. (U.S. Weather Bureau.)
During the warmer seasons some showery
convective rainfall may also occur within the
warm sector of southerly airflow to the south
of the cyclone center, where it is not as-
sociated with either a warm or a cold front
(Fig. 10.9).
Temperatures associated with cyclones
and anticyclones Simple rules of wide ap-
plication are difficult to make concerning the
temperature effects of cyclones and anti-
cyclones. Yet some generally applicable state-
ments may be made on these effects.
Since anticyclones develop in both cold
polar air and warm subtropical air, they may
bring either very cold or very hot weather.
Indeed the coldest weather in winter and the
hottest in summer are usually associated with
anticyclones. Also, the anticyclone is char-
acteristically accompanied by clear skies,
which adds to its tendency to produce
seasonal temperature effects. During the long
winter nights in middle latitudes the lack of
clouds permits a rapid loss of heat from the
earth; thus a cold anticyclone at such times
200
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
115 110 105 100 95 90 85 80 75 70 65
FIG. 10.13 Illustrating a warm anticyclone
which is relatively stagnant over southeastern
United States. Such a weather-map condition
produces unseasonably warm weather over the
central and eastern parts of the country.
may cause subzero temperatures (Fig. 10.12).
But the same clear skies in summer, when
days are relatively long, permits of strong
solar heating, so that at this time a warm
anticyclone of subtropical origin is accom-
panied by high daytime temperatures
(Fig. 10.13).
By contrast, the cloud cover which usu-
ally accompanies a cyclone tends to reduce
night cooling in winter and daytime heating
FIG. 10.14 Characteristic arrangement of
isotherms in a winter cyclone over the central and
eastern United States in winter.
in summer, so that less extreme temperatures
prevail. However, since the cyclone is com-
posed of unlike air masses, temperatures may
vary greatly in different parts of an individual
storm. Where northerly winds prevail (cP air)
as they do to the northeast, north, and west
of the cyclone center, relatively low tempera-
tures are to be expected (Fig. 10.14). Where
the air flow is from the south (cT or mT air)
as is commonly the case to the south of the
storm center, relatively high seasonal tem-
peratures are the rule.
MIDDLE-LATITUDE
WEATHER IN GEN ERAL
The very essence of middle-latitude
weather, which to a high degree is under the
control of a procession of eastward-moving
cyclones and anticyclones, is its irregular, or
aperiodic, changeability. The averages of
such weather elements as temperature and pre-
cipitation for a month or a year give a very
atypical and lifeless picture of the climate
unless supplemented by use of the daily
weather map where the various weather
episodes induced by cyclones and anticyclones
are represented. Especially in the colder
months weather variability reaches peak
strength (Figs. 10.15, 10.16, 10.17). Nor is
it absent in summer, although it is weaker
and sun control producing regular diurnal
FIG. 10.15 Barograph and thermograph
traces showing the approach and retreat of a
middle-latitude cyclone.
30.50"
30.00"
12 p.m.
Noon
12 p.m.
Pressure
Temperature-,
--Arrival of cold front
with wind shift
from S.E. to N.W.
Approach of storm | Retreat of storm
Atmospheric disturbances; air masses and fronts 201
THERMOGRAPH
60
50
40
30
20
10
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20
Feb. 6
Feb. 7
Feb. 8
Feb. 9
Feb. 10
Feb. 11
Feb. 12 | Feb. 13
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FIG. 10.16 Barograph and thermograph traces of a week of winter
weather at a middle-latitude station. Note the marked pressure changes
indicating the passage of well -developed cyclones and anticyclones with their
contrasting air masses. The temperature belt (as bounded by the dashed lines)
rises as pressure falls and sinks when the pressure rises. (From Ward.)
change is very important then. It should also
be repeated that the weather conditions of
cyclones and anticyclones described above
are idealized, and individual storms vary
greatly in the weather patterns they produce.
For instance, cyclonic weather is by no
means identical in various parts of the middle
latitudes. Even within the United States,
cyclones produce relatively different weather
patterns along the Pacific Coast from
those they produce over the interior and
eastern parts.
Cyclone tracks and their concentra-
tions All parts of the middle latitudes, as
well as those parts of tropical and high lati-
tudes which border the middle latitudes,
are affected by moving cyclones and anti-
cyclones, but not to the same degree.
Cyclones, for example, cross some extensive
areas more frequently than others, and as a
FIG. 10.17 Barograph and thermograph traces of a week of summer
weather at a middle-latitude station. Note the relatively flat barograph curve
indicating weak cyclonic-anticyclonic control. Regular daily temperature
changes induced by sun control are more conspicuous than those
nonperiodic changes associated with the contrasting air masses of cyclones
and anticyclones. (From Waul.)
THERMOGRAPH
202
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
> Principal tracks of extra-tropical cyclones
> Principal tracks of tropical cyclones
FIG. 10.18 Showing, in greatly simplified form, the principal regions of
cyclonic -storm concentration in both middle and low latitudes.
(From Sverse Fetter ssen, Introduction to Meteorology, 2d ed., McGraw-Hill Book Company,
Inc., New York, 1958.)
consequence weather in the former areas is
more variable.
Figure 10.18 shows in a generalized fashion
the principal cyclonic tracks of the earth. In
observing them, it should be remembered
that the weather effects of such extensive dis-
turbances are felt well beyond the path of
their centers.
The United States and adjacent parts of
Canada have the distinction of being the
world's continental area with the most
cyclonic activity. This may be because North
America east of the Rocky Mountains is a
region of numerous well-developed fronts re-
sulting from the clash of polar and tropical
air masses which move freely across extensive
lowlands from arctic to tropical latitudes. All
parts of the United States east of the Rockies,
except possibly peninsular Florida, have an
abundance of cyclonic weather. But it is in
the northeastern parts of the United States
and adjacent sections of Canada, where there
is a distinct bunching of cyclone tracks, that
cyclonic disturbances are most numerous in
North America.
TROPICAL WEATHER
DISTURBANCES
THE APERIODIC WEATHER ELEMENT
IN THE TROPICS
Not until recently has it been appreciated
how importantly large atmospheric disturb-
ances affect the weather and climate of the
tropics. Earlier it was assumed that the diurnal
and seasonal course of the sun controlled the
weather in the low latitudes to an unusual
degree and so produced the regularity that is
monotonous compared with the variety and
irregularity of weather characteristic of middle
latitudes. But while the effect of weather dis-
turbances is not so marked in the tropics as in
Atmospheric disturbances; air masses and fronts 203
Because they involve air convergence, these
tropical disturbances do produce clouds and
precipitation, most of it of a showery nature
falling from cumulonimbus clouds, and this
is their principal climatic effect. The progress
of most weak tropical disturbances can in-
deed be best detected by the moving areas of
organized cloud and rainfall which they
generate.
SEVERE TROPICAL DISTURBANCES:
THE HURRICANE OR TYPHOON
FIG. 10.19 Weak tropical disturbances of the
summer period in southern Asia. Such
disturbances are responsible for much of summer
precipitation in this region.
the middle latitudes, it is by no means absent.
Unfortunately, however, the variety of dis-
turbances which produce the modest aperiodic
weather changes in the tropics are not yet
satisfactorily understood, and discussion of
them must reflect this fact.
WEAK TROPICAL DISTURBANCES AND
THEIR WEATHER CONSEQUENCES
Most of the extensive tropical disturbances
appear to be relatively mild (Fig. 10.19).
Their organized pressure and wind systems
are frequently difficult to distinguish on the
daily surface-weather map, and some exist
only as above-surface phenomena. Numerous,
very weak disturbances appear to be concen-
trated in the vicinity of the ITC, where they
may move in either an easterly or a westerly
direction, depending on the general direction
of airflow. Others are characteristic of the
deep trade winds, where they move in a gen-
eral east-to-west direction. The temperature
effects of such tropical disturbances are small
and usually a consequence of their produc-
tion of increased cloudiness, which in turn
reduces the amount of incoming solar energy.
In addition to these numerous weak dis-
turbances, which are responsible for much of
the weather and rainfall in the low. latitudes,
there are very likely disturbances of all degrees
of intensity there. Certain it is that there are
a few of a much more violent nature, and
that the most violent of all is the hurricane,
or typhoon (Fig. 10.20). These severe vortex
storms, however, are of much less climatic
significance than the weak disturbances, for
F I G . 1 . 2 A West Indies hurricane, with the
barograph trace of this storm as recorded at
Miami, Florida.
204
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
they are far less frequent and characteristic of
much more restricted areas. Moreover, since
hurricanes originate and mature only over
the sea, the land areas that feel their concen-
trated effects are mostly only coasts and
islands.
Differences from middle-latitude cy-
clones The violent and destructive hurri-
cane resembles a middle-latitude cyclone in
having a central area of low pressure, a con-
verging vortex system of winds, and a rel-
atively widespread area of clouds and rain.
But it also differs in a number of respects.
(a) The isobars of the hurricane are more
symmetrical and more nearly circular, (b)
Pressure gradients are much steeper, so that
winds are stronger. Wind speeds in the storm
must reach at least 75 miles an hour for it to
be a genuine hurricane, (c) Rains are inclined
to be more torrential and somewhat more
evenly distributed about the center, (d) Tem-
perature distribution around the center is
relatively similar in every direction. Cold and
warm sectors and surface fronts are absent.
(e) There are no sharp wind shifts within the
violent parts of the storm. Instead, the winds
develop a perfect spiral whirl, with strong,
vertically ascending currents around the
center, or core. (/) The storms are most
numerous in the warm months of late sum-
mer and fall, rather than in winter, and where
frequent may give rise to a fall maximum of
rainfall, (g) Each has a relatively calm, rain-
less center of descending air 5 to 30 miles in
diameter, called the eye of the storm, (h) This
tropical cyclone has no anticyclone companion.
Although variable in size, the hurricane is
smaller than the cyclone of middle latitudes,
having a diameter of about 100 to 400 miles.
But its winds may reach such destructive
speeds as 90 to 130 miles per hour, resulting
in tremendous damage to shipping and coastal
settlements, and loss of life by drowning is
by no means rare. A considerable part of this
loss of life and property is due to two things:
great avalanches of sea water piled up and
driven onshore by the gale winds, and ex-
cessive rainfall and resulting floods that
accompany the storm.
Origin and concentration There is no
generally accepted theory of the origin of
hurricanes. But it is clear that they develop
only over warm water, probably 82 or
higher. Summer and fall, when they are most
numerous, is the period when the ITC is
farthest from the equator. Many apparently
have their beginnings as weak disturbances
which subsequently mushroom into the in-
tense and dreaded hurricane.
Severe tropical cyclones appear to occur
occasionally over the warmer parts of most
tropical oceans, but not in close proximity to
the equator, and probably nowhere in the
South Atlantic. There are a number of areas
of marked concentration of these storms (Fig.
10.18). These are (a) the China Sea, the
typhoons of which affect particularly the
Philippines, southeastern China, and southern
Japan; (b) the Arabian Sea and the Bay of
Bengal, on either side of peninsular India;
(c) the Caribbean Sea, the hurricanes of which
are felt in the West Indies, Yucatan, and the
southeastern United States; (d) the eastern
North Pacific in the region west of Mexico;
(e) the South Indian Ocean east of Madagas-
car; and (/) the tropical waters both north-
east and northwest of Australia.
THUNDERSTORMS
General characteristics A thunderstorm
is an intense convectional shower accom-
panied by lightning and thunder. In its ma-
ture stage it is characterized by several
Atmospheric disturbances; air masses and fronts 205
15,000 ft ..-vxl^-Jk: \-
Key to diagram of cumulonimbus cloud
A- Anvil top
B-Dark area
C-Roll cloud
C u -Advance cumulus clouds
D-Down drafts
U -Up drafts
R- Primary rain area
R'-Secondary area
W-Wind direction
FIG. 10.21 Vertical section through a thunderstorm and its cumulonimbus cloud.
chimneys of vigorously ascending warm air,
each surrounded by compensating cooler
downdrafts. This strong turbulence is evi-
denced in the seethings and convulsions that
can be observed in the awesome cumulo-
nimbus cloud, or thunderhead (Fig. 10.21).
Since rapid, vertical upthrusts of air com-
monly accompany high surface temperatures,
it is not surprising to find thunderstorms
most numerous in the warmer latitudes of the
earth, in the warmer seasons in the middle
latitudes, and in the warmer hours of the day.
Heat and thunderstorms are closely related.
But heat is not the only requirement for
thunderstorm development. The warm air
must also be relatively rich in water vapor,
for abundant heat of condensation released in
the rising air is the principal source of energy
for the storm. Indeed, the intensity of the
storm depends very largely upon this supply
of latent energy. Without exception the phe-
nomena commonly associated with thunder-
storms torrential local rain, hail, violent
squall winds, lightning, and thunder are
directly related to vigorous convectional over-
turning in warm, humid air.
Most thunderstorms appear to occur in
connection with some weather control which
favors an upward movement of air. This may
be a terrain obstacle or, more commonly, the
general convergence that is present in most
extensive atmospheric disturbances of the
low-pressure variety. Because of the latter,
much thunderstorm activity is organized in
206
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
character, and shifts position with the prog-
ress of the general disturbance of which it is
a part. In middle-latitude cyclones thunder-
storms are commonly concentrated in the
vicinity of fronts, especially vigorous cold
fronts, where upward movement of air is in-
tensified.
Precipitation As indicated earlier, rain-
fall in thunderstorms is likely to be more
vigorous but of shorter duration than rainfall
associated with cyclones. A cumulonimbus
cloud is so small in area that it quickly drifts
by and the rain ceases. One speaks of thunder-
showers rather than thunder rains. This
downpouring nature of the precipitation is
related to (a) the more rapid ascent of air in
thunderstorms than in most cyclones (a rise
of at least 2,400 ft per min must occur), and
(b) the higher temperature and thus higher
specific humidity of the air in the summer
season when thunderstorms are prevalent.
The vigorous and local nature of thunder-
storm rainfall, plus the fact that in middle
latitudes it is concentrated in the growing
season, has important economic consequences,
some beneficial, others not.
Hail Occasionally hail, the most destruc-
tive form of precipitation, is developed in very
intense thunderstorms. Fortunately it occurs
in only a few, and usually falls only in
restricted areas or belts in any particular
storm. When convection is most vigorous
and air is ascending at a rate of 25 to 50
miles an hour or more, raindrops caught in
the upward-surging currents are carried up
into atmospheric regions of below-freezing
temperatures, so that on mixing with snow
they freeze as globules of cloudy ice. These
ice pellets then may fall a long distance from
the high freezing level through a great thick-
ness of subfreezing cloud layers, striking and
capturing supercooled water droplets and
snow crystals on the way and thus growing
into hailstones. When the strong upward-
moving currents are temporarily halted, such
hailstones fall to earth, often doing serious
damage to crops and such structures as
greenhouses, and occasionally even killing
livestock in the fields.
Although hail falls only in conjunction
with thunderstorms, hail and thunderstorms
are not similarly distributed. For example, hail
is practically unknown in the tropics where
thunderstorms are most numerous, and is
rare in the warmer, subtropical parts of the
United States, such as Florida and the Gulf
Coast, where thunderstorms are at a maximum.
Hail and hail damage in the United States
are startlingly local in distribution but occur
most frequently over parts of the Rocky
Mountains and the Great Plains.
Lightning, thunder, and squall winds
Three other common phenomena of thunder-
storms need brief comment: lightning,
thunder, the squall wind, or thunder squall.
Lightning results from the disruption of rain-
drops, with consequent development of static
electricity, in rapidly ascending air currents.
Like hail, therefore, it is largely confined to
the vigorous convectional storms which are
most numerous in the warm season. As rain-
drops in a storm grow larger and larger, their
limit of cohesion is eventually passed, and in
the vigorous updrafts of air they begin to
break up. Then droplets with contrasting
electrical charges become concentrated in
different parts of a cloud. The lightning flash,
usually from one part of a cloud to another
but sometimes from cloud to earth, serves to
equalize these unlike electrical charges. The
thunder is produced by violent expansion of
the air caused by the tremendous heat of the
lightning.
Probably not more than 1 per cent of
Atmospheric disturbances; air masses and fronts 207
lightning flashes reach the earth's surface.
Yet in the United States several hundred per-
sons lose their lives each year as a result of
lightning, and twice as many are injured; and
fire losses due to lightning amount to millions
of dollars annually. Some of the greatest
losses result from the kindling of forest fires.
The thundersquall is the strong, outrush-
ing mass of cool air just in front of a thunder-
storm (Fig. 10.21). Its speed at times attains
hurricane violence; thus it may do serious
damage. The force of the squall is due in
part to the cool air which has been brought
down from aloft with the mass of falling rain
and spreads out in front of the storm, under-
running the warm air.
Tornadoes Very occasionally, severe
thunderstorm activity, of the sort that occurs
in association with well-developed cold fronts
and squall lines, may be accompanied by
scattered tornadoes, the most violent and
destructive of all atmospheric disturbances.
But spectacular and awesome as it is, the
tornado is of minor consequence in world
climates. Its distribution is limited to a few
regions, its occurrence is infrequent, and the
width of its destructive path usually does not
exceed */2 mile.
Distribution of thunderstorms Taking
the average of their occurrence at all longi-
tudes, thunderstorms are found to be most
numerous near the equator and decrease in
frequency poleward (Fig. 10.22). Beyond la-
titudes 60 -70 thunderstorms are few. This
distribution reflects chiefly the general decline
in air temperature from equator to poles
and the associated decline in the humidity
content of the atmosphere. But air temper-
ature is not the only control of their distri-
bution, for while thunderstorm frequency de-
clines sharply between latitudes and 20,
temperatures drop little if at all. Thus the
strong equatorial maximum is also a conse-
quence of the convergent nature of the air-
flow and its deep humidification near the
equator. The marked falling off in thunder-
storm frequency away from the equator re-
flects decreasing humidity and increasing sub-
sidence and horizontal divergence as the
subtropical anticyclones are approached.
Greater frequency of thunderstorms over
land areas than over oceans in similar lati-
tudes is to be expected because of the higher
summer temperatures of the former. Some
equatorial land areas record over 100 days
with thunderstorms during the year; a few
places even experience 200 such days.
In the United States the fewest thunder-
storms are experienced in the Pacific Coast
states, which are dominated by stable anti-
cyclonic air masses in summer (Fig. 10.23).
There are two regions of maximum occur-
rence: (a) the subtropical Southeast, and (b)
the Rocky Mountain area in New Mexico,
Colorado, and Wyoming. The eastern Gulf
Coast region in the United States is the most
thundery area outside of the tropics, having
70 to 80 days per year with thunderstorms.
In this region heat and humidity combine to
produce an environment ideal for the develop-
ment of convective overturning.
FIG. 10.22 Latitudinal distribution of the
average number of days per year with
thunderstorms, averaged for all longitudes.
(From Brooks.)
60 50 40 30 20
NORTH
10 20 30 40 50
SOUTH
208
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
FIG. 10.23 Distribution of the average number of days per year with
thunderstorms.
SELECTED REFERENCES
Blair, Thomas A., and Robert C. Fite: Weather Elements, 4th ed., Prentice-Hall, Inc., Engle-
wood Cliffs, N.J., 1957.
Kendrew, W. G.: Climatology, 2d ed., Oxford University Press, London, 1957.
Kimble, George H. T: Our American Weather, McGraw-Hill Book Company, Inc., New York,
1955.
Koeppe, Clarence E., and George C. DeLong: Weather and Climate, McGraw-Hill Book Com-
pany, Inc., New York, 1958.
Miller, Arthur Austin: Climatology, E. P. Dutton & Co., Inc., New York, 1943.
Petterssen, Sverre: Introduction to Meteorology, 2d ed., McGraw-Hill Book Company, Inc., New
York, 1958.
Riehl, Herbert: Tropical Meteorology, McGraw-Hill Book Company, Inc., New York, 1954.
Taylor, George F.: Elementary Meteorology, Prentice-Hall, Inc., Englewood Cliffs, N. J., 1954.
Trewartha, Glenn T.: An Introduction to Climate, McGraw-Hill Book Company, Inc., New
York, 1954.
Watts, I. E. M.: Equatorial Weather, University of London Press, Ltd., London, 1955.
CHAPTER 11
Classification
,t/
of climates
and their
distribution;
the tropical
humid climates
In the preceding chapters temperature and
precipitation, the two elements which in
combination largely determine any climate,
have been analyzed, and their distributions
have been described. In addition, the effects
of the great controls of climate on temper-
ature and precipitation have been examined.
It has been seen how the climatic controls
209
210
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
determine regional variations in the amount,
intensity, and seasonal distribution of both
temperature and precipitation, resulting in
changeful combinations of these two great
climatic elements. In this manner are created
the great variety of climates, which will now
be described. 1
Because of the exceedingly numerous com-
binations of temperature and precipitation
that exist on the earth, it becomes necessary,
in order to appreciate the general world
pattern of climate, to reduce the great variety
of regional and local climates to a relatively
few large groups and types having important
characteristics in common. This reduction is
essentially accomplished through classifica-
tion, a process common to all sciences.
1 Since ocean currents are a climatic control of at least
modest significance, even if not as important as the controls
already studied, it is suggested that the student acquaint
himself with the world patterns of the oceanic circulation
before proceeding to a study of the climatic types and their
distribution in this chapter and Chaps. 12 through 14. For
this discussion see Chap. 15, pp. 277-291 ff.
CLASSIFICATION: CLIMATIC REGIONS, GROUPS,
AND TYPES
A climatic region is any portion of the
earth's surface over which the broad climatic
characteristics are similar, though not neces-
sarily identical. Similar climatic regions, that
FIG. 11.1 Locations of the five great climatic
groups on a hypothetical continent of relatively low
and uniform elevation.
80
THE
GREAT
CLIMATIC
GROUPS
is, areas with similar climates, are found in
widely separated parts of the earth (Plate 2).
But they are commonly found in correspond-
F I G . 11.2 Locations of the types of climate,
which are subdivisions of the climatic groups, on a
hypothetical continent of relatively low and uniform
elevation.
Classification of climates and their
ing latitudinal and roughly corresponding
continental locations, which suggests that
there is order and system in the origin and
distribution of the climatic elements. The cor-
respondence indeed makes possible the gather-
ing of the numerous climatic regions into a few
principal climatic groups (Fig. 11.1) and
climatic types (Fig. 11.2), each type compris-
ing a number of separate regions.
Plate 2 shows that a recognizable world
pattern of climatic distribution exists, for
there the several climatic regions comprising
a type are seen to fairly repeat each other in
distribution; the tropical humid climates 211
terms of latitudinal and continental locations.
Nor should the existence of the pattern be
surprising: the greatest controls of climate are
in the distribution of solar energy and the
general circulation of the atmosphere, both
with clearly distinguishable world patterns.
In Figs. 11.1 and 11.2 an attempt is made
to show in diagrammatic form the climatic
arrangement as it might appear on a hypo-
thetical continent of relatively low and uni-
form elevation. The continent is designed to
show typical positions and arrangements of
the climatic types and groups divorced from
Classification of Climates*
Groups
A. Tropical humid climates
B. Dry climates
C. Humid mesothermal climates
D. Humid micro-thermal climates
E. Polar climates
//. Highlands
Types
I. Low latitudes (the tropics)
1. Tropical wet (Af, constantly wet)
(Am, monsoon variety)
2. Tropical wet-and-dry (Aw)
3. Low-latitude dry climates
. Low-latitude desert (BWh, arid)
b. Low-latitude steppe (BSh, semiarid)
II. Middle latitudes (intermediate zones)
4. Middle-latitude dry climates
a. Middle-latitude desert (BWk, arid)
b. Middle-latitude steppe (BSk, semiarid)
5. Dry-summer subtropical (Cs)
6. Humid subtropical (Co)
7. Marine (Cb, Cc)
8. Humid continental climates
a. Humid continental, warm summer (Da)
b. Humid continental, cool summer (Db)
9. Subarctic (Dc, Dd)
III. High latitudes (polar caps) or high altitudes
10. Tundra (ET)
11. Icecap (Ef)
Not divided into distinct types
* Temperature and precipitation data for representative stations are presented in
tables accompanying the text for each type of climate.
212
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
peculiarities associated with individual real
continents by reason of their size, shape, and
surface configurations.
In a study of the text materials on groups
and types of climate to follow, continuing
reference should be made to Figs. 11.1 and
11.2 as well as to Plate 2. In addition, Fig.
11.2 and Plate 2 should be compared, be-
cause the resemblances between them are
important.
The plan of climatic classification employed
in this book is a simplified and otherwise
modified version of the well-known Koppen
system, which uses letter symbols to desig-
nate climates. Five great climatic groups are
recognized, and each of these is subdivided
into a relatively few climatic types. In the
book the groups and types are named, and
the corresponding Koppen symbols follow
each name in parenthesis. Since the two clas-
sifications are similar but not identical, the
symbols indicate only comparable climates,
and should not be understood to imply
complete agreement.
The five groups of climate (Fig. 11.1) are
as follows: In the low latitudes near the
equator is a winterless region with adequate
rainfall. This group is called the tropical
humid climates (A). Poleward From this group
and extending far into the middle latitudes are
the dry climates (B). The humid middle lati-
tudes with their seasonal contrasts in temper-
ature are divided into two climatic groups,
one in which the winters are short and mild,
the humid mesothermal climates (C), and the
other in which they are severe and long, the
humid microthermal climates (D). Finally, in
the higher latitudes are the summerless polar
climates (E). 2 The outline of climatic classifi-
cation is presented in more detail in the
preceding table. The tropical humid climates
will be considered first.
THE TROPICAL HUMID CLIMATES:
LOCATION, BOUNDARIES, AND TYPES
The tropical humid climates (A) form a
somewhat interrupted belt 20 to 40 wide
around the earth astride the equator (Fig.
11.1, Plate 2). This belt is distinguished
from all other humid areas of the earth by
the fact that it is constantly warm; in other
words, it lacks a winter. On its poleward
margins the group of tropical humid climates
may be terminated either by diminishing
rainfall or by decreasing temperatures. Usually
it merges with dry climates in the western
and central parts of continents (Fig. 11.1).
On the more humid eastern sides it extends
poleward until a season of cold develops.
(The accepted boundary is where the average
temperature of the coolest month falls below
64.) Highlands, with their low temperatures,
are responsible for the principal interruptions
in the belt of tropical humid climates over
the continents.
Normally the tropical humid climates form
a wider belt and thus extend farther poleward
along the eastern side of a continent than
toward the west side. This reflects the facts
2 The Koppen definitions of the five great climatic groups
are as follows:
A temperature of coolest month over 64.4 (18C)
B potential evaporation exceeds precipitation
C temperature of coldest month between 64.4 (18 C)
and 26.6 (-3C)
D temperature of coldest month under 26.6 ( 3 C);
wannest month over 50 (10C)
E temperature of warmest month under 50 (10C)
The Koppen designations for types of climate will be
noted and explained as the types are defined in the discussion.
Classification of climates and their
(a) that the air is more stable in the eastern
than the western part of an oceanic subtrop-
ical anticyclone, and (b) that the ocean water
is cooler along the western tropical coasts.
Since temperatures are constantly high, the
two principal types of climate within the
tropical humid group are distinguished from
distribution; the tropical humid climates 213
each other on the basis of the annual distri-
bution of precipitation. One type, tropical
wet, has abundant rainfall throughout the
year with no marked dry season. The second
type, tropical wet-and~dry 9 usually has less
total rainfall, and there are a distinctly wet
and a distinctly dry season.
TROPICAL WET CLIMATE (TROPICAL RAINFOREST)
TYPE LOCATION
(a) Uniformly high temperatures and (b)
heavy precipitation distributed throughout
the year, so that there is no marked dry
season, are the two most distinguishing char-
acteristics of the tropical wet (Af) type of
climate. 3 When typically located, it is found
astride the equator and extending out 5 or
10 on either side, but the latitudinal spread
may be increased to 15 or even 25 along
the eastern margin of a continent (Fig. 11.2).
Tropical wet climate is sometimes called
the tropical rainforest climate. This climate
is closely associated with the doldrums,
or intertropical convergence zone (ITC),
where weak rain-generating disturbances are
numerous and the air is unstable (Fig. 11.3).
Characteristically, tropical wet climate is
bounded by the tropical wet-and-dry type on
its poleward side. Along the wetter eastern
margins of continents, however, it usually ex-
tends farther poleward to make contact with
the humid subtropical climate, one of the
humid mesothermal group of climates of
middle latitudes (Fig. 11.2).
Geographical location The Amazon
River Basin in northern South America and
the Congo River Basin and Guinea coast in
central and western Africa are the two largest
contiguous areas with tropical wet climate
3 In the Koppen system / = moist (feucht) throughout the
year: no month with less than 2.4 in. of rain.
(Plate 2). A third extensive but not con-
tiguous area includes much of the East Indies,
the Philippine Islands, and the Malay Penin-
sula in tropical southeastern Asia. There are
smaller areas of tropical wet climate in eastern
Central America, the windward parts of some
islands in the West Indies, western Colombia,
the coastal lowlands and slopes of sections of
eastern Brazil, and eastern Madagascar.
TEMPERATURE
Annual and seasonal temperatures
Lying as areas with the tropical wet type of
climate commonly do, athwart the equator,
and consequently in the belt of maximum
insolation, it is to be expected that tempera-
tures will be uniformly high, and yearly
averages do usually lie between 77 and 80
(following table). Moreover, since the sun's
noon rays are never far from a vertical posi-
tion, and since days and nights vary little in
length from one part of the year to another,
the annual-insolation curve remains not only
FIG. 11.3 Locations of tropical wet (Af),
tropical wet-and-dry (Aw), and dry (B), types of
climate on the zonal profile of sea-level pressure.
I. T. C.
214 FUNDAMENTALS OF PHYSICAL GEOGRAPHY
Climatic Data for Representative Stations with Tropical Wet Climate
M
D
Range
Singapore, Malay Peninsula
Temp.
78.3
79.0
80.2
80.8 81.5 81.1 81.0
80.6
80.4
80.1
79.3
78.6
80.1
3.2
Precip.
8.5
6.1
6.5
6.9 7.2 6.7 6.8
8.5
7.1
8.2
10.0
10.4
92.9
Nouvelle-Anvers, the Congo
Temp.
79.2
80.1
79.2
78.1 79.2 78.4 76.5
76.3
77.0
77.4
77.9
78.1
78.1
3.8
Precip.
4.1
3.5
4.1
5.6 6.2 6.1 6.3
6.3
6.3
6.6
2.6
9.3
67.0
high but also relatively constant, with the
result that there is little seasonal variation in
temperature (Fig. 11.4).
Annual temperature range The an-
nual temperature range, or difference between
the average temperatures of the warmest and
coolest months, is usually less than 5, and
it may be only 1 of 2 on islands and along
coasts. It is not the high monthly- temperature
averages but rather this constant succession
of hot months uniform, monotonous, with
no relief from the heat that characterizes
the tropical wet climate. Thus, the average
July temperatures of many nontropical Amer-
ican cities, such as Charleston, South Caro-
lina, with 82; Galveston, Texas, 83; and
Montgomery, Alabama, 82, may equal, or
even exceed by a few degrees, those of the
hottest months at stations near the equator.
Daily temperatures The daily, or
diurnal, range of temperature (difference be-
tween the warmest and coolest hours of the
day) is usually 10 to 25, several times
greater than the annual range. For example,
at Bolobo, the Congo, the average daily range
is 16, while the annual range is only 2.
During the afternoons the thermometer ordi-
narily rises to temperatures varying from 85
to 93 and at night sinks to 70 or 75. It is
commonly said that night is the winter of the
tropics.
During the day the heat, even though not
extremely high, combines with slight air
movement, intense light, and high relative
humidity to produce an atmospheric condi-
tion with low cooling power. It is oppressive
and sultry; the sensible temperature is very
high.
Even the nights actually give little relief
from the oppressive heat. Rapid nocturnal
cooling is riot to be expected where there are
such excessive humidity and abundant cloudi-
ness. The cooling is usually sufficient, how-
ever, to cause surface condensation in the
near-saturated air, so that radiation fogs and
heavy dew are common.
Daily march of temperature Figure
11.5 shows the daily march of temperature
for the extreme months at a representative
station within tropical wet climate. The graph
illustrates a temperature regime in which sun
is almost completely in control. There is a
Climatic Data for Calicut, India, a Representative Monsoon
Rainforest Station (Am)
J
F M
A
M J
J
A
S
O
N
D Yr
Range
Temp.
77.8
79.8 81.6
83.6
83.1 78.5
76.7
77.4
78.3
79.1
79.5
78.3 79.5
6.9
Precip.
0.3
0.2 0.6
3.2
9.5 35.0
29.8
15.3
8.4
10.3
4.9
1.1 118.6
Classification of climates and their distribution; the tropical humid climates 215
marked diurnal regularity and periodicity
about the changes, temperatures rising to
about the same height each day and falling
to about the same level each night, so that
one 24-hr period almost duplicates every
other. Irregular invasions of cool air, a feature
common in the middle latitudes, are rare.
28
2S
24
PRECIPITATION
Amount Rainfall, as has been said, is
both heavy and distributed throughout the
year, there being no distinctly dry season
(table, p. 214; Fig. 11.4). Indeed, taken as a
whole, tropical wet climate is coincident with
the belt of the world's heaviest precipitation
(Plate 1). Ward estimates the average an-
nual rainfall of the doldrum belt to be in the
neighborhood of 100 in., with less over the
continents and more over the oceans. In this
area close to the equator conditions are ideal
for rain formation. The air is warm, humid
up to great heights, and unstable. Horizontal
wind convergence with a consequent lifting
of the air prevails. Rain-generating atmos-
pheric disturbances are numerous. The result
is an abundance of cumulus cloud and
showery rainfall.
,, .. ,.
JFMAMJJASO N___
FIG. 11.4 Average monthly temperatures and
precipitation amounts for a representative station
with a tropical wet climate (Af). Monthly
temperatures are much more uniform than
monthly amounts of precipitation.
Seasonal distribution Although there
is no genuinely dry season in tropical wet
climate where it is developed in its standard
form, it should not be inferred that the rain-
F I G . 11.5 Daily maximum and minimum temperatures for the warmest
and coolest months at a representative station with tropical wet climate (Af).
Diurnal solar control is almost complete, as shown by the regular daily rise
and fall of temperature.
Temp. Day 5
90
10
25
216
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
-
FIG. 11.6 Average monthly temperatures and
precipitation amounts for a representative tropical
wet station where the monsoon control is strong
(Am).
fall is evenly distributed throughout the year.
Some months and seasons, while far from
being dry, are still less wet than the rainiest
(Fig. 11.4). In the rainier months precipita-
tion falls on a large majority of the days, al-
though there are usually a few days with
none. Fewer rainy days and less rain on each
day are characteristic of the less wet seasons.
Precipitation varies much more throughout
the year, and from one year to the next, than
does temperature.
In some areas with tropical wet climate,
rainfall is not so well distributed throughout
the year, and a short moderately dry season
even exists, usually at the time of low sun,
or winter. Such a modified form of tropical
wet climate is sometimes referred to as a
monsoon subtype (Fig. 11.6). On Plate 2
it is designated by the symbol Am, the m
standing for monsoon.
Nature and origin of the rainfall Most
of the precipitation originates in towering
cumulus clouds of great vertical thickness,
and is in the form of heavy showers of rela-
tively short duration, frequently accompanied
by lightning and thunder. More thunderstorms
occur here than in any other latitudinal belt
of the earth. There is relatively little rainfall
of the steady continuous type falling from a
uniformly gray overcast. Apparently, more
rain comes during the warm afternoon and
early evening hours when solar heating makes
the humid air more unstable and buoyant,
but night rains are by no means infrequent.
To what extent the tropical showers are
the result of random thermal convection as-
sociated with daytime surface heating is con-
troversial. Almost certainly, however, most of
the rainfall is associated with extensive travel-
ing atmospheric disturbances in which
horizontal convergence of air streams creates
an environment favoring vigorous convec-
tional currents. As a consequence the shower
activity is usually organized in belts or areas
which coincide with the moving disturbances,
and so shifts position from one day to another.
Showers so developed may occur at night as
well as day, although usually in a somewhat
weakened condition. Because most of the
shower activity takes place in conjunction
with extensive disturbances, rainfall occur-
rence does not have the daily regularity so
characteristic of temperature. Instead there
are likely to be spells of prevalently showery
days followed by others in which little or no
rain falls. Thus, while the irregular, nonperi-
odic daily weather element is by no means as
strong here as in the middle latitudes, it is
certainly not absent as far as rainfall is
concerned.
Classification of climates and their
RESOURCE POTENTIALITIES
OF THE TROPICAL WET REALM
Although approximately 10 per cent of the
earth's land surface has tropical wet climate,
by no means does this part of the land con-
tain a similar proportion of the earth's popu-
lation. Also the earth's tropical wet areas have
wide variations in population densities. The
New World tropics are far emptier than those
of the Old World.
Tropical wet is the most lavish and prolific
of all climates. There is no dormant season
for plant growth imposed either by a season
of cold or a season of drought, so plants
grow more continuously and rapidly than in
any other climate. Since plants provide the
ultimate food resource for human beings, this
would seem to suggest a potential maximum
food production within the rainforest areas.
An offsetting factor, however, is the handicap
which this climate is believed to impose upon
the health, comfort, and general well-being of
the people who live in it. Numerous tropical
diseases, among them malaria, sleeping sick-
ness, yellow fever, and tropical dysentery,
have been veritable scourges to the inhabitants
of the low latitudes. By some the constant
heat and humidity are considered to be
obstacles to the maintenance of mental and
physical vigor. Nevertheless, there is now a
growing hope that modern hygiene and san-
itation in association with improved medical
care, together with electrical refrigeration
which permits better food preservation, may
greatly reduce the hazards and discomforts
associated with living in a tropical climate.
Aside from the problem of human living
conditions, the low-grade residual soils of
tropical wet climate offer a serious counter-
balance to the bountiful climatic environ-
ment for plants by making difficult the growth
distribution; the tropical humid climates 217
of crops other than the deep-rooted bush and
tree crops. The strong leaching effects of
the abundant and warm rains continuing
throughout the entire year leave the soil de-
ficient in mineral plant foods and in organic
material. A very few years of cropping is
enough to exhaust the topsoil, so that the
native agriculturist is forced to migrate or at
least to shift his fields. On the other hand,
these same soils which are deficient in min-
eral plant foods are coarsely granular in
structure, so that they are friable and easy to
till. This characteristic recommends them to
the native agriculturist who operates with rel-
atively ineffective tools.
No other climate produces such a dense
growth of large trees, so that no other forest
region of the earth provides such a store-
house of wood and lumber, although the ex-
ploitation of this resource is associated with
many obstacles. To the agricultural setder who
is obliged to clear the land, this forest is much
more of a handicap than a resource, of course.
Given the advantages arid the disadvantages
of the tropical wet regions, the value of the
plentiful unoccupied land there for future
settlement is an actively debated question,
although there is more optimism at present
about the future of these areas than at any
previous time. Of the earth's three most ex-
tensive types of land with low or modest
population densities the cold, the dry, and
the humid tropics the last appears to possess
the greatest potentialities for future settlement.
TROPICAL WET-AND-DRY CLIMATE
(SAVANNA)
As previously stated, tropical wet-and-dry
climate (Aw) 4 differs in two principal respects
4 In the Koppen system w = dry season in winter, or low-
sun period: at least 1 month with less than 2.4 in. of rain.
218
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
TYPE Tropical Wet and Dry (Savanna) Aw
PLACE Champoton, Mexico
N D
FIG. 11.7 Average monthly temperatures and
precipitation amounts for a representative station
with a tropical wet- and -dry climate.
from tropical wet climate: (a) it usually has
less total precipitation, and (b) rainfall is un-
evenly distributed throughout the year, there
being a distinctly wet and a distinctly dry
season. These climatic contrasts result in re-
placement of the dense forest cover typical of
areas near the equator by open forest and
tree-studded grassland in the wet-and-dry
climate, which is sometimes called savanna
climate.
TYPE LOCATION AND BOUNDARIES
On the hypothetical continent of Fig. 11.2,
tropical wet-and-dry regions lie on the pole-
ward and interior sides of the tropical wet
climate, and between it and the dry climates.
This arrangement is also shown in Fig. 11.3.
Toward the rainier eastern side of a continent
the wet-and-dry climate commonly makes
contact on its poleward side with the humid
subtropical type of climate of the middle
latitudes.
The typical latitudinal location of the
tropical wet-and-dry climate is from about
5 -10 to 15 -20, and it may extend still
farther poleward on the eastern side of a con-
tinent. This places wet-and-dry in an inter-
mediate position between the ITC and its
unstable air masses on the equatorial side,
and the subtropical anticyclones with their
stable subsiding and diverging air masses on
the poleward side (Fig. 11.3). During the
course of the year, with the north-south shift-
ing of the sun and the pressure and wind
belts, latitudes 5 to 15 are encroached upon
by the wet ITC and its rainbringing disturb-
ances at the time of high sun, and by the
drier parts of the trades and the subtropical
anticyclones at the time of low sun. The re-
sult is rainy summers and dry winters.
Geographical location It becomes
evident from a comparison of Plate 2 and
Fig. 11.2 that most of the extensive areas
with tropical wet-and-dry climate are actually
located on the individual continents in ap-
proximately the positions suggested in the
analyses of type location just preceding. The
llanos of the Orinoco River Valley in
Colombia and Venezuela and the adjacent
parts of the Guiana Highlands in northern
South America, the campos of Brazil south
of the equator in the same continent, the ex-
tensive Sudan north of the Congo River
Basin and the veld south of it in Africa, the
great wet-and-dry area in northern Australia,
and that in tropical southern and southeastern
Asia all are situated approximately as rep-
resented on the hypothetical continent (Fig.
11.2). Most parts of these representative areas
lie between latitudes 5 -10 and 15 -20
and have a tropical wet climate on their
equatorward sides, a dry climate or a humid
subtropical climate on their poleward frontiers.
Classification of climates and their distribution; the tropical humid climates 219
Climatic Data for Representative Stations with Tropical
Wet-and-Dry Climate
J
F
M
A
M I
./ A S
JV
D
Yr
Range
Timbo, Guinea (1040'N)
Temp.
72
76
81
80
77 73
72 72 72
73
72
71
74
10
Precip.
0.0
0.0
1.0
2.4
6.4 9.0
12.4 14.7 10.2
6.7
1.3
0.0
64.1
Cuiaba, Brazil (IS'SCXS)
Temp.
81
81
81
80
78 75
76 78 82
82
82
81
80
7
Precip.
9.8
8.3
8.3
4.0
2.1 0.3
0.2 1.1 2.0
4.5
5.9
8.1
54.6
TEMPERATURE AND PRECIPITATION
The temperature differences between trop-
ical wet-and-dry climate and tropical wet
climate are not great. Constantly high tem-
peratures are still the rule in tropical wet-
and-dry, for the noon sun is never far from
a vertical position, and days and nights
change little in length from one part of the
year to another. In general, yearly temper-
ature ranges are somewhat greater than in
typical rainforest regions but still small, usu-
ally over 5 but seldom exceeding 15 (Fig.
11.7). These slightly larger ranges may result
from the fact that the high-sun months are
often slightly hotter and the low-sun months
are slightly cooler than in regions nearer the
equator (Figs. 11.7, 11.8).
Not infrequently the hottest period occurs
just before the time of highest sun and the
rainy season, which come together. Tropical
wet-and-dry climate, like tropical wet, shows
a remarkable diurnal regularity in its daily
march of temperature (Fig. 11.8), reflecting
the dominance of sun control.
Amount of rainfall Since regional tem-
perature variations are not great within the
wet tropics, rainfall is a more critical element
in setting apart the several climatic types of
the low latitudes. Characteristically, 40 to
FIG. 11.8 Daily maximum and minimum temperatures for the warmest
and coolest months at a station with tropical wet-and-dry climate (Aw). Note
the strong dominance of diurnal solar control and the weakness of any
nonperiodic control by atmospheric disturbances.
32
220
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
60 in. of rain falls a year in regions with
wet-and-dry climate, less than in regions with
tropical wet climate. But since the wet-and-
dry climate usually occupies transitional belts
between the constantly wet and the constantly
dry climates, there naturally is considerable
contrast between the amounts of rainfall on
its equatorward and poleward margins. As a
general rule there is a decrease poleward.
Seasonal rainfall It is not the total
amount of precipitation which chiefly dif-
ferentiates the two climates of the humid
tropics; it is rather the fact that there is much
more seasonal variation of precipitation in the
wet-and-dry than in the wet climates (Fig.
11.7). This contrast between the two types is
principally due to their latitudinal locations.
As previously stated, the tropical wet type is
constantly in or near the ITC where atmos-
pheric disturbances are numerous and there
is a large-scale ascent of warm, humid, un-
stable air masses; the wet-and-dry type, on
the other hand, is on the margins of the ITC
and therefore in an intermediate position be-
tween it and the dry settling air masses of the
subtropical anticyclone and the poleward
margins of the trades.
The seasonal distribution of rainfall in
tropical wet-and-dry climates is a complex
subject in itself, of course, which an example,
the Sudan of Africa north of the equator, will
be used to clarify.
As the sun's vertical rays move northward
from the equator after the spring equinox,
their thermal effects cause pressure and wind
belts to shift in the same direction but a
month or two behind in time. Thus the ITC
with its unstable air masses and heavy rains
gradually shifts northward over the Sudan,
and convectional showers and thunderstorms
begin to occur there in March and April. The
rainfall continues to increase in amount until
July or even August, when the ITC reaches
its maximum northward migration. With the
southward retreat of the ITC following the
sun the rains decline in amount, until by
October or November the dry, subsiding,
stable air masses associated with the subtrop-
ical anticyclone and poleward margins of
trades are prevailing over the Sudan, and
drought grips the land. The lengths of the
wet and dry seasons are variable, depending
upon distance from the equator.
There is no abrupt boundary between the
constantly wet and the wet-and-dry climates,
but a very gradual transition from one to the
other (Fig. 9.14). Thus on the equatorward
margins of the wet-and-dry regions the rainy
season persists for almost the entire year,
while on the poleward margins it is short.
Conversely, on the dry poleward margin the
period of absolute drought may last several
months, while on the rainy margin, where
wet-and-dry climate makes contact with trop-
ical wet climate, there may be no month
absolutely without rain. For emphasis it bears
repeating that the rainy season closely coin-
cides with the period of high sun and the
dominance of converging, unstable air masses,
whereas the dry season is identified with the
period of low sun when stable, diverging,
and subsiding air masses prevail. In short,
rainfall follows the sun.
Seasonal weather During the rainy
season tropical wet-and-dry weather is identi-
cal with that of tropical wet regions. Cloud
of the cumulus type is abundant, and spells
of showery rainfall with frequent thunder-
storms are numerous. In the low-sun, or dry,
season, on the other hand, the weather is like
that of the deserts. The humidity becomes
very low so low that the skin is parched
and cracked. (Yet the dry season is welcomed
after the humid, oppressive heat of the rainy
Classification of climates and their distribution; the tropical humid climates 221
period.) During the dry season the landscape
is parched and brown, the trees lose their
leaves, the rivers become low, the soil cracks,
and all nature appears dormant. Smoke from
grass fires and dust fills the air, so that vis-
ibility is usually low.
Rainfall reliability Rainfall in the trop-
ical wet-and-dry climate, besides being sparser
and more seasonal than in tropical wet climate,
is less reliable: there is wider fluctuation in
the amount from year to year (Fig. 11.9).
One year may bring enough rain to flood the
fields, rot the crops, and increase the ravages
of injurious insects and fungi; the following
year there may be even more severe losses
from drought.
RESOURCE POTENTIALITIES
OF THE TROPICAL
WET-AND-DRY REALM
Tropical wet-and-dry climate characterizes
close to 15 per cent of the earth's land area.
On a map showing the distribution of the
earth's inhabitants, many of the wet-and-dry
areas are conspicuous because of their dearth
of people. This is especially true of the ex-
tensive wet-and-dry regions of the New World
and of Australia. Peninsular India is the most
striking exception, for there human life is
abundant. Portions of the African Sudan and
of the upland wet-and-dry areas of eastern
Africa are intermediate in their population
densities.
Although temperatures are constantly high
in tropical wet-and-dry climate, the fact that
there is a dormant season imposed by drought
makes the productiveness of wet-and-dry
climate considerably less than that of tropical
wet climate. The smaller total amount of pre-
cipitation and its undependability emphasize
this contrast.
FIG. 11.9 Variations In amounts of annual
rainfall over a 20-year period at Nagpur, India, a
station with tropical wet-and-dry climate. Large
annual variations in precipitation are characteristic
of this type of climate.
Reflecting the reduced climatic energy, the
vegetation cover is one of tall, coarse grasses
with scattered trees, and of open forest with
grass, instead of the dense evergreen forest
that characterizes the tropical wet climate.
Much of the woodland is of little value com-
mercially, and the mature natural grasses are
too tall, coarse, and unnutritious to support
an important grazing industry. To the native
agriculturist the tough-grass sod offers a more
formidable obstacle than, does the luxuriant
rainforest.
Little is known about the mature soils of
222
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
the wet-and-dry climate, but generally they
appear to be leached and infertile. There is
some evidence that they are even inferior to
those that develop under the tropical rain-
forest. As in most regions of infertile or dif-
ficult soils, the fresh, young, unleached
alluvial surfaces are the most attractive sites
for cultivation.
UPLAND TROPICAL CLIMATE
In tropical latitudes on several continents,
especially Africa and South America, there
are extensive upland areas, possessed of many
of the normal characteristics of tropical wet
and tropical wet-and-dry climates but also
differing, chiefly in their somewhat lower
temperatures, which are the result of modest
altitude. Some of these uplands, such as those
of eastern Brazil and of eastern Africa, are
among the best-developed tropical areas, for
the lower temperatures make them more at-
tractive to agricultural settlers. These uplands
are included within the general wet-and-dry
type of climate but on Plate 2 are set apart
from the more standard lowland variety by a
light stippling. (Climatic modifications im-
posed by altitude are discussed in Chap. 14.)
CHAPTER 12
The dry
climates
CLASSIFICATION, BOUNDARIES,
AND CHARACTERISTICS
Definition; desert and steppe types A
dry climate (B) may be defined as one in
which the annual water losses by evaporation
from soils and vegetation potentially exceed
the annual water gains by precipitation. In
other words, there is a rainfall deficiency,
and as a result there is no surplus of water to
maintain a constant ground-water supply, so
that permanent streams rarely originate within
dry-climate areas.
Since the amount of water lost through
evaporation increases with temperature and
thus is greater in warm climates than in cold
ones, the amount of annual rainfall dis-
tinguishing between dry and humid climates
must also vary; that is, warm dry climates
can have more annual rainfall than those
which are cool.
Two types of dry climate are commonly
recognized: (a) the arid, or desert, type and
(b) the semiarid, or steppe, type. In general
the steppe is a transitional belt surrounding
the desert and separating it from the humid
climates beyond (Fig. 11.2). The boundary
between arid and semiarid climates is some-
what arbitrary, but in the Koppen system it
223
224
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
is defined as one-half the amount of annual
rainfall separating steppe from humid climates.
Type location Of all the climatic groups
dry climates are the most extensively devel-
oped over the continents, occupying over
one-quarter of the earth's land surface. As
Fig. 11.2 and Plate 2 show, they are to be
found both in the tropics (including the sub-
tropics) and in the middle latitudes.
In the tropics dry climates are concentrated
between about latitudes 15-20 and 30,
which are influenced by the subtropical anti-
cyclones. Here the dry climates character-
istically are shifted away from the eastern
side of a continent toward its western and
central parts, an asymmetry reflecting the fact
that subsidence and atmospheric stability are
greater in the eastern part of an oceanic sub-
tropical anticyclone than along its western
margins.
In the middle latitudes drought conditions
are best developed in the deep interiors of
the great land masses, areas which feel the
effects of a cold anticyclone in winter and
which are also farthest removed from the
oceanic sources of moisture.
TEMPERATURE
Since dry climates exist in a wide range of
latitudes, their average annual temperatures
vary a great deal. Some dry climates are hot,
others cold, still others intermediate in aver-
age annual temperature. On the other hand,
dry climates at any latitude are generally
characterized by seasonal temperatures that
are more severe than the average for the lati-
tude. In other words, summers in dry climates
are likely to be notably warm or hot and
winters notably cool or cold as compared
with the summers and winters of humid
climates in the same latitude. Large annual
ranges of temperature are therefore repre-
sentative. Such seasonal extremes and large
annual ranges are related to the leeward and
interior locations of most dry climates, and
to their prevailingly clear skies and dry
atmosphere.
Even more striking than these annual
ranges are the large daily ranges of temper-
ature in dry climates, where the cloudless
skies and relatively low humidity permit an
abundance of solar energy to reach the earth
by day and allow a rapid loss of earth energy
at night. In deserts large diurnal ranges are
also associated with the meager vegetation
cover, which permits the dry, barren surface
to become intensely heated by day.
Precipitation and humidity Rainfall
in the dry climates is always meager, and in
addition so extremely variable from year to
year that even the low average is not to be
depended upon (Fig. 12.1). Significantly also,
there are more years when rainfall is below
the average than above, for it is the occasional
humid year which tends to lift the average. It
is a general rule, worthy of memorization,
that dependability of precipitation usually
decreases with decreasing amount; thus (a)
meagerness and (b) unreliability of rainfall
seem to go together.
With a few exceptions, relative humidity is
low in the dry climates, 12 to 30 per cent
being usual for midday hours, and conversely,
potential evaporation is characteristically
high. Thus there is little cloudiness, and the
amount of sunshine is great: direct sunlight,
as well as that reflected from the bare, light-
colored earth, is blinding in intensity.
Absolute humidity, on the other hand, is
by no means always low, for desert air in
warm and hot climates usually contains a
considerable quantity of water vapor, even
when the air is far from saturated.
Winds Dry regions are usually windy
places, there being little friction between the
moving air and the low and sparse vegetation
The dry climates 225
FIG. 12.1 Rainfall variability is normally at a maximum in dry and
SUbhumid Climates. (From Biel, Van Roven, and others.)
cover. In this respect dry regions are like the
oceans. The daylight hours, when surface
heating and convective overturning are at a
maximum, are especially windy; nights are
much calmer.
Because of the strong and persistent day-
time winds, desert air is often murky with
fine dust which fills the eyes, nose, and
throat, causing serious discomfort. Much of
this dust is carried by the winds beyond the
desert margins to form the loess deposits of
bordering regions. The heavier, wind-driven
rock particles, traveling close to the surface,
are the principal tool of the wind in sculptur-
ing the land forms of the deserts themselves.
Geographical classification The
already-explained division of dry climates
into desert and steppe types is supplemented
in the classification of climates employed
here by recognition of two great geographical
divisions related to temperature contrasts.
They are (a) the dry climates of the tropical
and subtropical low latitudes, or the hot
steppes and deserts, and (b) the dry climates
of the middle latitudes, or the cold (in winter)
steppes and deserts.
LOW-LATITUDE DRY CLIMATES
TYPE LOCATION
As indicated earlier, low-latitude dry
climates (BWh and BSh) 1 owe their origin
chiefly to the stabilizing effects of the sub-
1 In the Koppen system, W = desert ( Wustc), S = steppe,
and h = hot (heiss): annual temperature over 64.4 (18C).
sidence and horizontal wind divergence as-
sociated with the subtropical anticyclones
(Fig. 11.3). It was also pointed out that since
subsidence and divergence are concentrated
in the eastern and central parts of an oceanic
anticyclonic cell, drought conditions ordi-
226
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
narily do not extend over to the eastern
margins of a continent (Fig. 11.2).
Along west coasts in these tropical and
subtropical latitudes, by contrast, drought
conditions reach down to the sea margins
and even far beyond over the ocean. Here
the drought-producing effects of strong sub-
sidence in the eastern parts of an oceanic
anticyclone are augmented by those of the
cool ocean currents with upwelled water
which characteristically parallel tropical west
coasts. The cold water acts to chill and
stabilize the surface air and to intensify the
aridity, so that along these west coasts dry
climates may be carried 5 to 10 farther
equatorward than normal. Indeed it appears
to be a general rule that while tropical humid
climates extend unusually far poleward along
the eastern sides of the continents (eastern
Brazil, eastern Central America, and eastern
Madagascar), tropical dry climates are carried
equatorward beyond their normal latitudes
along the western littorals (western Peru and
western Angola in southwestern Africa).
PRECIPITATION
Annual amount and dependability
The deserts of low latitudes are the most
nearly rainless regions of the earth. Here the
persistent anticyclonic control consistently
holds at bay the rainbringing disturbances that
attempt to encroach from north, south, and
east. In the steppes that usually surround the
deserts, the same disturbances are present
often enough to add appreciably to the total
rainfall.
Probably most of the tropical desert has an
annual rainfall below 10 in., and extensive
areas receive less than 5 in. In parts of the
Chilean desert no rain may fall for 5 to 10
years in succession. Over the steppe lands
10 to 25 in. of rain is more common.
Actually, averages are of little value in pro-
viding a correct impression of the year-by-year
amount or the seasonal distribution of desert
and steppe rainfall, for it is not only small
but also undependable in amount, and un-
certain in its time of occurrence. As an illus-
tration, no rain fell at Iquique in northern
Chile during a period of four years; then in
the fifth year one shower provided 0.6 in.,
which made the average annual rainfall for
the five-year period 0.12 in.
In the dry tropics, as in the humid tropics,
most of the rainfall is of the showery convec-
tive type which falls from cumulus clouds. It
is chiefly in the poleward parts of the low-
latitude dry climates, which lie closest to the
cyclone tracks of middle latitudes, that wide-
spread general rains falling from a gray over-
cast are likely.
Seasonal distribution In the deserts,
where total annual rainfall is so meager, it is
almost useless to speak of a seasonal distribu-
Climatic Data for Representative Stations in Low-latitude Deserts
J F M A M J J A S N D Yr Range
Jacobabad, Pakistan
Temp. 57 62 75 86 92 98 95 9.2 89 79 68 59 79 41
Precip. 0.3 0.3 0.3 0.2 0.1 0.2 1.0 1.1 0.3 0.0 0.1 0.1 4.0
William Creek, Australia
Temp. 83 83 76 67 59 54 52 56 62 70 77 81 68 31
Precip. 0.5 0.4 0.8 0.4 0.4 0.7 0.3 0.3 0.4 0.3 0.4 0.3 5.2
The dry climates 227
Climatic Data for a Representative Low-latitude Steppe Station with
High-sun Rainfall
F M
D
Kayes, Sudan
Temp. 77 81 89 94 96 91 84 82 82 85 83 77 85
Precip. 0.0 0.0 0.0 0.0 0.6 3.9 8.3 8.3 5.6 1.9 0.3 0.2 29.1
Range
19
Climatic Data for a Representative Low-latitude Steppe Station with
Low-sun Rainfall
,/ F M A M J J A S O N D Yr Range
Benghazi, Libya
Temp. 55 57 63 66 72 75 78 79 78 75 66 59 69 24
Precip. 3.7 1.8 0.7 0.1 0.1 0.0 0.0 0.0 0.1 0.3 2.1 3.1 12.0
tion or to distinguish wetter and drier seasons.
This is less true of the steppe climate, where
annual rainfall is greater. As a rule, those
steppes lying on the poleward margins of a
tropical desert and therefore closest to the
dry-summer subtropical climate 2 (in northern-
most Africa, for example) receive their max-
imum rainfall in winter, or the time of low
sun, when the cyclonic belt has shifted
farthest equatorward (Plates 1, 2). By con-
trast, those steppes situated on the equator-
ward borders of the tropical desert and there-
fore adjacent to tropical wet-and-dry climate,
such as parts of the Sudan, experience most
of their rainfall in summer, or the time of
high sun. This is the season when the ITC
and its disturbances are displaced farthest
poleward (table, p. 227: compare data for
Benghazi and Kayes).
As might be expected cloudiness is meager
and sunshine abundant in dry climates. In
the Sonora desert of northwestern Mexico
and adjacent parts of the United States, about
75 per cent of the possible sunshine is ex-
2 Dry-summer subtropical, or Mediterranean, climate will
be considered in detail in the next chapter.
perienced in winter and 90 per cent in the
other seasons. The blinding glare of direct
and reflected sunlight is a characteristic
feature.
TEMPERATURE
Seasonal temperatures It has been
pointed out earlier that dry climates as a
group are characterized by large seasonal and
diurnal extremes of temperature. In the low-
latitude dry climates specifically, scorching,
dessicating heat prevails during the period of
high sun. Average hot-month temperatures are
usually between 85 and 90, and those of the
winter season between 50 and 60 (Fig. 12.2).
Thus an annual range of 25 to 30 or even
more is to be expected. Such relatively large
seasonal differences are not to be found in
any other tropical climates. They reflect the
greater seasonal extremes of solar energy
here, as well as the clear skies, low humidity,
and sparseness of vegetation.
Daily temperatures The temperature
difference between day and night (diurnal
range) may equal or even exceed that between
winter and summer. Midday readings of 100
228
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
TYPE L ow Latitude Desert (BWh)
IN.
PLACE Aswan, Egypt
F
100
90
80
70
60
50
40
30
20
10
-10
-20
-30
-40
28
oc
*S
9M
t
N
s
X
x
>)A
/
/
s
s
22
20
18
16
14
12
10
8
6
4
2
X
x
x'
v
s
Preci pita
tio
h-t
ial
n
Not a p prec
mi i i
,le
mim
JFMAMJJASOND
FIG. 12.2 Average monthly temperatures for
a low-latitude desert station with an interior
location. Note the relatively large annual range for
the tropics.
to 110 are common in summer, and over
130 has been recorded (Fig. 12.3). At Yuma,
Arizona, the daily maximum exceeded 100
on 80 consecutive days in one summer, except
for 1 day. On summer nights the temperature
may drop to 75 or 80, a welcome relief
after the parching daytime heat, but still rela-
tively warm.
During winter midday temperatures are
pleasantly warm, averaging 65 to 75, while
nights are distinctly chilly, the minimum
temperatures dropping to 45 to 55. Occa-
sionally, light frosts are experienced in the
tropical deserts. Thus, diurnal ranges of 25
to 35 are characteristic.
Daily weather Weather in the tropical
dry climates is dominated by a diurnal regu-
larity, which bespeaks solar control (Fig.
12.3). For any locality one day is markedly
like another, with temperature rising to about
the same level during the period of afternoon
FIG. 12.3 Daily maximum and minimum temperatures for the warmest
and coolest months at a station with low-latitude desert climate (BWh). The
station, at Yuma, Arizona, is located in the subtropics rather than in the real
tropics, so that while diurnal sun control is dominant, there is some
evidence of nonperiodic cyclonic, or air-mass, control also.
Temp.
Day 5
10
15
20
25
The dry climates 229
heat and falling to nearly the same minimum
at night. Some nonperiodic variety in weather
is added in the more marginal parts, where
disturbances bring spells of showery or rainy
weather on the low-latitude frontier in sum-
mer and the poleward frontier in winter. The
poleward margins likewise experience some
nondiurnal temperature variations derived
from passing middle-latitude fronts accom-
panied by invasions of polar air.
COOL MARINE DRY CLIMATES
The previously mentioned extension of
drought conditions to tropical west coasts
produces an unusual subtype of desert along
several tropical coasts paralleled by cool
ocean currents a desert with cool marine dry
climate (Bn). 3 The locations of these deserts
are shown on Fig. 12.4. The normal features
of low-latitude dry climates hot summers,
large annual and daily temperature ranges,
low humidity, and meager cloudiness are
strikingly modified in these locations.
Temperature Along these cool- water
desert coasts average summer-month temper-
atures are 10 to 20 below those of interior
locations. For example, the hottest month at
Callao on the Peruvian coast has an average
temperature of only 71 (as does July at
Madison, Wisconsin); Port Nolloth in south-
western Africa has 60 (as does July at
Archangel, U.S.S.R.). The result is daily and
annual temperature ranges which are only a
third to a half as great as at interior locations.
Observation of the table of climatic data for
Lima, Peru (p. 330), and comparison of Figs.
12.2 and 12.3 with Figs. 12.5 and 12.6 will
illustrate these contrasts.
Precipitation and fog Rainfall along
these cool tropical desert coasts is exceedingly
3 In the Koppen system n = frequent fog (Ntbet).
FIG. 12.4 Distribution of coo! marine dry
climates (Bn) in the tropics. Here fog is prevalent.
Characteristically this subtype of low-latitude dry
climates is located along coasts paralleled by cool
ocean currents.
low, even lower than in much of the interior
desert, because of the combined effects of two
drought makers, (a) the stable eastern end of
an oceanic subtropical anticyclone, and (b)
the cool coastal water. For a distance of
nearly 2,000 miles along the desert coasts of
FIG. 12.5 Average monthly temperatures for
a marine desert station located on a tropical west
coast paralleled by a cool ocean current.
Temperatures are abnormally low and the annual
range is very small. Compare with Fig. 12.2.
TYPE Cbo/ Marine Dry C/imate (Bn)
IN.
PLACE PORT NOLLOTH, U. So. AFRICA
F
100
90
80
70
60
50
40
30
20
10
-10
28
f)(L
24
22
20
18
16
14
12
10
8
6
4
2
B
=
*
^
^
w
. -
\ +
M
I
-20
-30
-40
J F M A
M J
J A
S
N D
230
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
Temp. Day 5
10
15
32
FIG. 12.6 Daily maximum and minimum temperatures for a
low-latitude desert station located on a tropical west coast paralleled by a
cool ocean current. Note the abnormally low average temperatures and the
small daily ranges. Compare with Fig. 12.3.
Peru and northern Chile, annual rainfall is
held to about 1 in.
But although precipitation is meager, fog
and low stratus clouds are abundant, so that
the persistent sunshine so common to most
deserts is greatly reduced, especially in winter.
At Swakopmund in southwestern Africa fog
is recorded on 150 days of the year. Much of
the surface fog is attributable to the chilling
effects of the cool water upon air moving
landward from the sea. The gray overcast
aloft is associated with the strong temperature
inversion produced by subsidence in the anti-
cyclone. At times a drizzle falls from the
stratus cloud.
Climatic Data for a Representative Desert Station on a Cool-water Coast
J F M A M J J A S O N D Yr Range
Lima, Peru
Temp. 71 73 73 70 66 62 61 61 61 62 66 70 66 12
Precip. 0.0 0.0 0.0 0.0 0.0 0.2 0.3 0.5 0.5 0.1 0.0 0.0 1.6
MIDDLE-LATITUDE DRY CLIMATES
TYPE LOCATION
Deserts and steppes in the middle latitudes
(B Wk and BSk) 4 are usually found in the deep
4 In the Koppen system k = cold (kalt): average annual
temperature below 64.4 (18C).
interiors of the great continents far from the
oceans, which are the principal sources of the
atmosphere's water vapor (Fig. 11.2; Plate 2).
Significantly also, the driest regions usually
have a basin configuration. Thus the aridity of
some interior parts of the two great Northern
The dry climates 231
Hemisphere continents is intensified by their
being largely surrounded by highland barriers
that block the entrance of humid maritime air
masses. Indeed, where high mountains closely
parallel a coast, as in western North America,
arid climates approach relatively close to the
sea. The winter thermal anticyclone likewise
acts to reduce the precipitation of the colder
months.
Although tropical dry climates character-
istically extend down to the ocean margins
on the leeward (western) side of a continent,
the leeward side of a continent in the mid-
dle-latitude westerlies (here the eastern side)
may be far from dry, as in eastern North
America and Eurasia, for example. This
shifting of middle-latitude dry climates inland
from the east coast is associated with (a) the
presence along the eastern side of a large con-
tinent in the westerlies of air masses which
are humid and not too stable, and (b) rain-
generating cyclonic storms. Owing to an un-
usual combination of circumstances, dry
climates do reach the east coast in Argentine
Patagonia, but this is the exception. There
the land mass is so narrow that all of it lies
in the rain shadow of the Andes, where
descending currents make for drought con-
ditions.
TEMPERATURE
The interior location on large continents
of most middle-latitude dry climates assures
their having relatively severe seasonal tem-
peratures and consequently large annual
ranges. However, because they have such a
wide latitudinal spread (15 or 20 in both
North America and Asia), it is difficult to
speak of typical temperature conditions, for
the conditions are very different on their
equatorward and poleward margins. Yet for
any given latitude temperatures are severe.
TYPE Middle Latitude Sieppe (BSk)
PLACE Laram/e, Wyoming
F
100
90
80
70
60
50
40
30
20
10
-10
-20
30
FMAMJJASOND
FIG. 12.7 Average monthly temperatures and
precipitation amounts for a representative station
having a middle-latitude steppe climate.
Summers are inclined to be warm or even hot,
and winters are correspondingly cold (Figs.
12.7, 12.8; table, p. 232). Diurnal ranges
are large for the same reasons noted previously
for tropical steppes and deserts (Figs. 12.3,
12.8). Patagonia in Argentina is again some-
what the exception. There the narrow land
mass and the cool waters offshore result in
temperatures that are more marine than con-
tinental, so that summers are unusually cool
and winters relatively mild.
PRECIPITATION
Probably no parts of middle-latitude deserts
are so rainless as the most arid tropical des-
erts, and some precipitation, in all likelihood,
falls every year. Unlike the dry climates of
low latitudes, these of middle latitudes receive
a part of their total precipitation in the form
of snow, although the amount is small and
232 FUNDAMENTALS OF PHYSICAL GEOGRAPHY
Climatic Data for Representative Stations in Middle-latitude Steppes
J
F
M
A
M J J A
S
o
N
D
Yr
Range
Williston, North Dakota
Temp.
6
8
22
43
53 63 69 67
56
44
27
14
39.2
62.7
Precip.
0.5
0.4
0.9
1.1
2.1 3.2 1.7 1.7
1.0
0.7
0.6
0.5
14.4
Quetta, Pakistan (5,500 ft)
Temp.
40
41
51
60
67 74 78 75
67
56
47
42
58.1
38.2
Precip.
2.1
2.1
1.8
1.1
0.3 0.2 0.5 0.6
0.1
0.1
0.3
0.8
10.0
Urga, Mongolia (3,800 ft)
Temp.
-16
-4
13
34
48 58 63 59
48
30
8
-17
28
79
Precip.
0.0
0.1
0.0
0.0
0.3 1.7 2.6 2.1
0.5
0.1
0.1
0.1
7.6
the snow cover is of variable duration, de-
pending chiefly on latitude.
Again, it is not easy to generalize about
the seasonal distribution of precipitation. Cer-
tainly much the larger parts of the middle-
latitude dry climates, located as they are in
the deep interior of a large land area, have the
characteristic continental strong summer max-
imum and strong winter minimum (Fig. 12.7;
table, p. 232, data for Williston). However,
along western subtropical margins where the
subtropical anticyclone strengthens in sum-
F 1 G . 12.8 Dally maximum and minimum temperatures for the warmest
and coolest months at a station with middle-latitude steppe climate. Note
the relatively stronger nonperiodic air-mass control, especially in January.
25
30
32
20
10
SALT LAKE CITY
The dry climates 233
Climatic Data for Representative Stations in Middle-latitude Deserts
Temp.
Precip.
Temp.
Precip.
Temp.
Precip.
M
M
S
JV D
Yr
0.6 0.5 0.5 0.4 0.6 0.3 0.1 0.2 0.3 0.4 0.3 0.6 4.8
Range
Santa Cruz, Argentina
59 58 55 48 41 35 35 38 44 49 53 56 47.5
0.6 0.4 0.3 0.6 0.6 0.5 0.7 0.4 0.2 0.4 0.5 0.9 6.1
Turfan, Smkiang, China (-56 ft)
13 27 46 66 75 85 90 85 74 56 33 18 56
No data
Fallon, Nevada (3,965 ft)
31 36 41 50 56 65 74 72 61 51 40 32 50.6
24
77
mer, a reversed situation, with a winter
maximum, may prevail (table, p. 232, data
for Quetta).
As in the dry tropics, precipitation amounts
vary so greatly from year to year in middle-
latitude dry climates that the average is not
to be depended upon. This unreliability of
precipitation is most serious in the semiarid
steppe lands, which are marginal between
agricultural and grazing economies. During a
series of relatively humid years crop yields
may be bountiful, but drought years with
associated crop failures and lean incomes are
bound to follow, so that agriculture is a pre-
carious occupation (Fig. 12.9).
The weather element Although sun
control, with its diurnal regularity of weather,
remains strong (Fig. 12.8), it is by no means
as dominant as in the dry tropics. Traveling
cyclones and anticyclones are fairly numerous,
and these disturbances induce nonperiodic
episodes of alternating cold and warmth, rain
and fair weather.
RESOURCE POTENTIALITIES OF THE DRY REALM
Dry climates, as previously stated, are char-
acteristic of over one-quarter of the earth's
land surface more land than any other
climatic group occupies. It is unfortunate that
such an unproductive climate should be so
extensively distributed because for the most
part dry lands are coincident with great blank
spaces on the world-population map, like
parts of the wet tropics and nearly all of the
cold polar and subarctic lands. Indeed, these
three climates the dry, the cold, and the
constantly hot offer the greatest obstacles to
a large-scale redistribution of population on
the earth.
Owing to the insufficiency and extreme
year-to-year variability of the rainfall in dry
climates, it appears that a large part of the
earth's land surface is doomed to remain
relatively unproductive. Recent successes
attained in artificially producing rainfall
by cloud-seeding methods and in converting
sea water into fresh water have created a
heightened optimism concerning expanded
utilization of the dry lands. Yet no large-
234
FIG. 12.9 Wide fluctuations during a period of
5 years in the location of the boundary separating
dry from humid climates in the interior United
States east of the Rocky Mountains. (From Kendall.)
scale increase in dry-land agriculture seems
likely to result from these methods of creat-
ing new water sources within the near future.
It would appear, then, that the expansion
of settlement in lands with dry climates will
be associated with (a) an increased use of
irrigation methods and (b) the further devel-
opment and greater use of drought- resistant
plants and their cultivation by dry-farming
methods. But again, it is hard to be optimistic
about the promise that either of these methods
holds for opening up extensive areas of dry
land for future agricultural settlement.
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
The niggardly climate is responsible for a
sparse vegetation cover which has relatively
low resource value. Some deserts are almost
barren wastes, practically devoid of plants
having economic value. Other deserts have a
thin mantle of widely spaced woody shrubs
with some short desert bunch grass, but even
the grazing value of this vegetation is very
low. Over the desert area of the southwestern
United States, for instance, more than 75
acres are required to supply natural forage for
one steer. In the semiarid regions, short,
shallow-rooted, widely spaced grasses prevail,
and this steppe vegetation has a considerably
higher grazing value than has the desert
shrub; i.e., it is capable of supporting more
livestock per unit area. This grass, the great-
est natural asset of the steppes, forms the
basis for a grazing industry, but grazing is an
economy which is able to support only a small
population.
Soils are of little consequence in deserts
largely because the deficient rainfall makes it
impossible to use them for agricultural pur-
poses. Soils of the tropical steppes appear to
be inferior to those of the steppes in middle
latitudes, where the very modest amount of
leaching and the humus (organic material)
derived from the root mat of the grasses make
for dark fertile soils of high resource value.
Unfortunately this admirable soil resource of
middle-latitude semiarid lands cannot be
exploited to anything like its capacity because
of the precipitation handicap. Thus, although
the humid margins of some middle-latitude
steppes have been brought under the plow, it
appears that the meager and unreliable rain-
fall will go on tending to keep the larger part
of the world's steppe lands out of cultivation
and in natural grasses in the future. It is the
old story of fruitful soils and prolific climates
seldom being areally coincident.
CHAPTER 13
Humid
mesothermal
climates
GENERAL CHARACTER AND TYPE LOCATIONS
In the tropics, where temperatures are
constantly high, climates are distinguished on
the basis of rainfall contrasts and seasons are
designated as wet or dry. But in the middle
latitudes temperature becomes coequal with,
in places even superior to, rainfall as an aid
in differentiating climates (excepting the mid-
die-latitude dry climates, of course), and the
dominant seasons are designated as winter
and summer. Here the dormant season for
plant growth is usually the season of low
temperature, not of drought. Thus it is that
in the humid middle latitudes there are two
great groups of climate which are distin-
guished from each other on the basis of
temperature: the milder humid mesothermal
(C) which will be considered in this chapter,
and the more severe humid micro thermal (D).
Since winters in mesothermal climates must
be relatively mild, the three types compris-
235
236
FIG. 13.1 Locations on a hypothetical
continent of the three mesothermal (C) types of
climate.
ing this group are found only where severe
and long-continued winter cold is precluded.
They are, as a consequence, restricted to two
locations: (a) the equatorward margins of the
middle-latitude continents where the latitude
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
ensures winter mildness, and (b) marine loca-
tions farther poleward on the windward or
western side of a continent (Figs. 11.1, 13.1).
Two of the mesothermal climates, dry-summer
subtropical and humid subtropical, occupy the
first of the type locations mentioned above,
while the third, marine climate, is typically
found in the second (Fig. 11.2).
Throughout the discussion of middle-
latitude climates to follow it should be borne
in mind that in the middle latitudes the
changeableness of the weather is a striking
characteristic. It should be remembered that
as the natural region of conflict for contrast-
ing air masses expelled from the tropical and
polar source regions, the middle latitudes are
/ones of horizontal air convergence, and that
as a consequence, cyclonic storms and fronts,
with their accompanying weather changes,
are numerous.
DRY-SUMMER SUBTROPICAL CLIMATE
(MEDITERRANEAN)
GENERAL FEATURES
For land areas as a whole it is common for
summer to have more rainfall than winter,
even where winter is far from being dry: it is
a general rule that rainfall follows the sun.
Consequently, where seasonal rainfall distri-
bution is reversed, and winter is not only
wet but summer is also dry, a most unusual
climatic condition is present. Actually, a
unique climatic condition is present, for
dry-summer subtropical, or Mediterranean,
climate (Cs) 1 has the distinction of being the
earth's only type of humid climate in which
the seasons of heat and drought coincide.
This climate with its bright and sunny
weather, blue skies, relatively few rainy days,
1 In the Koppen system s = dry season in summer.
and mild winters, and its usual association
with abundant flowers and fruit has quite
deservedly acquired a glamorous reputation.
The distinctive features of the type are
marked, and not easily forgettable, and they
are duplicated with notable similarity in the
five regions where this climate occurs, viz.,
the borderlands of the Mediterranean Sea,
southern California, central Chile, the south-
western tip of Africa, and parts of southern
Australia.
TYPE LOCATION
Mediterranean climate characteristically is
located on the tropical margins of the middle
latitudes (30 -40) along the western side of
a continent (Fig. 11.2; Plate 2). Situated thus
on the poleward slopes of the subtropical
high, it is intermediate in location between
the dry subsiding air masses of the sub-
tropical anticyclone and the rainbringing
fronts and cyclones of the westerlies. With
the north-south shifting of wind belts during
the course of the year, these Mediterranean
latitudes are joined to the dry tropics at one
season and to the humid middle latitudes at
the other season. Tropical constancy there-
fore characterizes them in summer, middle-
latitude changeability in winter. Emphatically,
this Mediterranean, or dry-summer subtrop-
ical, type is a transitional climate between the
low-latitude steppe and desert, and the cool,
humid, marine climate farther poleward.
In addition the Mediterranean climate,
confined as it usually is to the western side of a
continent roughly between latitudes 30 and
40, in summer feels the especially strong
subsidence characteristic of the eastern limb
of an oceanic anticyclone.
In both central Chile and California,
mountains terminate the type abruptly on the
land side. The farthest poleward extent of
southern Africa and southwestern Australia
carries them barely to Mediterranean latitudes,
so that on these continents the dry-summer
subtropical climate occupies southern and
southwestern extremities rather than distinctly
west-coast location. Only in the region of the
Mediterranean Sea Basin, which is an im-
Humid mtsothermal climates 237
portant route of winter cyclones, does this
type of climate extend far inland; there it
prevails inland for 2,000 miles or more, this
most extensive development being responsible
for the climate's regional name.
The interior and the eastern margin of a
continent, where the summer anticyclone is
relatively weak and where there is a tendency
toward a monsoon wind system, are not con-
ducive to the development of Mediterranean
climate, especially its characteristic seasonal
rainfall regime.
TEMPERATURE
Winter temperatures It is for its char-
acteristically mild, bright winters with their
pleasant living temperatures that Mediter-
ranean climate is deservedly famed. People of
the colder, higher latitudes seek it out for
comfortable winter living. Absence of severe
winter cold is to be expected here because of
both the subtropical location and the prox-
imity to the sea on the windward western
side of the continent. Usually the winter
months have average temperatures between
45 and 55, with coastal locations somewhat
milder than those inland (Figs. 13.2, 13.3).
During midday hours the temperature com-
monly rises to 60 or even higher; at night
it may drop to 40 or 45 and at times even
Climatic Data for Representative Dry-summer Subtropical Stations
D Yr
Range
Red Bluff, California (interior)
Temp.
45
50
54
59
67
75 82 80 73
64
54
46
62.3
37
Precip.
4.6
3.9
3.2
1.7
1.1
0.5 0.0 0.1 0.8
1.3
2.9
4.3
24.4
Santa Monica, California (coast)
Temp.
53
53
55
58
60
63 66 66 65
62
58
55
59.5
13
Precip.
3.5
3.0
2.9
0.5
0.5
0.0 0.0 0.0 0.1
0.6
1.4
2.3
14.8
Perth, Australia (coast)
Temp.
74
74
71
67
61
57 55 56 58
61
66
71
64
19
Precip.
0.3
0.5
0.7
1.6
4.9
6.9 6.5 5.7 3.3
2.1
0.8
0.6
33.9
238
TYPE Dry Summer Subtropical (Mediterranean) CSa
IN - PLACE Athens. Greece
MAMJJASOND
FIG. 13.2 Average monthly temperatures and
precipitation amounts at an interior station north of
the equator having a Mediterranean climate with a
hot summer (C.va).
reach freezing (Fig. 13.4). Alternating spells
of cooler and of warmer weather result from
the advection of air from higher and from
lower latitudes which is brought by the
moderately frequent cyclonic storms of winter.
Frost It is not because of its frequency and
severity that frost (which was discussed gen-
erally in Chap. 7) is so much dreaded in
these regions of sensitive fruit and vegetable
crops, but rather because of its infrequency.
It is this infrequency that tempts the farmers
to take a chance on the frost hazard, with the
result that on occasion large losses are
sustained. The growing season is ordinarily
not quite the whole year, for frosts do oc-
casionally occur during the 3 winter months;
but they usually occur on only a few nights,
and rarely are they severe freezes (Fig. 13.4).
Never does the thermometer remain below
freezing during the daytime hours. For ex-
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
ample, during a period of 41 years at Los
Angeles, California, near the Pacific Coast,
there were 28 in which no killing frost
occurred and the growing season was 12
months long, whereas at Sacramento, away
from the coast, the temperature usually drops
below 32 on several winter nights each year.
Such frosts as do occur in the Mediter-
ranean climate are generally the result of an
invasion of cool cP air followed by further
surface night cooling. Especially in Mediter-
ranean climates, the freezing temperatures are
confined to a shallow layer of cold surface
air which tends to collect in low places. Thus
preparations for frost are not extensive but
sensitive crops are characteristically planted
on slopes. Also, oil heaters occasionally
are lighted in citrus groves in order to pre-
vent frost damage.
FIG. 13.3 Average monthly temperatures and
precipitation amounts at a coastal station south of
the equator having a Mediterranean climate with a
cool summer (Csb).
TYPE Dry Summer Subtropical (Mediterranean) Csb
IN - PLACE Sant/ago.Chile
JFMAMJJASOND
Humid mesothermal climates 239
Temp.
Day
10
15
40
32
RED BLUFF (Csa)
FIG. 13.4 Temperature conditions at an interior dry-summer
subtropical station (Csa). Note the hot summer and the large diurnal range
of temperature. Solar control is dominant in summer, but irregular,
nonperiodic air-mass control is conspicuous in winter.
Summer temperatures On the basis of
varying intensity of the summer heat as de-
termined by proximity to the coast and cool
water, two subdivisions of Mediterranean
climate are recognized, the hot-summer sub-
type (Csa) and the cool-summer subtype
(Csb). 2 Along coasts, particularly those
bordered by cool ocean currents, average
summer-month temperatures are low for the
latitude, usually between 65 and 70 (Fig.
13.3). Elsewhere summer months are warm
to hot, commonly 75 to 80 (Fig. 13.2). In
the cool marine location daytime temperatures
may reach only into the 70s and the diurnal
range is small. Night fog and low stratus
clouds are frequent. By contrast midday
summer temperatures in the hotter interiors
commonly reach 85 to 90, sometimes even
2 On Plate 2 the Koppen designations Csa and Csb are
used; a = temperature of warmest month above 71.6
(22 C), and b temperature of wannest month below 71.6.
100 (Fig. 13.4). The dry summer heat of
interior stations in California, for instance,
greatly resembles that of a tropical steppe or
desert.
PRECIPITATION
Amount As a general rule Mediter-
ranean climate errs on the side of having too
little rather than too much rainfall. Much of
the area where this climate prevails just
escapes being semiarid, and the normal 15 to
25 in. of annual precipitation in this climate
justifies its being designated as subhumid
rather than genuinely humid. It is usually
bordered by steppe climate along its equator-
ward margin, so that it is driest along this
frontier and rainfall amounts increase pole-
ward. Thus San Diego in southernmost
California receives only 10 in. of rain; Los
Angeles, less than 100 miles farther north,
15 in.; and San Francisco, about 250 miles
240
FIG. 13.5 The precipitation of dry-summer
subtropical climate, concentrated in the cooler
months, originates chiefly in cyclonic storms.
still farther north, 20 in. With increasing
distance from the influence of the subtropical
anticyclone, rainfall mounts. The general
deficit in water and the variation in rainfall
amounts from year to year are reflected in the
large-scale use of irrigation water. Most of
the modest rainfall on lowlands originates in
cyclonic storms or along fronts, disturbances
which are characteristic features of the
middle-latitude westerly winds (Fig. 13.5).
Seasonal distribution To an unusual
degree the year's rainfall is concentrated in
the cooler half of the year (Figs. 13.2, 13.3).
Winter characteristically is the rainiest of the
4 seasons, while summer is desertlike in char-
acter. Thus at Los Angeles over three-quarters
of the year's rain comes during the 4 months
from December to March and only 2 per cent
from June to September. The rainfall regime,
therefore, is that of the deserts in summer
and that of the cyclonic westerlies in winter
when rain is relatively abundant.
This seasonal alternation of drought and
rainfall is, as previously indicated, a conse-
quence of the north-south migration of the
planetary winds and rainfall belts following
the course of the sun. A poleward shifting of
the sun in summer brings these subtropical
latitudes along west coasts under the influ-
ence of the stable eastern margin of a sub-
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
tropical anticyclone. Subsiding and diverging
air, a near absence of atmospheric disturb-
ance, and cool ocean water along the coast
all conjoin to produce aridity (Fig. 13.6). In
winter, by contrast, when the sun and the
wind and rainfall belts have reached their
southernmost limits, these same latitudes
largely escape the effects of the anticyclone,
and instead are encroached upon by the
westerlies with their cyclonic storms and
frontal systems (Fig. 13.5).
Seasonal weather Although winter is
the rainy season in dry-summer subtropical
climate, it is by no means so dismal and
gloomy as it is farther poleward along the
coast in the marine climate. More often than
not these subtropical latitudes lie on the
FIG. 13.6 The drought of summer in
dry-summer subtropical climate is the result of
stable air which originates in the eastern end of a
subtropical oceanic anticyclone.
HIGH
1020
1017
equatorward margins of most of the passing
cyclonic storms, so that sunshine is abundant
even in winter, and long-continued periods
of cloud and rain are not characteristic.
Nevertheless, since cyclones are most frequent
in winter, it is to be expected that nonperiodic
weather changes in both precipitation and
temperature will be at a maximum during
that season. The periodic diurnal rise and
fall of temperature following the daily course
of the sun is modified by the irregular impor-
tation by the cyclonically induced winds
from south and north of alternating spells
of warmer and cooler weather (Fig. 13.4).
Short periods of cloudy, rainy weather sepa-
rate others that are bright and sunny. Even
in winter, however, it is common for these
subtropical latitudes to experience 50 to 70
per cent of the possible sunshine.
Summer weather is less fickle and change-
able than that of winter because cyclonic
control is largely lacking. With the sub-
tropical anticyclone dominating and sun con-
trol imposing a diurnal regularity upon the
weather, one day is much like another and
clear cloudless skies and bright sunny weather
are the rule (Fig. 13.4). In interior California,
for instance, midsummer months have over
90 per cent of the possible sunshine.
Snowfall In all Mediterranean regions
snow is so rare on the lowlands that it is
something of an event when it does fall. Over
the lowlands of central and southern Cali-
fornia, for example, annual snowfall averages
less than I in., and it is absent along the
coast from San Luis Obispo southward. On
adjacent highlands in Mediterranean regions,
however, a moderate to heavy snow cover
may be present, and meltwater from it pro-
vides an invaluable source of irrigation for
the nearby drier lowlands.
Humid mesothermal climates 241
RESOURCE
POTENTIALITIES OF THE
MEDITERRANEAN REALM
This, the most restricted of all the principal
climatic types, embracing less than 2 per cent
of the earth's land surface, is nevertheless
one of the most unusual and alluring. Its
abundance of sunshine, fruit, and flowers; its
mild and relatively bright, sunny winter
weather offering resort and outdoor-sport
attractions; and its blue skies and even
bluer waters create for the Mediterranean
climate a reputation and renown far out of
proportion to its small area. The Mediter-
ranean realm is attractive as a winter play-
ground for peoples from more severe climates
especially, for here snow, ice, and frigid tem-
peratures are left behind, and much out-of-
doors living can be enjoyed throughout even
the coolest months. The proximity of the sea
is a further, year-round attraction. Certainly,
in providing amenities conducive to pleasant
living, climate looms large among the realm's
resources.
In the dry-summer subtropical climate is
found a unique combination of elements, some
of which make for high agricultural productive
capacity, and some of which have the opposite
effect. Particularly the temperature elements
long, warm-to-hot summers with abundant
sunshine; mild, bright winters; and an almost-
year-round frost-free season underlie the
high agricultural potentialities of this climate.
The Mediterranean realm and the humid
subtropical realm, which will be studied
next, approach the bountiful temperature
regime of the tropics more nearly than any
other part of the middle latitudes. This close
approach to tropical temperature conditions,
242
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
combined with their lying within the middle
latitudes and profiting by proximity to the
markets of these latitudes, gives to these two
realms a considerable part of their distinctive
character. In them is permitted the develop-
ment of certain heat-loving or frost-sensitive
crops, some of a luxury type citrus, figs,
viniferous grapes, rice, sugar cane, cotton
which can be grown in few other parts of the
middle latitudes. The subtropical climates
likewise allow the production of out-of-season
vegetables and flowers for the markets of
regions farther poleward, where a season of
severe cold imposes a long dormant period.
The possibility, within the Mediterranean
realm specifically, of utilizing the middle-
latitude winter as an active growing season
offers unusual agricultural opportunities.
On the other hand, (a) the relatively meager
total precipitation and (b) the long summer
drought place definite climatic limitations
upon production within the Mediterranean
realm. This total water deficiency and its
heightening in the warm months make
summer, in spite of its abundant heat, a
naturally dormant season. The relatively
meager total precipitation and the arid
summers also tend to place limitations upon
the kinds of crops grown, causing emphasis
on drought-resistant perennials, such as the
olive and the vine, and on those annuals
which mature quickly, such as barley and
wheat. The large-scale development of irriga-
tion within the Mediterranean realm is evi-
dence of the people's attempt to overcome
the handicap of the dormant summer and
provide a year-round growth for crops, and
to take away the limitations on kinds of crops.
It is fortunate, since there is a season of
drought, that it occurs in summer, i.e., that
the usual 15 to 25 in. of rain is concentrated
in the cooler months of the year when evapo-
ration is at a minimum. If the same modest
amount fell during the hot summer, when
evaporation is excessive, much less of it
would be effective for plant growth and the
climate would be semiarid.
The modest precipitation and the summer
drought produce a vegetation cover char-
acterized by woody shrubs and widely spaced,
stunted trees; in some Mediterranean regions
scattered patches of desert bunch grass are
also present. This plant cover is of some
value for grazing, particularly of sheep and
goats. But only on the higher hill lands and
the mountain slopes are the forests of gen-
uine commercial value, the stunted trees and
the bushes of the valleys and the lower slopes
being useful chiefly as checks to erosion.
Because of the widespread occurrence of
hill land in this realm, mature residual soils are
not extensively developed. On the slopes
soils are inclined to be thin and stony, and
to a considerable extent they remain uncul-
tivated. It is the young alluvial soils of the
valleys which are the attractive sites for
cultivation. .
\
HUMID SUBTROPICAL CLIMATE
Humid subtropical climate (Ca), 3 the other
subtropical climate in the mesothermal
3 In the Koppen system a = temperature of the warmest
month over 71. 6 (22C).
group, differs from dry-summer subtropical
climate in three principal ways: (a) it is char-
acteristically located on the eastern rather
than on the western side of a continent, (b) it
has more total precipitation, and (c) its pre-
cipitation is concentrated in the warmer
months, although in most areas winter is by
no means dry.
TYPE LOCATION
As previously stated, the two subtropical
climates are similar in latitudinal location,
both lying on the tropical margins of the
middle latitudes roughly between 30 and
40, a fact which in itself would make for
climatic likeness (Fig. 13.1; Plate 2). It is the
dissimilar location of the humid subtropical
climate on the eastern side of a land mass
(Fig. 11.2) which fosters climatic difference.
Thus the subtropical east-side location favors
more rainfall than occurs on the subtropical
west coast, for the east side of a continent in
these latitudes feels the effects of the weaker
western end of a subtropical oceanic anticy-
clone, where the air varies from being mildly
stable to actually unstable. In addition, along
the subtropical eastern side of a large conti-
nent, where the anticyclone is relatively weak,
there is a tendency for humid, onshore mon-
soon winds to prevail in summer. The associ-
ated advection of tropical heat and humidity
in turn favors summer precipitation.
These subtropical east coasts are paralleled
by warm ocean currents originating in tropi-
cal latitudes, whose general effect is to stimu-
late the rain-making processes. This contrasts
with the cool currents and their stabilizing
effects along west coasts.
Although both subtropical types of climate
lie on the tropical margins of the middle
latitudes, they are flanked by quite unlike cli-
mates to north and south. Thus, while the
dry-summer subtropical characteristically
passes over into dry climate on its equator-
ward side, humid subtropical is bounded by
tropical humid climates on that frontier. Sim-
Humid mesothermal climates 243
ilarly, dry-summer subtropical usually merges
into mild, rainy marine climate on its pole-
ward side, while, in Asia and North America
at least, humid subtropical makes contact
with severe continental climate on the north.
TEMPERATURE
Summer temperatures In many of its
temperature characteristics humid subtropical
climate resembles its west-side counterpart.
Both are mild climates, lacking long and
severe winter cold, and characterized by a
long growing season without killing frost.
Still there are significant contrasts.
Completely lacking in the humid sub-
tropics are the cool-summer coastal climates
which are so characteristic of the subtropical
west sides where cool ocean currents prevail.
Along the subtropical east sides, coast as well
as interior, the summer heat is like that of the
humid tropics, with the average temperature
of the warmest month reaching 75 to 80
(Fig. 13.7). This rather resembles the situa-
tion at inland stations in Mediterranean cli-
mates. But there is a difference, for in the
humid subtropics the air is so moist that the
heat is sultry and oppressive, while dry
desertlike heat is more the rule in warm
Mediterranean climates. Because of the
humid atmosphere with more cloud, night
cooling is usually less marked in the humid
subtropics, so that the diurnal range of tem-
perature is somewhat less than at interior
locations in its west-side counterpart (com-
pare Figs. 13.4 and 13.8).
Winter temperatures In these subtrop-
ical latitudes winters must be mild, but there
are important regional variations (Fig. 13.7).
Thus in subtropical eastern Asia the large-
ness of the continent causes average January
temperatures of 40 and even lower, while in
Argentina and Australia the coldest month
244
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
TYPE Humid Subtropical (Caf)
IN - PLACE Tokyo, Japan
30
28
26
24
22
20
18
16
14
12
. .
J J A S N D
FIG. 13.7 Compare with Fig. 13.2. Seasonal
rainfall distribution contrasts strikingly in the two
subtropical types.
may be 10 warmer. The annual range of
temperature is only moderate, but again it
varies from region to region: only 23 at
Buenos Aires, Argentina, 32 at Montgomery,
Alabama, and over 40 at Shanghai, China.
The larger the land mass, the colder the
winters and the larger the annual ranges.
Midday temperatures in winter are likely
to be pleasantly warm (50 to 60) and on
many more than half the winter nights the
thermometer remains above freezing (Fig.
13.8). But the passage of cyclonic storms
with their accompanying southerly and
northerly winds results in irregular spells of
warmer and colder weather, the latter often
very uncomfortable because of the inefficient
heating systems characteristic of many homes.
Frost It is to be expected that the grow-
ing season, or the period between killing
frosts, will be long, usually from 7 or 8
FIG. 13.8 Daily maximum and minimum temperatures for the warmest
and coolest months at a humid subtropical station. Note the strong periodic
(solar) control in summer. By contrast, winter shows stronger nonperiodic
(air-mass) control.
Humid mesothermal climates 245
Climatic Data for Representative Humid Subtropical Stations
./ F M A M J J A S O N D Yr Range
Charleston, South Carolina
Temp.
50
52
58
65
73 79 82 81
77
68
58
51
66.1
32
Precip.
3.0
3.1
3.3
2.4
3.3 5.1 6.2 6.5
5.2
3.7
2.5
3.2
47.5
'
Shanghai, China
Temp.
38
39
46
56
66 73 80 80
73
63
52
42
49
42
Precip.
2.8
2.0
3.9
4.4
3.3 6.6 7.4 4.7
3.9
3.7
1.7
1.3
45.7
Rosario, Argentina
Temp.
77
76
70
62
56 49 51 52
57
62
69
75
63
28
Precip.
3.7
3.2
5.3
3.1
1.8 1.5 1.0 1.5
1.6
3.5
3.4
5.3
34.9
months up to the entire year. In the Asiatic
and North American humid subtropics, which
are bordered on the north by harsh conti-
nental climates, winters are more severe and
killing frosts much more frequent than in
their Southern Hemisphere counterparts
where broad continents are lacking to pole-
ward. In the southeastern United States pro-
tective east-west highland barriers are lack-
ing, so that strong thrusts of cold northerly
air sweep unhindered from the Arctic to the
Gulf of Mexico, resulting in occasional spells
of severe cold. Temperatures as low as 10
have been recorded along the ocean margins
of all of the northern Gulf states (Fig. 13.9).
PRECIPITATION
Amount The total annual rainfall is
more abundant (30 to 60 in.) in the humid
subtropics than in the dry-summer subtropics.
This contrast is to be expected, given the fact
that the subtropical east side is affected by
the weaker western part of a subtropical anti-
cyclone and a warm ocean current parallels
the coast, while cold water and the stable
east side of a high-pressure cell dominate the
subtropical west side.
To be sure, there are variations in rainfall
amounts within the humid subtropics. As a
rule rainfall decreases inland, so that the
driest parts are usually along the western
margins where humid subtropical climate
makes contact with steppe climate (Fig. 11.2).
Nature and origin Much of the summer
precipitation is of the local showery type as-
sociated with convective overturning and fall-
ing from cumulus clouds. Strong heating of
the invading maritime air over the warm land
surface operates to make the air unstable and
FIG. 13.9 Weather controls giving rise to a
spell of severe subfreezing night temperatures in
the American humid subtropics. A cold anticyclone
advancing southward from arctic Canada as a
mass of cold c/*air produced a minimum
temperature of 20 at New Orleans and 8 at
Memphis. The isotherm of 20 approximately
coincides with the south Atlantic and Gulf Coasts.
246
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
buoyant, so that it is ripe for precipitation.
Thunderstorms are numerous, the south
Atlantic and Gulf margins of the United
States having an unusually large number.
However, most of the showery rain appears
to occur in conjunction with extensive, though
weak, atmospheric disturbances. As a result
the showers and thunderstorms are organized
in character, occurring in belts or areas, and
the shower areas move with the disturbance
that generates them. Random convective
showers are less common, and in spite of the
prevailing heat all days are not showery.
Winter precipitation, by contrast, is more
general and widespread in character, and
convective showers are infrequent. In this
season the land surface, being colder than
any maritime tropical air moving landward,
chills this air at its base and makes it stable
and nonbuoyant. It is not inclined to rise un-
F I G . 1 3 . 1 A common weather type of the
summer season in the humid subtropics of Asia.
Weak cyclonic storms are responsible for a consid-
erable part of the summer precipitation. The area
of precipitation has a stippled shading. (From
Japanese weather map.)
HIGH
less forced upward by terrain barriers or by
colder, denser air along a front. Most of the
winter precipitation occurs in association
with well-developed cyclonic storms and falls
from a dull, gray, uniform cloud deck. Snow
occurs now and then when a vigorous winter
cyclone takes a course which carries it well
equatorward, but it rarely stays on the ground
for more than a few days.
Seasonal distribution Over most of the
humid subtropics precipitation occurs
throughout the year and there is no season of
genuine drought, in contrast to the situation
in the dry-summer subtropics along the west
side of a continent (Fig. 13.7). As a rule
more rain falls in the warmer parts of the
year when the air contains the most moisture
and the warm land increases the instability of
the surface air. Yet winter is far from being a
dry season in most areas. It is chiefly in parts
of the humid subtropics of Asia, where the
dry winter monsoon is best developed, that
winter may be a genuinely dry season.
Seasonal weather Irregular, non-
periodic weather changes are usually less
marked in the humid subtropics than they
are farther poleward, where the conflict be-
tween air masses is more marked and cyclonic
storms more numerous. In summer when the
storm belt is farthest poleward, irregular
weather changes are at a minimum (Fig. 13.8).
The sun largely controls the weather and so
a diurnal regularity of temperature is a char-
acteristic feature; humid, sultry days are the
rule. Frequent spells of showery weather ac-
companying weak disturbances alternate with
short periods of several days in which no
rain falls (Fig. 13.10). To an unusual degree
the weather resembles that of the wet tropics.
Late summer and fall are the dreaded hur-
ricane season, and, although these storms are
not numerous, their severity more than makes
up for their infrequency. Sunny autumn days
furnish delightful weather, although the
equatorward-advancing cyclonic belt pro-
duces a gradually increasing number of gray,
cloudy days and begins to import unseason-
able temperatures as winter approaches.
In winter the belt of cyclonic storms is
farthest equatorward, so that irregular weather
changes are more frequent and extreme. The
arrival of tropical air masses may push the
day temperatures to well above 60 or even
70, only to be followed by northerly winds
of polar origin which may reduce the tem-
perature as much as 30 within 24 hr, result-
ing occasionally in severe freezes. Bright,
sunny winter days are distinctly pleasant and
exhilarating out of doors. Spring again sees
the retreat of the cyclonic belt and the grad-
ual reestablishment of regular, diurnal sun
control (Fig. 13.8).
RESOURCE POTENTIALITIES
OFTHE HUMID SUBTROPICS
Without doubt the humid subtropical is
the most productive climate of the middle
latitudes. Temperature and rainfall here
combine to produce the closest approach to
humid tropical conditions outside the low
latitudes. The bountiful temperature regime
of the Mediterranean realm, which was dis-
cussed earlier in this chapter, is, as indicated
then, closely duplicated in this other sub-
tropical climate. But the more abundant pre-
cipitation of the humid subtropics, in con-
junction with the lack of a genuinely dry
season, makes this realm potentially more
productive climatically than its subhumid
counterpart. To be sure, its sultry tropical
Humid mesothermal climates 247
summers are far from being ideal for human
comfort, but they are nonetheless excellent
for a luxuriant plant growth.
The abundant climatic energy, expressed
in rainfall as well as temperature, induces an
equally abundant vegetation, usually of
forests, although in regions of more modest
precipitation grasses may replace trees.
Grasses are particularly prevalent in the
westernmost parts of the American humid
subtropics, in the Argentine Pampa, and in
parts of Uruguay. The character of the forests
varies so greatly among the humid subtropical
regions that generalizations are difficult to
make. Trees grow more rapidly in the humid
subtropics than they do in other climates of
the middle latitudes, so that natural or arti-
ficial reforestation is a quicker process than it
is farther poleward.
The mature forest soils of the humid sub-
tropics are characteristically of low fertility,
which tends to offset seriously the effects of
the realm's climate; and it seems like a geo-
graphic imperfection of first magnitude that
the most productive climate of the middle
latitudes should be associated with such in-
fertile mature soils. The soil inferiority is not
surprising, however, considering the high
leaching power of the climate and the low
humus-producing character of the forest veg-
etation. The red and yellow soils of the
humid subtropics generally resemble those of
the wet tropics, although the humid subtropical
soils are not so completely leached. Under
cultivation they deteriorate rapidly.
Where grasses replace forests, as they do
in the subhumid portions of the subtropics,
the soils are darker in color and much more
productive. The lower rainfall results in less
leaching, and the grasses provide a greater
abundance of organic matter.
248
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
MARINE CLIMATE
TYPE LOCATION
Unlike the other two mesothermal climates,
the marine, or maritime, type (Cb) 4 owes its
mildness not to subtropical location but to its
position on the windward, or western, side of
a continent in the westerly winds (Fig. 13.1). It
has a position which is poleward of the two
subtropical types, extending from about 40 to
60 or even beyond and thus actually reach-
ing relatively high latitudes. But even these
latitudes can be temperate where winds from
the sea rather consistently carry onshore the
generally mild oceanic conditions and add to
them the effects of the warm ocean currents
which characteristically parallel the western
margins of middle-latitude continents. An ex-
tensive development of this marine type on
the eastern, or leeward, side of a large con-
tinent in middle latitudes is, of course, im-
possible. Even Japan, an island group, is
4 In the Koppen system b = temperature of warmest
month below 7 1.6 (22 C).
continental rather than marine in its temper-
ature characteristics.
Where mountains closely parallel west
coasts, as in western North America, Scandi-
navia, and South America, the marine climate
does not extend far inland. But where exten-
sive lowlands prevail, as in western Europe,
the oceanic effects penetrate much deeper
(Plate 2).
On its equatorward margins marine climate
normally makes contact with the dry-summer
subtropical type. On its poleward side it
passes over into the subarctic or the tundra
type (Fig. 11.2, Plate 2).
TEMPERATURE
Since during much the larger part of the
year marine climate has its temperatures
brought to it from the ocean by the westerly
winds, it is not surprising to find that large
seasonal extremes of temperature are absent
(Fig. 13.11). Summers are likely to be rela-
Climatic Data for Representative Marine Stations
./
f
M
A
M J J A
S
N
D
Yr
Rangr
Valentia, Ireland
Temp.
44
44
45
48
52 57 59 59
57
52
48
45
50.8
15
Precip.
5.5
5.2
4.5
3.7
3.2 3.2 3.8 4.8 4.1
5.6
5.5
6.6
55.7
Seattle, Washington
Temp.
40
42
45
50
55 60 64 64
59
52
46
42
51.4
24
Precip.
4.9
3.8
3.1
2.4
1.8 1.3 0.6 0.7
1.7
2.8
4.8
5.5
33.4
Paris, France
Temp.
37
39
43
51
56 62 66 64
59
51
43
37
50.5
29
Precip.
1.5
1.2
1.6
1.7
2.1 2.3 2.2 2.2
2.0
2.3
1.8
1.7
22.6
Hokitika, New Zealand
Temp.
60
61
59
55
50 47 45 46
50
53
55
58
53
16
Precip.
9.8
7.3
9.7
9.2
9.8 9.7 9.0 9.4
9.2
11.8
10.6
10.6
116.1
Humid mesothermal climates 249
lively cool and winters mild, considering the
latitude, and the annual range is small.
Summer temperatures In having a
cool summer the marine climate resembles the
oceanic littorals of the Mediterranean realm,
but is quite in contrast to most parts of the
two subtropical climates with their greater
summer heat. Average summer-month tem-
peratures of about 60 to 65 are 5 or 10
lower than are those of the continental inter-
ior in similar latitudes. Only occasionally are
midday temperatures uncomfortably warm
(Fig. 13.12).
Winter temperatures Winter isotherms
show a strong tendency to parallel the coast
line, with temperatures decreasing inland from
the sea evidence that the land-water control
is stronger than latitude especially in winter.
Thus, compared with stations at the same
latitudes but in the interior of the continent,
the winter mildness is much more striking
than the summer coolness (Fig. 13.12). For
example, while Seattle is only 5 cooler than
Montreal in July, it is 27 warmer in January.
Frost Freezing temperatures are more fre-
quent and more severe than in the dry-summer
TYPE Marine (Cb)
IN - PLACE Dub/in. Ire/and
30
28
JASOND
FIG. 13.11 Temperature and rainfall char-
acteristics of a lowland marine station in western
Europe. Note the small range of temperature
and the modest amount of precipitation well
distributed throughout the year.
subtropical climate farther south (Fig 13.12).
Still, the growing season is long considering
the latitude, 6 to 8 months being character-
F I G . 13.12 Daily maximum and minimum temperatures for the warmest
and coldest months at a marine station on the Pacific Coast of Canada. Note
the small diurnal range, especially in winter when skies are prevailingly cloudy.
32
VICTORIA, B. C
250
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
istic of the American North Pacific Coast
region. At Paris, France, frost normally occurs
only on about half the nights during the 3
winter months. However, winter usually is
severe enough to produce a dormant season
for plant life. During occasional cold spells
the temperatures may remain constantly be-
low freezing for several days in succession.
Such cold spells occur when there is a west-
ward and southward thrust of polar continen-
tal air from the interior of the continent (Fig.
13.13).
While irregular temperature variations are
by no means as striking in these marine
climates as they are in the continental inte-
riors, still the passage of cyclones and anti-
cyclones, with the resulting changes in wind
direction, is bound to cause some degree of
weather variability (Fig. 13.12).
FIG. 13.13 Weather controls favoring unsea-
sonably low winter temperatures in western Europe.
A cold anticyclone to the north and east is deliver-
ing cold cP air to the regions west and south of
its Center. (From Kcndrew.)
PRECIPITATION
Annual amount Marine climates are
humid, and there is usually adequate precipi-
tation in all seasons (Fig. 13.11). However,
depending upon the amount of relief, the
total varies greatly from region to region.
Where lowlands prevail, as they do in western
Europe, rainfall is only moderate, usually 25
to 35 in. per year, but the humid marine
climate extends well inland; where highlands
are present, as in Chile and western North
America, there may be heavy precipitation on
the windward slopes, but eastward from the
mountains dry climates prevail.
Snow Snow is more common than in the
subtropical climates of the humid mesother-
mal group, but on lowlands it ordinarily lies
on the ground for only 10 to 15 days during
the year. On highlands, however, snowfall is
heavy, and a deep snow cover persists for
several months. Such mountain snowfields
have in the past created numerous valley
glaciers, the erosion effects of which have been
responsible for the characteristically irregular
fiorded coasts of Norway, southern Chile, and
British Columbia.
Annual distribution Sufficient precipi-
tation at all seasons and no marked period of
drought are typical of the marine climate
(Fig. 13.11). Normally there is no dormant
season imposed upon vegetation because of
rainfall deficiency. Both in the most definitely
marine locations and in those parts lying
closest to the dry-summer type, winters com-
monly are rainier than summers, although
a summer month is really dry only in a few
places.
Origin Over lowlands, where oro-
graphic effects are absent, the precipitation
is chiefly frontal, or cyclonic, in origin. Much
of it falls as long-continued steady but light
rain from a gray, leaden overcast (Fig. 13.14).
Humid mesotkermal climates 251
Because of the general lack of high tempera-
tures, thunderstorms are few, but showery rain
is nevertheless fairly common in the fresh
westerly maritime air following the passage of
a cyclonic center.
As previously stated, the total precipitation
on lowlands is only modest or moderate in
amount; but the number of days on which rain
falls is unusually high. This can only mean
that the precipitation is usually light or mod-
erate in its rate of fall. Thus, while Paris
receives only about 23 in. of precipitation a
year, this amount is spread over 188 rainy
days.
As might be expected, the amount of cloud
is great, the marine climate being one of the
earth's most cloudy types. Dark, gloomy,
overcast weather is very common. Over ex-
tensive areas in western Europe cloudiness is
greater than 70 per cent, the sun sometimes
remaining hidden for several weeks in succes-
sion, especially in winter and fall.
The weather element Since numerous
cyclonic storms, each with its accompanying
converging system of winds and its cloud
deck, cross these marine west-coast areas,
nonperiodic weather changes are a dominant
feature of the climate (Fig. 13.12). Fall and
winter, in spite of mild temperatures, are
stormy seasons, so that periods of gloomy,
dripping cyclonic weather are frequent (Fig.
13.13). Spells of bright, sunny anticyclonic
weather associated with cP air masses are the
exception, but when they do occur this cli-
mate is likely to experience its most severe
freezes.
As spring advances, cyclones become fewer
and sunshine more abundant. The air is still
cool, but the sun is strong; thus in western
Europe, for example, late spring is acclaimed
the most delightful season. Summer tempera-
tures make for a sense of physical well-being,
F I G . 1 3 . 1 4 A strongly occluded storm in
western Europe, producing light but steady and
widespread rainfall, a low cloud ceiling, and low
visibility. Most of the cyclones which affect west
coasts in middle latitudes are in an advanced stage
of occlusion. Such storms, while they produce
much cloud, yield only a modest amount of
precipitation over lowlands.
and where sunny days are numerous, as in
the American Pacific Northwest, a more
pleasant summer climate would be hard to
find. When cloudy, rainy days do occur in
summer, they may be unpleasantly chilly.
RESOURCE POTENTIALITIES
OF THE MARINE REALM
Two of the most significant climatic ele-
ments affecting the potential productivity of
the marine realm are (a) its unusually long
frost-free season, considering its latitude, and
(b) its relatively mild winters. To be sure, there
is a marked dormant season imposed by kill-
ing frosts, so that the growth of sensitive and
of out-of-season crops characteristic of both
252
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
the dry-summer subtropics and the humid
subtropics is excluded from this realm. Never-
theless, the frost-free period of 6 to 8 months
and the relatively mild winters permit many
cereal crops to be sown in fall, and animals
can graze out of doors nearly 1 2 months, if
not the entire year. Large-scale storage of
animal feeds for winter use is therefore much
less necessary than in the more severe con-
tinental climates.
Somewhat offsetting the advantages of the
mild winters and the long frost-free season in
marine climate is the deficiency of summer
heat. Just as the winters are marine, so also
are the summers. Thus, while warm-month
temperatures of 60 to 65 are ideal for human
comfort, and may represent the optimum con-
ditions for physical activity as well, they are
not ideal for many crops; for instance, they
may be too low for the best growth of some
grain crops, especially maize. On the other
hand, grass finds almost ideal conditions here,
so that pastures are usually excellent and hay
and forage crops thrive.
The adequate amount of rainfall and the
fact that there is no season of marked drought
are climatic assets of the first magnitude for
crop growth generally. Another asset is the
dependability of the precipitation year in and
year out which is reflected in high uniformity
of crop yields.
In these mild, humid, west-coast regions
the original vegetation cover was chiefly forest,
and because of the hilly and mountainous
nature of large parts of the realm, trees still
cover extensive areas. In the North American
marine region, for instance, is the earth's
finest coniferous (needle-leaf) forest, the
world's principal source of high-grade soft-
wood lumber. In Europe the original forest
was composed largely of broadleaf deciduous
trees, with oaks predominating. Conifers
occupied chiefly the highland and sandy areas.
Here, however, centuries of occupance by
civilized peoples have resulted in a removal
of the forest cover from the plains, and even
that of the highlands has been greatly modi-
fied. Forests of the Southern Hemisphere
marine regions are moderately dense and
luxuriant, but they are composed of species
most of which produce inferior lumber.
The podzolic forest soils which are rather
characteristic of lowlands with this type of
climate are the best of the world's forest soils.
They are by no means the equal of the dark-
colored grassland soils, for they have been
moderately leached and the supply of organic
matter from the forest cover is not abundant.
On the other hand, they are distinctly better
than the red and yellow soils of the wet trop-
ics and subtropics. Under constant cultivation
they deteriorate, to be sure, but less rapidly
than the other light-colored soils, and with
less care and attention they can be kept in
good condition and fitted for a variety of
crops.
CHAPTER 14
Humid
microthermal,
polar, and
highland
climates
THE HUMID MICROTHERMAL CLIMATES
TYPE LOCATION
The humid microthermal climates (D) stand
in contrast to the humid mesothermals by
reason of having (a) a colder winter, (b) a
longer-lasting snow cover, (c) a shorter frost-
free season, and (d) a greater annual range of
temperature. In other words, the microther-
mals are more severe than the temperate
mesothermals. This greater severity results
primarily from locational differences in both
253
254
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
latitude and position on the continents, that
is, from the facts that the humid microthermal
climates lie poleward from the subtropical
types and occupy more interior and leeward
locations on the great land masses than the
marine climate (Fig. 11.1; Plate 2).
These specific locational differences suggest
the basic locational facts about the micro-
thermal climates, that they are land-controlled
and can develop only in large continents in
the higher middle latitudes, and thus are
confined exclusively to the Northern Hemi-
sphere, only Eurasia and North America being
able to produce them. Of the Southern Hem-
isphere continents, South America alone
extends poleward far enough to permit the
development of severe climates; but the nar-
rowness of that land mass south of latitudes
35 -40 prevents genuinely extreme seasonal
conditions in spite of the latitude.
Microthermal climates are excluded from
the western, or windward, side of the two
Northern Hemisphere continents because of
the dominance there of maritime air masses.
They occupy, instead, the interiors of these
continents and extend down to tidewater on
their leeward or eastern sides where, in spite
of proximity to the sea, modified continental
conditions prevail.
Unlike the mesothermal climates, those of
the microthermal group have temperature
regimes which differ substantially from one
another largely in the degree of summer heat
and winter cold. Here, moreover, there are no
strongly contrasting seasonal rainfall regimes.
Therefore, the general aspects of microthermal
climates as a group will be discussed before
the individual types of climate are analyzed.
TEMPERATURE
Because they extend through a wide range
of latitude, there are marked temperature
contrasts within the regions classed as humid
microthermal. However, these climates are
sure to have relatively severe seasons for any
particular latitude, so that annual ranges are
large. The winter cold is more characteristic
and distinctive than the summer heat, but
summers are warm for the latitude. Further-
more, the seasons are not only extreme but
also variable in temperature from one year to
another. In marine climate one year's winter
is likely to be much like another's, but wide
departures from the normal seasonal tempera-
ture are characteristic of severe continental
climates.
Effects of a snow cover on tempera-
ture In the microthermal, polar, and high-
land climates and, indeed, only in these
climates the snow cover is of sufficiently
long duration to have a marked effect upon
winter temperature. In these climates once a
region is overlain by such a lasting white
snow mantle, the ground itself underneath
the snow cover has little influence upon air
temperature: sunlight falling upon the snow
is largely reflected back to space continually,
so that little of the solar energy is effective
in heating the ground and then the atmos-
phere. Moreover, while loss of energy by
earth radiation goes on very rapidly from the
top of a snow surface, the low conductivity
of snow tends, as long as the snow lasts, to
retard the flow of heat upward from the
ground to replace that which is lost. Clearly,
then, the total effect of a lasting snow cover
is markedly to reduce winter temperatures.
On the other hand, the snow cover operates
to keep the soil underneath the snow warm
and prevents deep freezing.
PRECIPITATION
Annual precipitation in most lowland re-
gions of microthermal climate errs on the side
Humid microthermal, polar, and highland climates 255
of being too meager rather than too great.
Well over half the area of such regions has
under 20 in., and the area with more than
40 or 45 in. is very limited. Considering both
the latitudinal and geographical locations of
microthermal climate, this modest amount of
precipitation is to be expected. Characteris-
tically the amount declines (a) from the sea
margins toward the interior, and (b) north-
ward with the decline in temperature and
specific humidity.
Although winter is not without precipita-
tion, summer is normally a season of a strong
precipitation maximum in microthermal cli-
mates, and this concentration of much of the
year's precipitation in the warmer months is
as distinct a hallmark of continental climates
as their severe winters and large annual ranges
of temperature.
This seasonal distribution is related to the
following conditions: (a) Low temperatures
make the specific humidity, or reservoir of
water vapor in the atmosphere, markedly
lower over the continent during the winter
than it is in summer when temperatures are
much higher, (b) During winter the settling
air in the continental seasonal anticyclone is
also conducive to low specific humidity, and
makes for increased stability of the atmos-
phere as well, (c) The continental anticyclones,
which develop over the colder, more northerly
parts of the land masses in winter, are areas of
diverging air currents, and thus antagonistic
to the development of fronts and cyclones.
This applies particularly to the more severe
microthermal climates, such as the subarctic,
where the winter anticyclone is best devel-
oped. In summer, although cyclones may be
fewer and weaker, they can, nevertheless,
penetrate deeper into the continents, (d)
Convection is at a minimum during the winter
months, for at that season the cold snow sur-
face tends to increase the stability of air
masses. In summer, on the other hand, the
warm land surface has a tendency to make
unstable the air masses moving over it. (e)
Consequent upon the seasonal extremes of
temperatures and hence of pressure, a tend-
ency toward a monsoon system of winds is
developed, which leads to an outflow of dry,
cold cP air in winter, and to an inflow of trop-
ical maritime air with high rainfall potentiali-
ties in summer.
The occurrence of these conditions making
for drier winters and wetter summers is a
fortunate thing, for in severe climates with a
short frost- free season it is of the highest
importance for crop agriculture that rainfall
be concentrated in the warm growing season.
This is especially true where the total amount
of precipitation is relatively modest, as it is
over extensive areas within this group of
climates. In the tropics, where it is constantly
hot, it matters not at all when the rain falls,
and even in the subtropics winter rainfall is
effective for plant growth. But in the micro-
thermal climates, where the severe winter
creates a completely dormant season for
plants, it is essential that periods of sufficient
heat and sufficient rainfall to foster growth
coincide.
TYPES
Two principal types of climate are included
within the microthermal group: (a) the humid
continental climate, including both warm-
summer and cool-summer subtypes, and (b)
the subarctic climate. The first type, an im-
portant agricultural climate, characteristically
lies on the equatorward margins of the sub-
arctic type, which itself occupies such high
latitudes that agriculture ceases to be of great
importance.
256
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
HUMID CONTINENTAL CLIMATE
TYPE LOCATION
Humid continental climate, as might be
expected, has interior and lee location in both
North America and Eurasia, with a latitudi-
nal spread of 10 to 20 extending from about
40 on the south to 50 -60 on the north
(Fig. 11.2; Plate 2). But there are differences
in location on the two continents. In Eurasia,
where lack of mountain barriers on the west
side allows the oceanic air to enter freely, this
climate is positioned both to the west and to
the east of the dry interior, while in North
America it lies only to the east of the dry cli-
mates. In both eastern Asia and eastern North
America humid continental climate is bor-
dered by subarctic climate on the north and
humid subtropical on the south. But in
Europe and western Asia it meets Mediter-
ranean and dry climates on its southern
margins.
It may seem somewhat surprising that this
land-controlled climate should extend east-
ward to the ocean margins in both Asia and
North America, but not if it is remembered
that the prevailing west-to-east atmospheric
circulation in these latitudes makes deep and
persistent entrance of maritime air on the lee,
or east, side unlikely.
TEMPERATURE
Seasonal temperature Summers that
vary from warm in the south to cool on the
northern frontier, and winter always cold but
actually varying more in degrees of tempera-
ture, are characteristic of humid continental
climate. Depending largely on latitudinal
location, the average July temperature may
vary from 75 or even more (in the south) to
65 (in the north). The January average shows
a much greater variation: from zero or below
in the north to 25 or above in the southern
parts. As a consequence, the annual range of
temperature is everywhere large, and it in-
creases both to the north and with distance
from the sea. At Peoria, Illinois (41N), for
example, the January and July averages are
24 and 75, so that there is an annual range
of about 50. But at Winnipeg, Canada, at
about 50 N, the comparable figures are 4
and 66 and the range is 70. Similarly, New
York City on the Atlantic Coast has a range
of 42, while Omaha, Nebraska, at a com-
parable latitude but inland, records 55.
In correspondence to the varying tempera-
ture regime, the growing season changes in
length from south to north, approaching 200
days on the southern margins and declining
to about 100 days on the subarctic side.
Seasonal gradients Summer and winter
present remarkable contrasts in the rate of
change in temperature in a north-south di-
rection (Fig. 14.1). In the cold season the
isotherms are much more closely spaced than
in summer, so that in the central and eastern
United States, for instance, the temperature
gradient is two to three times as steep in
January as in July. Thus sudden and marked
temperature changes associated with shifts in
wind direction are much more likely in winter
than in summer.
PRECIPITATION
Amount and seasonal distribution
Modest amounts of annual precipitation, with
a seasonal concentration in summer, are the
rule in the humid continental climate as in
the humid microthermal group as a whole
(pp. 261, 263). With increasing distance from
the sea, the total amount of precipitation
declines as the inner frontier where humid
continental makes contact with dry climate
(a)
Humid microthermal, polar, and highland climates
is approached, as well as a greater annual
range of precipitation, with winter becoming
more of a dry season. An example of this
range is Omaha, Nebraska, which receives
29 in. of rain a year: in July it receives 4.7
in., in January only 0.7.
Another feature of the less humid interior,
especially in the United States and central
Europe, is a shift of the precipitation max-
imum to early summer, so that June is wetter
than July.
Winter precipitation Cool-season pre-
cipitation is largely frontal or cyclonic in
origin. In North America, mTGult air masses
move poleward up the Mississippi River
Valley with no relief obstacles to interfere,
being chilled and thus stabilized by the cold
land surface. Occasionally the tropical air may
flow poleward at the surface as far north as
Iowa and the southern shores of the Great
Lakes. More commonly, however, it comes into
conflict with colder, heavier air masses before
reaching so far inland and is forced to ascend
over them, and widespread frontal precipitation
results. The North American continental cli-
mate, therefore, has a moderate amount of
winter precipitation. The amount increases
eastward as the ocean is approached, until
along the Atlantic seaboard winter is equally
as wet as summer. In northeastern Asia where
the outward-flowing winter monsoon is
stronger, raTair is unable to advance so far
poleward, so that winter precipitation in
northern China and Manchuria is very meager;
for example, Peking, which receives 25 in. a
year, has 9.4 in. in July, and only 0.1 in. in
January.
A portion of the winter precipitation is in
the form of snow, and a permanent snow
cover, varying from a few weeks to several
months in duration, is typical (Fig. 14.10).
In those parts of the northeastern United
States and southeastern Canada where winter
257
(6)
FIG. 14.1 Surf ace- temperature gradients in
microtherma! climates are much steeper in winter
(a) than in summer (ft).
cyclones are particularly numerous and well
developed the Great Lakes region, the St.
Lawrence River Valley, northern New Eng-
258
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
land, and the Canadian Maritime Provinces
snow becomes very deep. Thus northern New
England and northern New York have more
than 7 ft of snowfall during an average winter,
and the snow cover remains on the ground for
more than 4 months.
Summer precipitation While steady,
long-continued cyclonic rain falling from a
gray overcast is not absent in summer, more
common by far is showery rainfall, much of
it associated with lightning and thunder. This
is because the warmer, more humid summer
air has been made buoyant and unstable by
heating from below while passing over a
warm land surface. For example, 75 to 90
per cent of the summer rainfall at Madison,
Wisconsin, occurs in association with
thunderstorms. Yet even most of the con-
vective showers and thunderstorms of summer
occur in conjunction with some form of exten-
sive atmospheric disturbance such as a front
or a weak cyclonic storm.
The nonperiodic weather element In
no other type of climate are rapid and marked
nonperiodic weather changes so characteristic
as in the humid continental, for it is here
that the conflict between polar air masses and
tropical air masses reaches a maximum
development.
This nonperiodic control of weather is
strongest in the cold season, when the sun,
accompanied by the jet stream and the storm
belt, has retreated farthest south. At this
season, with weather conditions dominated
by moving cyclones and anticyclones associ-
ated with the invading, rapidly shifting air
masses and the fronts along their boundaries,
the daily rise and fall of temperature with the
sun is often obscured by larger nonperiodic
oscillations (Figs. 14.7, 14.9). The central
and eastern United States, which are freely
open to the movements of air masses both
from north and south, are regions of unusual
storminess; storm control is less marked in
eastern Asia.
In summer throughout the humid conti-
nental regions, air masses show weaker tem-
perature contrasts and move less rapidly, so
that fronts are fewer and weaker, and the
weather is somewhat more diurnally regular
and sun-controlled (Fig. 14.7).
Special seasonal weather types The
normal cycle of middle-latitude weather
changes with the passage of a well-developed
cyclone, as well as the normal effects of the
associated anticyclone, was described in
Chap. 10. But at the same time it was pointed
out that there is almost an infinite variety of
weather variations related to these storms in
these latitudes, depending upon the season,
the size and intensity of the atmospheric dis-
turbance, the nature of the air masses involved
in the storm, the track followed by the storm,
and the contrasting patterns of high-level
atmospheric circulation.
As a consequence of this great variety of
weather combinations no universally satis-
factory classification of types of weather has
ever been developed. Yet even the layman is
aware that there are some weather types
which are sufficiently distinctive to have been
given names. Warm wave, cold wave,
Indian summer, blizzard, and January thaw
are such, and much more numerous are the
unnamed ones. Furthermore, it is certain that
no real comprehension of humid continental
climate is possible without an appreciation
of the variety of weather types which in com-
bination produce the seasonal climates. But
this requires more than a layman's effort; in-
deed, this requires a study of the daily
weather map in conjunction with a firsthand
observation of weather conditions.
A very few of the numerous weather types
characteristic of humid continental climate
are shown in Figs. 14.2 to 14.5. These
F I G . 1 4 . 2 A common winter weather type.
Here a cyclone traveling on a northern track is pro-
ducing cloudy, mild weather and light precipitation
over extensive areas of the north central United
States. Temperatures shown are for 1:30 A.M.
sketches of synoptic weather charts of parts
of North America are worthy of careful study.
Winter, the season of strongest temperature
gradients and of greatest air-mass contrasts, is
the period of greatest weather variety.
A well-developed anticyclone arriving from
arctic Canada as a mass of fresh cP air may
produce bitterly cold weather, even subzero
temperatures (Fig. 10.2). This sharp drop in
temperature brought by the northwest wind
is the well-known cold wave. Blizzardlike
conditions with violent winds may even usher
in the anticyclone if it is characterized by un-
usually steep pressure gradients. But if an in-
vading cold anticyclone is composed of
modified mP air from west of the Rocky
Mountains, skies will be clear and temper-
atures only moderately low; and this control
produces some of the finest winter weather.
A deep cyclonic storm, especially if it
originates in the Texas area and takes a route
northeastward across the country, is more
than likely to bring extensive and heavy
snowfalls to the humid continental climates
of the Mississippi River Valley and the East
Humid microthermal, polar, and highland climates 259
(Fig. 14.3). If the vigorous cyclone travels a
more northerly route, the weather will be
milder and the rain area more extensive. A
weak low following a route to the north of
the Great Lakes may bring generally gray,
overcast weather but only very modest
amounts of rain or snow (Fig. 14.2). But
these are only a few of the far more numerous
weather types which in combination produce
the winters of humid continental climates.
In summer, temperature gradients are
weaker, air-mass contrasts less striking, and
the weather element, as controlled by passing
atmospheric perturbations, is altogether less
well developed. But while sun control and
diurnal regularity are stronger than at other
seasons, this period of high sun is by no
means lacking in nonperiodic weather irregu-
larities. A somewhat stagnant anticyclone to
the south and east may envelop the humid
continental area of the United States in a
prolonged heat wave with a succession of
days when the daily temperature maxima rise
to between 90 and 100 (Fig. 14.4). Such a
heat wave may be suddenly brought to an
F I G . 1 4 . 3 A well-developed winter storm
originating in the Texas area and moving northeast-
ward across the United States. Such storms are
likely to bring heavy precipitation, much of it in the
form of snow.
260
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
JJL
FIG. 14.4 A July heat wave a summer
weather typeover the central and eastern United
States. Temperatures shown are the maxima for
the 12 hr preceding. Tropical southerly air from a
warm subtropical anticyclone controls the weather.
end by the passage of a V-shaped cyclonic
storm with a well-developed cold front and
associated cold-front thunderstorms. Then,
following the passage of the cold front with
its strong convectional activity, there may be
F I G . 1 4 . 5 A spring weather type. Here a cold
anticyclone advancing southward as a mass of cold
iPa\r with northwest winds carries low tempera-
tures well into the subtropics and results in a severe
spring freeze in the north central states.
several days of delightfully cool weather, as
an anticyclone with air of polar origin dom-
inates the weather.
Spring and fall, the transition seasons,
witness a more even struggle between storm
and sun control. First the one and then the
other is in the ascendancy, so that there is
something of an oscillation between summer
and winter conditions. Mild, warm days in
April and early May, with a regular diurnal
rise and fall of the thermometer resembling
summer, may be followed by a reestablish-
ment of winter conditions, as a passing
cyclone lays down a snow cover and the
following invasion of cP air drops the tem-
peratures to an unseasonable frost (Fig. 14.5).
Continental climates are particularly famous
for the fickleness of their spring weather.
Autumn brings some of the loveliest days
of the entire year but likewise some of the
rawest, gloomiest weather. Bright, clear
weather, with warm midday temperatures and
crisp, frosty nights, comes with anticycionic
control. A reestablishment of hot-wave gra-
dients with south winds in October and No-
vember, after severe frost and perhaps even
snow have been experienced, may cause a
temporary return of summer conditions, re-
sulting in those much-cherished spells of
warm weather with hazy, smoky atmosphere
known as Indian summer. But well-developed
cyclonic storms of this season may bring raw,
gray days with chilly rain, and occasionally
they produce a temporary snowy winter land-
scape as early as October.
WARM-SUMMER
AND COOL-SUMMER
SUBDIVISIONS OF HUMID
CONTINENTAL CLIMATE
Two principal subdivisions of humid con-
tinental climate are here recognized: (a) the
TYPE Humid Continent I -Warm Summer (Da)
IN - PLACE Peoria. III.
..
.
.
TT
Humid microthermal, polar, and highland climates
less severe warm-summer subtype (Da), and
(b) the more extreme cool-summer subtype
(D6). 1 The latter is characteristically located
farther poleward, that is, on the northern
side of the warm-summer subtype (Fig. 11.2).
As Plate 2 shows, the warm-summer sub-
type is to be found in three far-separated
locations, the central eastern United States,
central Europe, and eastern Asia. The cool-
summer subtype is likewise represented in
North America, Europe, and Asia.
Temperature and precipitation In the
warm-summer subtype summer months are
only 5 to 10 warmer than in the cool-
summer subdivision, but this is sufficient to
make for marked differences in their agricul-
tural potentialities (following table). More-
over, the frost-free season of 5 to 6 months in
the warm-summer climate is shortened to 3
to 5 months in the other (Figs. 14.6 and
14.8).
Summers in warm-summer areas are likely
to have so many uncomfortably hot and
humid days that human comfort is better
261
JFMAMJJASO
FIG. 14.6 Data of a station representing the
warm-summer subtype of humid continental climate.
A large annual range of temperature and concen-
tration of precipitation in the warm season
are characteristic.
1 In the Kcippen system a = temperature of warmest
month above 71.6 (22 C), and b = temperature of wann-
est month below 71.6.
served by the cooler summers farther north,
even though they may offer more handicaps
to agriculture. Winter temperatures, however,
are usually 15 to 20 colder in the cool-
Climatic Data for Representative Stations in the Humid Continental
Warm-summer Subtype (Da)
JFMAMJJASOND Yt Range
Peoria, Illinois
Temp.
24
28
40
51
62 71 75 73
65
53
39
28
51
51
Precip.
1.8
2.0
2.7
3.3
3.9 3.8 3.8 3.2
3.8
2.4
2.4
2.0
35.1
New York City
Temp.
31
31
39
49
60 69 74 72
67
56
44
34
52
43
Precip.
3.3
3.3
3.4
3.3
3.4 3.4 4.1 4.3
3.4
3.4
3.4
3.3
42.0
Bucharest, Rumania
Temp.
26
29
40
52
61 68 73 71
64
54
41
30
51
47
Precip.
1.2
1.1
1.7
2.0
2.5 3.3 2.8 1.9
1.5
1.5
1.9
1.7
23.1
Peking, China
Temp.
24
29
41
57
68 76 79 77
68
55
39
27
53
55
Precip.
0.1
0.2
0.2
0.6
1.4 3.0 9.4 6.3
2.6
0.6
0.3
0.1
24.8
262
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
Temp.
90
Day 5
10
15
20
FIG. 14.7 Daily maximum and minimum temperatures for the warmest
and coldest months for a station with humid continental warm-summer
climate. Nonperiodic air-mass control is conspicuous, especially in winter.
TYPE Humid Continental- Cool Summer (Db)
IN - PLACE Montreal, Canada
J F M A
N D
summer areas, and subzero temperatures and
long spells of cold weather are more common
there (Figs. 14.6-14.9).
The two subtypes are not conspicuously
different in amount and seasonal distribution
of precipitation. However, in those parts
farther north a larger part of the winter pre-
cipitation is in the form of snow, and as
a consequence the snow cover is more dur-
able and long-continued there (Fig. 14.10).
FIG. 14.8 The cool-summer subtype of humid
continental climate. Note the large annual range
of temperature. At this station there is an absence
of any seasonal concentration of precipitation, a
feature characteristic of the northeastern United
States and adjacent parts of Canada where winter
cyclones are numerous.
Humid microthermal, polar, and highland climates 263
Temp.
Day 5
10
15
20
25
10
-10
-20
-30
FIG. 14.9 The cool-summer subtype of humid continental climate. Note the very large and irregular
temperature changes, evidence of strong air-mass control associated with cyclones and anticyclones.
Climatic Data for Representative Stations in the Humid Continental
Cool-summer Subtype (Db)
J F M A M J J A S O N D Yr Range
Madison, Wisconsin (marginal in location)
Temp.
17
20
31
46
58 67 72 70
62
50
35
23
46
55
Precip.
1.2
1.3
1.9
2.6
3.7 3.4 3.5 3.3
4.1
2.3
2.0
1.4
30.7
Montreal, Canada
Temp.
13
15
25
41
55 65 69 67
59
47
33
19
42
56
Precip.
3.7
3.2
3.7
2.4
3.1 3.5 3.8 3.4
3.5
3.3
3.4
3.7
40.7
Moscow,
Union of Soviet Socialist Republics
Temp.
12
15
23
38
53 62 66 63
52
40
28
17
39
54
Precip.
1.1
1.0
1.2
1.5
1.9 2.0 2.8 2.9
2.2
1.4
1.6
1.5
21.1
Harbin, Manchuria
Temp.
-2
5
24
42
56 66 72 69
58
40
21
3
38
74
Precip.
0.1
0.2
0.4
0.9
1.7 3.8 4.5 4.1
1.8
1.3
0.3
0.2
19.3
264
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
NUMBER OF DAYS
WITH
SNOW COVER
FIG. 14.10
RESOURCE POTENTIALITIES
OF THE
HUMID CONTINENTAL REALM
Climatically the humid continental realm is
less bountiful than the humid subtropics,
chiefly because of the shorter growing season
and the cooler summer. This deficiency of
heat tends to exclude many of the more sen-
sitive crops and those requiring a long period
between frosts. Greater dependence upon
quick-maturing annuals is the result. Again
in comparison with the subtropics, there is a
shorter period during which animals can
forage for their food and a much longer one
during which they must be protected against
the cold and fed from feeds stored in barns,
silos, and granaries.
A further climatic handicap grows out of
the fact that over extensive areas rainfall is
only modest in amount, and is inclined to be
undependable. The relatively wide fluctua-
tions in crop yields from year to year reflect
these disadvantages. But the seasonal concen-
tration of the precipitation in the period of
greatest heat somewhat compensates for
them.
The original vegetation cover was largely
forest in the more humid sections and prairie
grass in the less humid parts. In their virgin
state the prairies provided some of the earth's
finest natural grazing land. Almost all of the
prairie land has long since been brought
under cultivation, however, for it is some of
the world's best agricultural land. The forests
of the more humid sections of the realm were
various in nature. Thus a representative
north-south cross section of the forests would
show conifers predominating toward the
northern margins of the realm, with mixed
Humid microthermal, polar, and highland climates 265
forests and purer stands of deciduous broad-
leaf trees prevailing farther south. Without
doubt the virgin forests of the humid conti-
nental realm were among the finest and most
extensive of the earth. For decades they were
the world's principal source of lumber, and
they are still important producers. But, be-
cause they were composed of such superior
lumber trees and readily accessible, they have
suffered rapid cutting. Much of the forest
was removed by settlers in search of farm
land; in less desirable agricultural regions the
great lumber companies logged off the forest
and left behind a desolate cutover country.
Soils in the humid continental climates
show wide variations in quality. Under the
broadleaf forest podzolic soils have developed
which, although of only average quality, are
still the best of the forest soils. Farther north
in the coniferous region, poor, gray soils, the
true podzols, prevail. In the less humid sec-
tions of the realm, where prairie grasses pre-
dominated, are to be found some of the
earth's finest soils. There the lower rainfall
results in less leaching, and the grasses pro-
vide an abundance of organic matter, so that
the soils are high in soluble minerals and
dark in color. Such excellent soils help to
compensate in part for the less abundant and
also less reliable rainfall of these sections.
Considerable areas in both the North
American and European sections of the realm
have been subjected to recent glaciation by
continental ice sheets. In the parts of such
areas where the relief is relatively great or the
bedrock resistant, as, for example, in New
England, northern New York State, and parts
of Norway, Sweden, and Finland, ice erosion
has been dominant, so that soils are thin and
stony and lakes are numerous. In other parts
where ice deposition prevailed, the drainage
lines have been disrupted, so that both lakes
and swamps are numerous, and a rolling and
somewhat patternless terrain arrangement of
rounded hills and associated depressions is
characteristic. Here, although the soils may
be deep, they vary greatly in composition and
quality.
SUBARCTIC CLIMATE
TYPE LOCATION
Subarctic climate (Dc, Dd), 2 the most severe
of the humid microthermal climates, is char-
acteristically located in the broad northern
parts of Eurasia and North America between
about latitudes 55 and 70 N. On its northern
margins, approximately at the poleward limit
of forest growth, it makes contact with polar
climate. On its equatorward side it is bordered
by either humid continental or dry climate
(Fig. 11.2, Plate 2).
2 In the Koppen system c = cool summers, with only 1
to 3 months above 50 (10C); d = cold winters, with the
temperature of the coldest month below -36.4 (-38C).
TEMPERATURE
Long and bitterly cold winters, very short
summers with brief intervening falls and
springs, and unusually large annual ranges
these are the prime temperature character-
istics of subarctic climate (Fig. 14.11). Sub-
arctic thus is land-controlled climate at its
maximum development.
Winter Winter, by reason of both its
length and its severity, dominates the climatic
calendar. Frosts may arrive in late August,
and ice begins to form on ponds in Sep-
tember. At Yakutsk, Siberia, the average
monthly temperature drops 37 (from 16 to
266
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
Climatic Data for Representative Subarctic Stations
./ F M A M J J AS O N D Yr Range
Temp.
Precip.
Temp.
Precip.
Temp.
Precip.
Fort Vermilion, Alberta, Canada (58'27'N)
-14
0.6
-6
0.3
8
0.5
30
0.7
47
1.0
55
1.9
60
2.1
57
2.1
46
1.4
32
0.7
10 -4
0.5 0.4
-4 -2
1.3 0.9
10
1.1
Moose Factory, Canada (5116'N)
28 42 54 61 59 51 39 22 5
1.0 1.8 2.2 2.4 3.3 2.9 1.8 1.1 1.1
Yakutsk, Siberia, Union of Soviet Socialist Republics (6521'N)
_46 -35 -10 16 41 59 66 60 42 16 -21 -41
0.9 0.2 0.4 0.6 1.1 2.1 1.7 2.6 1.2 1.4 0.6 0.9
27
12.2
30
20.9
12
13.7
74
65
112
21) between October and November.
Within an extensive area in northeastern
Siberia, near the center of the great thermal
winter anticyclone, the average January tem-
perature is below 50 and minimum Jan-
uary temperatures of 76 and lower have
been recorded. At Oimyakon, Siberia, the
F I G . 1 4 . 1 1 A cool summer, a severe winter,
a large annual range of temperature, and modest
precipitation concentrated in summer are char-
acteristic of subarctic climate.
TYPE Subarctic (Dc)
IN- PLACE Moose Factory, Canada
30
JFMAMJJASOND
F
100
90
80
70
60
50
40
30
20
10
-10
-20
-30
-40
1-50
cold pole or region of lowest minimum tem-
peratures, the thermometer has fallen as low
as 95 below zero. However, these are the
extremes in subarctic winters, for over most
of the North American and European sectors
average January temperatures of zero to 15
below are the rule. It is common, however,
for the average temperatures of 6 or 7 months
to be below freezing. The low temperatures
combined with the long daily period of dark-
ness (which is, obviously, partly responsible
for them) make the winter weather depressing
and hard to bear.
Summer The striking characteristic of
the period of warmth in the subarctic is
its briefness rather than its coolness. Typically
the warmest month, July, has an average tem-
perature in the 60s, which is no lower than
that of many stations in marine climates
farther south. Moreover, it is not uncommon
for midday temperatures to reach 80 and
above (Fig. 14.12). But July's modest warmth
is very brief, for June and August averages
are between 50 and 60, and May and Sep-
tember are in the 40s. As a rule the period
between killing frosts is only 2 to 3 months
in length, and many stations experience freez-
ing temperatures even in July and August in
some years.
Humid microthermal, polar, and highland climates 267
Temp.
Day 5
10
FORT VERMILION, CANADA
i I
-40
-50
FIG. 14.12 Data from a subarctic station in Canada. Note the
unusually strong nonperiodic air-mass control of temperature changes in
winter. Summer shows greater diurnal regularity.
Somewhat compensating for the briefness
of summer is the unusually long period of
daylight in these higher latitudes. Thus at
60 N June days have an average 18.8 hr of
possible sunshine.
PRECIPITATION
The characteristically meager annual pre-
cipitation, usually amounting to no more than
15 to 20 in., is related to (a) the great breadth
of Eurasia and North America in subarctic
latitudes, (b) low temperatures and associated
low specific humidity, and (c) the well-devel-
oped thermal anticyclone of the colder part
of the year with its settling air and diverging
winds.
The year's precipitation is concentrated in
the warmer months when the humidity con-
tent of the air is highest and atmospheric
stability is least (Fig. 14.11). The especially
low winter temperatures and the strong winter
anticyclone mentioned above both operate to
268
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
F I G . 1 4 . 1 3 A type of subarctic winter weather. A well-developed
cyclone accompanied by strong winds and extensive snowfall prevails over
the subarctic and tundra region of northeastern Canada. A cold anticyclone
is conspicuous over northernmost Canada.
inhibit the processes making for precipitation.
Over northeastern Siberia winters are espe-
cially dry, the precipitation ratio of the
wettest summer month to the driest winter
month being more than 10: 1.
To a very great extent the year's precipi-
tation originates in extensive disturbances,
such as fronts and cyclonic storms. Thunder-
storms are few. Disturbances are numerous
enough at all seasons to give rise to marked
nonperiodic weather changes, although in
winter there is much settled anticyclonic
weather (Figs. 14.2, 14.3).
RESOURCE POTENTIALITIES
OF THE SUBARCTIC REALM
In spite of the fact that the subarctic realm
is one of the most extensive of the earth's
geographic realms, it is also one of the least
productive. Like the dry lands and parts of
the wet tropics, the subarctic realm is coinci-
dent with relatively blank areas on the world-
population map. The extractive industries,
such as hunting, fishing, mining, and logging,
which rank high in relative importance, are
capable of supporting only a meager pop-
ulation. The landscape, therefore, is one
composed predominantly of natural features:
man has left but a modest imprint.
In productive capacity the realm is funda-
mentally handicapped by its niggardly cli-
mate, which sets very definite low limits
upon agricultural development, the primary
difficulties being associated specifically with
(a) the briefness of the summers and (b) the
relatively low summer temperatures. At pres-
ent commercially successful agriculture is not
likely in regions where the frost-free season is
shorter than 80 or 90 days, and this condi-
tion prevails in all except the most southerly
portions of the subarctic realm.
Humid microthermal, polar, and highland climates 269
Subarctic Eurasia and North America are
largely covered by primarily coniferous virgin
forests. In their immensity and monotony
these subarctic forests are like the sea, and
travelers are impressed with their emptiness
and silence. Even animal life is sparse. Coni-
fers usually occupy in the neighborhood of
75 per cent of the forest area, with such
deciduous trees as the birch, poplar, willow,
and alder comprising most of the remainder.
Yet neither in the size of the trees nor in
the density of the stand is the subarctic
forest impressive, and in the ice-scoured
Canadian subarctic extensive areas of lake,
swamp, and bare rock are even practically
without forest. As a result the subarctic
forest does not represent nearly so great a
potential supply of forest products as its area
might seem to indicate. Most subarctic timber
is more valuable for firewood and pulpwood
than for good lumber. Moreover, the inacces-
sibility of these northern forests to world
markets severely reduces their resource value.
An impoverished soil environment is char-
acteristic of the subarctic realm, and this
infertile soil, combined with a climate of low
potentialities, causes the subarctic lands to
offer what appear to be almost insurmount-
able difficulties to the agricultural settler.
The needles from the coniferous forest pro-
vide a very meager supply of organic material
for the soils, and the ground water, high in
organic acids derived from the raw humus,
leaches the soil minerals excessively.
Ranking after climate and soils as a third
handicap to agricultural settlement within the
subarctic realm is deficient drainage. Poorly
drained land is prevalent, partly because of
the permanently frozen subsoil which exists
throughout the higher latitudes of the realm.
Over most of subarctic North America and in
Scandinavia, Finland, and western Soviet
Russia the abundance of lakes and swamps is
a consequence of continental glaciation.
POLAR CLIMATES
LOCATION AND BOUNDARIES
As the tropics lack a cool season, so do the
polar climates (E) lack a genuine warm season.
While monotonous heat characterizes the low
latitudes, in the polar regions monotonous
cold prevails.
Polar climates are confined to the high
latitudes of the earth, largely poleward of
latitude 60. The poleward limit of forest
growth is commonly accepted as the equator-
ward boundary of polar climates, and over
the great continents this vegetation boundary
coincides approximately with the 50 iso-
therm (line of 50 average temperature) for
the warmest month. Here during much of the
winter the sun is constantly below the hori-
zon, so that darkness prevails and cold is in-
tense and long-continued. Moreover, while
in summer the sun may never set and con-
stant daylight prevails, the sun is never far
above the horizon, and its oblique rays
deliver little energy at the earth's surface.
In the Southern Hemisphere the only ex-
tensive nonoceanic area with polar climates is
the Antarctic Continent, the approximate
center of which is at the South Pole. Since
the Arctic is almost a landlocked sea, except
for the frozen ocean, the polar climates there
are confined to the northern borders of
Eurasia and North America and to the island
continent of Greenland.
270
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
TYPE Tundra (ET)
IN.
28
26
24
22
20
18
16
14
12
10
8
6
4
2
PLACE Barrow, Alaska
F
100
90
80
70
60
50
40
30
20
10
-10
-20
-30
-40
-
/
/
s
S
/
s
s
/
>
\
/
s
/
S
Si
^
/
i
>
\
^s
.
I
F
M
A
M
I
A
S
N
D
FIG. 14.14 Data of a tundra station at a
severe continental location. Note the large annual
range of temperature and the meager precipitation.
TYPES
Polar climates may be divided into two
types, tundra and icecap, with the warmest-
month isotherm of 32 serving as the bound-
ary between them. Where the average tem-
perature of all months is below freezing
(32), vegetation is absent and a permanent
snow-and-ice cover prevails. Such locations
have icecap climate. Where 1 or more months
in the warmer period has an average temper-
ature above 32 (but not over 50), there is
tundra climate.
TUNDRA CLIMATE
Tundra climate (.ET) 3 on land areas is al-
most exclusively limited to the Northern
Hemisphere, for in the Southern Hemisphere
oceans prevail in those latitudes where tundra
climate normally would develop. The most
extensive tundra areas are the Arctic Ocean
margins of both Eurasia and North America
and the coastal borders of Greenland (Fig.
11.2, Plate 2).
Temperature A long cold winter and a
very short and cool summer are the rule in
tundra climate (Fig. 14.14). With the average
temperature of the warmest month between
32 and 50 by definition, even the warmest
months are raw and chilly, resembling March
and April in southern Wisconsin, or January
in the Gulf states. Killing frosts may occur at
any time, although it does not freeze on most
July nights (Fig. 14.15). The continuous but
weak summer sun frees the land of its snow
cover for a few months, but the subsoil re-
mains frozen, so that the surface remains
3 In the Koppen system T warmest month below 50
(10C) but above 32 (0C).
Climatic Data for Representative Tundra Stations
JFMAMJJASOXD
Yr
Range
Sagastyr, Siberia, Union of Soviet Socialist Republics (73 N, 124E)
Temp.
Precip.
_34 -36 -30 -7 15 32 41 38 33 6-16 -28
0.1 0.1 0.0 0.0 0.2 0.4 0.3 1.4 0.4 0.1 0.1 0.2
1
3.3
77
Upernivik, Western Greenland (73N, 56 W)
Temp.
Precip.
-7 -10-6 6 25 35 41 41 33 25 14 1
0.4 0.4 0.6 0.6 0.6 0.6 1.0 1.1 1.0 1.1 1.1 0.5
16
9.2
51
Humid microthermal, polar, and highland climates 271
FIG. 14.15 Daily maximum and minimum temperatures for the
warmest and coldest months at a tundra station in Greenland.
wet and poorly drained. Tundra vegetation
consists of lichens, mosses, sedges, and
bushes.
Precipitation Given the low tempera-
tures of these high latitudes, the modest pre-
cipitation, usually less than 10 or 15 in., is
not surprising. Likewise anticipated is the
concentration of the year's precipitation,
nearly all of it cyclonic in origin, in the
warmer months of the year when the humid-
ity content of the air is highest (Fig. 14.14).
The meager winter snowfall is dry and pow-
dery in character, so that the strong winds
sweep the level surfaces bare and concen-
trate the snow in depressions and on the lee
side of low eminences. It has been estimated
that 75 to 90 per cent of the surface of the
Arctic tundra lands is nearly free of snow at
all seasons.
ICECAP CLIMATE
Icecap climate (Ef),* the least well-known
of the earth's climatic types, is characteris-
tically developed over the great permanent
continental ice sheets of Antarctica and
Greenland and over the perpetually frozen
ocean in the vicinity of the North Pole.
Only fragmentary climatic data have been
obtained from these deserts of snow and ice
where the average temperature of no month
rises above freezing.
Temperature The average annual tem-
peratures of the icecaps are without doubt
the lowest for any part of the earth. At
Eismitte, located at nearly 10,000 ft elevation
4 In the Koppen system F = warmest month below 32
272
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
Climatic Data for an Icecap Station (EF), Eismitte (Wegener),
Interior Greenland (7054'N, 4042'W, 9,941 ft)
/
F
M
A
M
J
J A
S
N D
Yr
Range
Temp.
Precip.
-42 -
No data
53
-40
-24
-4
4
12 1
-8 -32
-46 -37
-22
65
on the ice plateau of interior Greenland,
average winter-month temperatures are be-
low 40 and the warmest month is about
12 (preceding table). During 9 months the
averages are below zero. In all probability
temperatures in inland Antarctica are even
more severe.
Precipitation If little is known about
temperature conditions of icecap climates,
still less information is available about gen-
eral weather conditions and precipitation.
Certainly annual precipitation is meager
(under 5 in., water equivalent), and all of it
falls in the solid form. It appears to originate
in cyclonic storms that pass over the ice
plateaus or skirt their margins. One year at
Eismitte, Greenland, there were 204 days
with precipitation, and there was no con-
spicuous seasonal concentration.
HIGHLAND CLIMATES
There is no such thing as a highland type
or clearly distinct types of climate. Instead,
mountains exhibit an almost endless variety
of climates depending on contrasts in altitude
and in exposure to sun and winds. The val-
ley contrasts with the exposed peak; wind-
ward slopes differ from leeward slopes; and
southern exposures are unlike those facing
north. And each of these climatic contrasts
is in turn multiplied, as it were, by differences
in latitude and continental location.
The resulting great complexity of the
variety of climates within highlands has
caused them to be grouped together under a
single designation on Plate 2. Generally re-
gions characterized by an elevation above
4,000 or 5,000 ft are included: lower regions
are not so climatically different from the sur-
rounding lowlands that they need to be
differentiated from them.
ATMOSPHERIC PRESSURE IN MOUNTAINS
At low elevations the minor changes in air
pressure from day to day, or from season to
season, cannot be directly perceived by the
human body. But the very rapid decrease in
the atmosphere's weight with increasing ele-
vation and the related very low atmospheric
pressure that prevails on high mountains and
plateaus cause the pressure element to be
genuinely important in highland climates.
Physiological effects (faintness, headache,
nosebleed, nausea, weakness) of decreased
pressure aloft are experienced by most people
at altitudes above 12,000 or 15,000 ft.
Sleeplessness is common and exertion is dif-
ficult. Yet mountain sickness is usually a
temporary inconvenience that passes away
after a week or so of residence at high
altitudes.
SOLAR RADIATION AND TEMPERATURE
Solar energy Intensity of sunlight in-
creases with elevation in the cleaner, drier,
thinner air of mountains. This is to be ex-
Humid microthermal, polar, and highland climates 273
Climatic Data for a Highland Station in the Tropics
s
o
N D
Range
Quito, Ecuador (9,350 ft)
Temp. 54.5 55.0 54.5 54.5 54.7 55.0 54.9 54.9 55.0 54.7 54.3 54.7 54.7 0.7
Precip. 3.2 3.9 4.8 7.0 4.6 1.5 1.1 2.2 2.6 3.9 4.0 3.6 42.2
pected, since dust, clouds, and water vapor,
the principal scattering, reflecting, and
absorbing elements of solar radiation in the
atmosphere, are concentrated at lower eleva-
tions. On a clear day probably three-fourths
of the solar energy penetrates to 6,000 ft,
but only one-half to sea level. This greater
intensity of sunlight at high altitudes has an
important effect upon soil temperature, and,
both directly and indirectly, upon plant
growth.
Air temperature Most important of the
climatic changes resulting from increased
elevation is the decrease in air temperature
(on the average, about 3.6 per 1,000-ft rise,
as stated earlier) which occurs in spite of the
increased intensity of solar energy. Quito,
Ecuador, on the equator at an elevation of
9,350 ft (preceding table), has an average
annual temperature of only 55, which is
25 lower than that of the adjacent Amazon
lowlands. Because the rare air at Quito is
incapable of absorbing and retaining much
solar energy, the air remains chilly. Yet for
the same reason the sunlight itself is strong.
The climate is therefore one of cool shade
and hot sun.
Importance of vertical change The verti-
cal rate of temperature change along moun-
tain slopes is several hundred times greater
than the winter north-south horizontal grad-
ient over continental lowlands, a fact that
has important consequences. In the tropics,
where the lowlands are characterized by
continuous and oppressive heat, the cooler
highlands may be so attractive to settlers that
in some regions they become the centers of
population concentration. Such is the case
in much of Latin America. There is a strik-
ing vertical zonation not only of contrasting
climates but also of agricultural and vegeta-
tion belts between tropical lowlands and
highlands (Fig. 14.16). Thus in tropical
valleys where there is a luxuriant rainforest
such heat-requiring crops as rubber, bananas,
and cacao thrive. Somewhat higher they may
give way to an economy based on coffee, tea,
maize and a variety of food crops. On the
still higher and cooler slopes middle-latitude
cereals and potatoes become more important,
as does the grazing of animals, the natural
pastures for which are terminated along their
upper margins by the permanent snow fields.
By contrast, highlands in middle latitudes
are less attractive climatically and have fewer
vertical zones of contrasting vegetation and
agriculture (Fig. 14.16). Here even the low-
lands are none too warm, so that any reduc-
tion in temperature with altitude, resulting
in a cooler summer and a shorter growing
season, materially reduces the opportunities
for agricultural production.
Diurnal and seasonal temperatures The
thin, dry air characteristic of mountains and
high plateaus permits not only the entry of
strong solar radiation by day but also the
rapid loss of earth energy at night, resulting
in rapid heating by day and rapid cooling
274
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
18,000 ft
18,110
. . . Upper altitude limit*
17,000
/\
Lower altitude limits
16,000
/ \
Zone of
permanent
15,000
/ \
snow
14,000
/Snow line \14,600
Zone of Alpine -
13,000
/ i Pine \nilft
meadows
12,000
/ fir \
7/\rm nf
11,000
/ Broadleaf-trees \1 1,400
uncleared
A"' 800 1 Zone of
10,000
/Wheat 10,100^
forests
/\
^perma-
nent
9,000
/Maize and beans 9,900 \
Xsnow linc\9 200
snow
-WL_Whea
7,000
*. /-Apples 8,200 \
Tierra Fria or
/ \
Zone of
alpine
/ \
zone of the
grains
/ Spruce \7,500
meadows
and dwarf
trees
6,000
/ Manioc 6600 \
/- Fir and pine \6,500
j~Complete forest cover "^\t2QO
JL.., Wheat and apples-------------- 6'000--------X
5,000
/ Sugarjand bananas S^OO
V
/Ssti!s--B55; HJjpl 1
Forest
zone
4,000 /
f Coffee 5,000
Tierra Templada
/ Darle y Rye 'i$-\
I u/u o->t anrl fliir ll ftflfl ., .I,.II,,M
,
nirn 1 Afif\
or zone of
.__*__
1-i
3,000 / -
Coffee*
2000 /
raran.nlantatinne n IAA
\
/- Maize 2,500
Agri-
kultural
Iffl^ii:
r
Rubber and banana plantations 1,300
Coffee* i;oOO
.Tierra Caiiente
/
zone\
tropical crops\
/ J
\
FIG. 14.16 Vertical temperature zones and the altitude limits of certain
crops and types of vegetation on a tropical mountain (left) and a
middle-latitude mountain (right). (From Sapper.)
by night. Thus large diurnal ranges of tem-
perature are characteristics of highland cli-
mates (Fig. 14.17).
At high altitudes in tropical highlands this
results in numerous days on which night
freezing and daytime thawing occur, and this
frequent and rapid oscillation between the
two has a marked effect upon vegetation and
soil characteristics. The great temperature
difference between day and night in tropical
highlands stands in contrast to the very
small difference between the average temper-
atures of the warmest and coldest months,
or the annual range. One of the distinctive
features of high plateaus and mountains in
the tropics is this combination of a large
daily and a small annual range of temperature.
Although the thermometer stands lower on
a tropical mountain than it does on an ad-
jacent lowland, the two locations are alike
in having uniform daily and monthly mean
temperatures and small annual ranges.
Monotonous repetition of daily weather be-
longs alike to tropical highlands and plains
(Figs. 11.5, 14.17). For instance, at Quito
the temperature difference between the
warmest and coolest months is only 0.7,
which is very similar to the difference in the
Amazon lowlands in the same latitude (Fig.
14.18). Mexico City at 7,474 ft has an aver-
Climatic Data for a Representative Highland Station in Middle Latitudes
J F M A M J J A S O N D Yr Range
Longs Peak, Colorado (8,956 ft)
Temp. 23 22 26 33 41 51 55 55 48 39 31 24 37 33
Precip. 0.7 1.2 2.0 2.7 2.4 1.6 3.6 2.2 1.7 1.7 0.9 0.9 21.6
Humid microthermal, polar, and highland climates 275
Temp.
Day 5
10
15
20
25
AREQUIPA, PERU, 7550 ft
20
FIG. 14.17 Daily maximum and minimum temperatures of the
warmest and coldest months at a tropical mountain station located at a
moderate altitude. Note the diurnal regularity of temperature change
indicating sun control. Diurnal range is greater in July, the dry season, when
the least cloud is present.
age annual temperature 17 below that of
Veracruz, in the same latitude but on the
coast; yet their annual ranges are almost
identical 11.5 and 11 respectively. One
climatologist has tersely described this tem-
perature relationship between lowlands and
highlands as follows: "The pitch changes;
the tune remains the same."
Highland climates in the tropics are unique
among cool or cold climates in having a
small annual range of temperature.
Farther away from the equator the annual
range characteristic of highlands increases in
magnitude, another similarity to lowlands. In
fact, the annual range for highland stations
and lowland stations in similar latitudes is
approximately the same.
INCREASE OF PRECIPITATION IN MOUNTAINS
AND ITS IMPORTANCE IN DRY CLIMATES
Precipitation is heavier in highlands than
on the surrounding lowlands, as was ex-
plained in the discussion of orographic pre-
cipitation in Chap. 9. This is a fact of great
importance, especially in dry climates. There,
no matter what the latitude, this heavier pre-
cipitation (including snowfall) of highlands is
critical. Not only are settlements attracted to
the humid slopes and to the well-watered
mountain valleys, but streams, descending
from the rainier highlands, carry the influence
FIG. 14.18 A comparison of the annual
marches of temperature at Iquitos, a tropical lowland
station in Peru, and at Quito, a tropical highland
station in Ecuador. Note the generally lower tempera-
ture at Quito. On the other hand, a small annual
range of temperature is characteristic of
both stations.
80
70 9
60
50f
40f
QUITO, ECUADOR. 0.10 S., 9350 feet
Annual march of temperature
JFMAMJJASOND
276
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
of highland climate far out on the dry low-
lands. There the mountain waters (including
meltwater from the snow fields) are put to
multiple uses irrigation, power development,
and, in some places, transportation.
In addition, mountains in regions of
drought, because they are ?f islands" of heavier
precipitation, are likewise islands of heavier
vegetation cover, and sometimes of more
abundant agricultural production as well. In
both arid and semiarid lands, highlands are
likely to bear a cover of forest in contrast to
the meager grass and shrub vegetation of the
surrounding drier lowlands.
WINDS AND WEATHER
On exposed mountain slopes and summits,
where the effects of ground friction are re-
duced, winds are strong and persistent. By
contrast, protected mountain valleys may be
particularly quiet areas. In general, highland
areas are particularly subject to numerous
local winds and accompanying weather, oc-
casioned by the great variety of relief and
exposure present. Common in valleys and
along heated slopes is the upslope wind by
day, when convection is at a maximum, and
the downslope wind at night. Where well-
developed cyclonic storms are present, as in
the middle latitudes, the passing low-pressure
system may induce a downslope wind, non-
diurnal in character, known as the foehn or
chinook. Such winds are characterized by
great dryness and unseasonable warmth.
In highlands the weather changes within a
24-hr period are likely to be greater than on
adjacent lowlands. Violent changes from hot
sun to cool shade, from chill wind to calm,
from gusts of rain or possibly snow to intense
sunlight these give the daily weather an
erratic nature. Even in the tropics the complex
sequence of weather within a day stands in
marked contrast to the uniformity of the
average daily and monthly temperatures.
SELECTED REFERENCES
Blair, Thomas A.: Climatology: General and Regional, Prentice-Hall, Inc., Englewood Cliffs,
N.J., 1942.
Critchfield, Howard J.: General Climatology, Prentice-Hall, Inc., Englewood Cliffs, N. J., 1960.
Haurwitz, Bernhard, and James M. Austin: Climatology, McGraw-Hill Book Company, Inc.,
New York, 1944.
Koeppe, Clarence E., and George C. De Longe: Weather and Climate, McGraw-Hill Book
Company, Inc., New York, 1958.
Miller, Arthur Austin: Climatology, 8th ed., E. P. Button & Co., Inc., New York, 1954.
Trewartha, Glenn T: An Introduction to Climate, McGraw-Hill Book Company, Inc., New
York, 1954.
Trewartha, Glenn T.: The Earth's Problem Climates, University of Wisconsin Press, Madison,
Wis., 1961.
C HAPTER 15
Water and
the seas
WATERS OF THE EARTH
Of all the substances that are familiar to
nan, probably none is more vital, more
ibiquitous, or more changeable under ordi-
lary conditions than water. Water covers
nore than two-thirds of the solid earth,
t occurs in quantity beneath the solid sur-
ace, and it is a normal constituent of the
itmosphere above the surface. Unlike any
)ther substance that occurs commonly near
he earth's surface, water may occur as a
iolid, liquid, or gas within the range of tem-
>eratures that are normal there.
THE HYDROSPHERE
It is sometimes convenient to refer to all oc-
currences of water on, above, or beneath the
earth's surface collectively as the hydrosphere,
in analogy to the atmosphere) or gaseous
envelope of the earth, and the lithosphere, or
solid crust of the earth. The term is not
wholly satisfactory, for the three "spheres"
interpenetrate one another. In this book,
therefore, it has been felt proper to discuss
much of that portion of the hydrosphere
277
278
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
which is ice as a part of the lithosphere, and
most of that which is vapor as a part of the
atmosphere. However, because water may
pass freely from one place and form of
occurrence to another, it is also valuable to
consider the hydrosphere in its entirety as a
distinct set of closely interlinked phenomena.
The fact that consideration of the hydrosphere
has, almost of necessity, been somewhat frag-
mented in this book in no way denies the im-
portance or the fundamental unity of the
hydrosphere. Instead it emphasizes the fact
that each element of the earth's physical
geography exists not by itself but in close re-
lationship with the others, so that even in a
survey study the individual elements cannot
and should not be treated in complete
isolation.
IMPORTANCE AND
OCCURRENCE OF WATER
It would be difficult to overemphasize the
importance of water in physical geography. It
is one of the three major elements of weather
and climate; it is the prime agent of degrada-
tion and aggradation; and it is an indispensa-
ble ingredient of the natural environment for
the existence of life. Its place in the organic-
complex is well summarized as follows:
"Where there is life there must be water. There is
no organism today, plant or animal, which is not
highly dependent on it. ... A seed will not sprout
without water. Indeed the cells which make up a
seedling ... are largely water. Water has a basic role
in the formation of the protein molecule, the funda-
mental material for all living matter, plant and ani-
mal. No less than light it is essential to photosyn-
thesis, the biochemical process by which . . . plants
obtain the principal raw materials for their growth. . . .
Apart from fat, the tissues of all animal bodies are
70 to 90 per cent water." *
1 Edward A. Ackerman, Water Resources in the United
Slates, reprint 6, Resources for the Future, Inc. 1958, p. 2.
Man, being an animal organism, must of
course have water to sustain his own life and
to produce those organisms he uses for food.
To these uses, man has added a long list of
others. In our modern industrial and com-
mercial society a large supply of water, far
beyond that needed for drinking and for
nourishing plants and other animals, has be-
come a virtual necessity. There is no more
important economic resource, and to meet all
the demands on this resource a large, con-
tinuous supply must be available.
Even if the water held in chemical bond
in the materials of the deep interior is ignored,
there is a vast amount of water in the hydro-
sphere. In the outermost 3 miles of the earth
there is three times as much water as all
other substances put together, and six times
as much as the next most abundant com-
pound, feldspar.
As the table (p. 279) shows, most of the
water near the earth's surface is in the great
ocean reservoir. Less than 8 per cent exists
elsewhere. An exceedingly small portion is
in the atmosphere as vapor, as cloud-forming
droplets, or as ice particles, and the re-
mainder occurs as liquid or ice on and
beneath the ground surface.
THE CYCLICAL
BEHAVIOR OF WATER
The hydrologic cycle Practically all
the water near the surface of the earth is in
some sort of vertical motion, a result of a
vast, continuous distilling process called the
hydrologic cycle. (Though not named there,
the hydrologic cycle was briefly discussed in
relation to precipitation in Chap. 9.) Wher-
ever water and the requisite energy meet,
some of the water evaporates and enters the
altmosphere. This happens at the soil surface,
Water and the seas 279
Approximate Distribution of the Waters of the Earth in Cubic Miles of
Liquid Equivalent*
In the lithosphere:
Above sea level 1,085,000
Below sea level to a depth of 2^ miles 1,255,000
In the subcrustal zone 19,400,000
On the lithosphere:
In soil, plants, and animals 3,400
In lakes and streams 53,000
In icecaps and glaciers 3,200,000
In the ocean basins 315,000,000
In the atmosphere:
In solid, liquid, and vapor 3,600
Total 340,000,000
* Estimated from various sources.
the surfaces of plants and animals, and the
water surfaces of lakes and streams, but
mostly at the ocean surface. Insolation is the
ultimate source of energy for these processes,
which together are called evapotrampiration.
As a constituent of maritime air masses the
vapor thus fonned moves with the advec-
tional and convectional currents of the atmos-
phere, wherein it condenses, sometimes as
dew, but mostly aloft as water droplets or ice
particles. Some of these coalesce and fall as
precipitation. A portion returns to vapor
again before it reaches the earth's surface.
The rest falls directly back into the ocean
reservoir or onto the land surface.
Much of the water that falls on the land is
destined to return ultimately to the ocean
reservoir, but its movement in that direction
is not likely to be direct. Some falls upon
lake or stream surfaces and thus starts back
immediately, while some is shed on the
ground and must find its way to a stream. In
either case some of this surface water will be
evaporated again before it returns to the
ocean. The stream, responding to the pull of
gravity, may flow either toward the ocean or
toward some enclosed basin on the land. In
such a basin it may collect as a salt lake,
from which it either evaporates again or sinks
beneath the land surface. Where there is no
such topographic interruption to its return to
the ocean, the surface water continues on its
way, sometimes being delayed in a fresh-
water lake or swamp but always moving
downward. On the way some may leave the
stream by sinking into its bed.
A portion of the water that falls on the
land neither evaporates again nor enters a
stream, but instead sinks downward into the
soil. Some of this is taken up by plant roots,
passed upward through the stems, and evap-
orated from pores in the leaves, a process
known as transpiration. A fraction may move
upward by capillary force, but some con-
tinues downward to become part of the
ground-water, i.e., the water that saturates the
cracks and pore spaces of the regolith (un-
consolidated surface material) and bedrock
to the bottom of the fracture zone. Within
this reservoir it moves in response to the pull
280
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
FIG. 15.1 The principal aspects of the hydro-
logic cycle.
of gravity either directly or indirectly as a re-
sult of pressure differences, and consequently
it may move laterally as well as vertically
within the ground. Some of the ground-water
reservoir discharges directly into the ocean,
below sea level, but by far the larger part
drains by seepages and springs to feed the
streams and rivers that flow back to the
ocean.
If, instead of summarizing the general
functioning of the hydrologic cycle, a person
tries to follow the history of a specific mass
of water during a long period, he finds many
complications. For example, a molecule may
start life anew. A minute amount of new
water escapes from the hot interior; this adds
to the supply at the surface, since it is be-
lieved that none is lost to space. Again, some
of the water may become locked up in ice
masses or in mineral crystals, or be trapped
in the pore spaces of sedimentary rocks being
formed at the bottom of the sea, there to re-
main for uncounted years or perhaps even
whole geologic eras. It may thus be held out
of circulation for a time and consequently
change to a small degree the relationship
among the quantities of water in the surface
portion of the hydrosphere enumerated in the
preceding table. However, this does not
greatly affect the short-term geographical
operation of the hydrologic cycle, i.e., the
movement of water via evaporation and pre-
cipitation from the sea to the land and back
which is diagrammatically illustrated in
Fig. 15.1.
Variations over the earth The varia-
tions over the earth in the performance of
the hydrologic cycle are very complicated as
a result of the variations of all the factors
that cause weather and climate to differ from
place to place and from time to time, plus
the variations of vegetation cover, soil and
bedrock character, saltiness of water surfaces
and many other factors. Nevertheless, some
geographical generalizations are possible. It
is estimated that some 60,000 cubic miles of
water is evaporated into the atmosphere each
year, the larger amount of it equatorward of
the middle latitudes because the energy
supply from the sun is greater there. In the
higher latitudes, poleward from about 38, it
is thought that the total precipitation exceeds
the evaporation. Thus there must be a net
zonal transfer of atmospheric water from the
lower to the higher latitudes that is compen-
sated by return ocean flow.
Unfortunately, the measurement of the
amount of water that evaporates from the
land and sea surface is not as easy as the
measurement of precipitation. Consequently
an accurate world map of the distribution of
actual evapotranspiration amounts cannot yet
be made. When it is, it will bear a strong
resemblance to a map of world precipitation.
On the other hand, evaporation from the
water surfaces of subtropical latitudes no
doubt exceeds that from water surfaces in
other parts of the earth, as a consequence of
the greater surface receipt of insolation and
the divergent surface airflow characteristic of
these regions.
At any given latitude those land areas that
have more water on the surface or in the soil
and a denser vegetation cover will furnish
proportionately more water to the atmosphere.
Likewise, the clearer, windier, and warmer
an area, the greater will be the possibility of
transfer. Thus the climatically dry land areas,
Water and the seas 281
generally being clear, windy, and warm, have
a high potential evapotranspiration, but, be-
cause of having little surface or soil water
and meager vegetation, naturally have a rel-
atively low actual rate.
THE SEAS
SIZE AND SIGNIFICANCE
The land surfaces upon which man lives
and from which he derives the greater part of
his sustenance in reality occupy what may
seem a surprisingly small fraction of the
whole surface of the globe. Nearly 71 per
cent of the earth is covered by the oceans,
and all of the land masses are completely
surrounded by water, forming huge islands
in the continuous sea. Moreover, the wide
sea is also deep, its volume being many
times as great as that of the portions of the
continents that lie above sea level. It is the
sea, not the land, that is the prevalent en-
vironment on the earth, foreign though this
is to the human point of view. Though man
does not live in the sea, he has much to do
with it, for it serves him as a route of trans-
port, as a source of food and, increasingly,
of minerals, as a modifier of his climate, and
as a partitioner of his lands. A study of the
earth as the home of man cannot properly
neglect so great and so significant a part of
that earth.
NATURE OF SEA WATER
Sea water is a substance of highly complex
composition. To be sure, only 3.5 per cent
of the substance, by weight, is anything but
pure water, and the greatest part of this
small amount of impurity is common salt
(sodium chloride) which is present in solu-
tion, with most of the small remainder being
dissolved salts of magnesium, calcium, and
potassium. But there are minute quantities of
an immense number of other substances so
many that a complete listing here is impos-
sible. Most of these impurities have probably
been brought into the sea at a very slow rate
by streams, though some may be derived
from other sources. Since evaporation leaves
the salts behind, a gradual concentration of
soluble materials in the sea is occurring.
The degree of concentration of dissolved
salts, called the salinity of the water, varies
somewhat from place to place, being affected
principally by the relative rate of precipita-
tion and evaporation. Heavy rainfall lowers
the surface salinity by dilution; strong evap-
oration raises the salinity by removal of
water and concentration of salts.
The highest salinities in the open sea are
found in the dry, hot subtropics, where evap-
oration is great. Nearer the equator salinities
decrease because of heavier rainfall. In the
cooler middle latitudes salinities are relatively
low because of the decrease in evaporation
and the considerable rainfall. But the varia-
tions are small in the open sea, generally less
than 5 per cent on either side of the mean.
282
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
In coastal waters and nearly enclosed seas,
on the other hand, the salinity often departs
greatly from the mean. In hot, dry, nearly
isolated seas such as the Red Sea and
Persian Gulf, the salinities reach very high
figures because the water in them is subject
to strong evaporation but cannot mix freely
with less saline waters from the depths of the
open sea. On the other hand, in the neigh-
borhood of the mouths of large rivers or in
nearly enclosed seas into which large rivers
flow, such as the Black Sea and the Baltic
Sea, the dilution by fresh water reduces the
salinity to relatively low figures.
The mean density of sea water is slightly
higher than that of pure fresh water because
of the presence in sea water of dissolved
salts, and the density becomes greater with
increasing salinity, as well as with decreasing
temperature. Therefore very saline or very
cold surface water tends to sink and to be
replaced by water from beneath. Thus in
middle-latitude waters, where winter cooling
of the surface is extreme, there commonly
occurs during the winter an "overturning,"
with the chilled surface waters becoming
cold enough to sink, while slightly warmer
waters from below come to the top. Where
warm and cold surface waters meet, the
colder sinks beneath the warmer.
MOVEMENTS OF OCEAN WATERS
WAVES
Waves are the smallest and most localized
of the several kinds of movements in which
the ocean waters are involved. Most waves
are originated by the wind, though they may
continue to travel beyond the area stirred by
the wind and long after the wind has ceased
to blow. The importance of waves lies
chiefly in their effect upon the operation of
seagoing ships and, more pertinent to the
present discussion, in their function of
eroding along the coasts and distributing the
eroded material.
In deep waters, with low or moderate
wind velocities, wave movements are smoothly
progressive, with each water molecule de-
scribing essentially a circle as the wave im-
pulse passes. The water rises on the front of
the wave, moves forward as the crest passes,
drops down the rearward slope, and moves
backward in the succeeding trough (Fig. 15.2).
However, with high wind velocities the crest
of the wave is tipped forward and breaks,
forming a whitecap.
The height of waves in the open sea
appears to depend upon the velocity of the
wind, the length of time the wind has blown,
and the distance the wind has driven the
waves across the surface. Up to a certain
point, the height becomes greater with in-
creasing values of each of these controls.
Near the shore, where the depth of water
decreases, an approaching wave is slowed by
friction from below. The crest rises, steepens,
and finally crashes forward as a breaker,
which may hurl tons of water against the
bottom or, if close enough inshore, against
the land (Fig. 3.31). It is here that the
erosional effect of the waves, which was
described in Chap. 3, is greatest.
THE TIDES AND THEIR CAUSES
Nearly all shores of the open seas ex-
perience the distinct periodic rises and falls
of sea level known as the tides. Like most
familiar natural phenomena, they have been
known and studied from very early times,
and along the way have become a symbol of
the certainty and inflexibility of natural
processes. But understanding of how and
why the tides vary from place to place has
Wafer and the seas 283
Forward at
crest of wave
Backward in
trough of wave
FIG. 15.2 How a water particle moves in a circle during the passage of
a wave in the open sea.
been slow in coming, and even now it is not
securely grasped. The basic factors that pro-
duce the tides are not especially obscure, but
the actual mechanism of tidal activity on the
earth is exceedingly complex. A full discus-
sion of the causes of the tides is beyond the
scope of this book, but they will be briefly
summarized here.
The moon and the tides The principal
tide-producing forces are (a) the gravitational
attraction of the moon and (b) the centrifugal
force of the earth's revolution about the
center of gravity of the earth-moon system. It
may be shown that the first of these forces is
directed toward the moon and is strongest
on the side of the earth toward the moon,
while the second force is directed away from
the moon and has the same strength at every
point on the earth. At the center of the earth
the two forces are in balance. On the side
toward the moon, the lunar attraction exceeds
the centrifugal force, producing a net pull
toward the moon. On the opposite side of
the earth, the centrifugal component exceeds
the gravitational, producing a net force di-
rected away from the moon (Fig. 15.3). As
the earth rotates within this field of forces,
the waters of each small area of the oceans
are subjected to successive pulls away from
the center of the earth which reach maximum
strength at intervals of about 12 hr, 25 min,
the time it takes for earth rotation to carry a
point on the surface from a position facing
the moon to a position opposite the moon.
Because the seas are not continuous but
form a series of interconnected basins of
many shapes and sizes, the tides do not
actually behave as simple progressive bulges
moving westward on the earth as it rotates
toward the east. Instead, this type of move-
ment appears to be combined with various
oscillatory or swashing movements of the sort
that may be produced by tilting or swinging
a basin full of water. Each major ocean basin
and bordering sea has its own pattern and
style of tidal movement, and movements set
up in each of two adjacent seas may, in the
zone between them, interfere with and affect
one another. The result is extreme variety
from place to place in height of successive
tides, intervals between tides, amount of rise
and fall, and various associated phenomena.
In most places there are two high tides per
day, at approximately the expected 12 hr, 25
min interval, and the amount of change is
usually a few feet. Along most Atlantic
shores successive high tides are roughly of
equal height, while in the Pacific every second
284
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
Centrifugal
exceeds
gravitational
Gravitational
exceeds
centrifugal
To MOON
Centrifugal force of revolution
of Earth Moon system (directed
away from Moon )
Gravitational attraction
of Moon (directed toward
Moon)
FIG. 15.3 The principal forces involved in the production of tides.
high tide is distinctly lower than the preced-
ing one (Fig. 15.4). On some shores, notably
parts of southern Asia and the Caribbean
and Gulf Coasts of North America, there is
but one high tide per day.
Tidal range The average difference in
water level between low and high tide at a
given place is called tidal range. Common
tidal ranges on exposed coasts vary from 5 to
10 ft. In sheltered waters, such as the Gulf
of Mexico and the Caribbean Sea, the range
is usually less than 2 ft; and in nearly en-
closed seas, such as the Mediterranean and
the Baltic, the tides are so slight as to be
negligible. In a few localities, mostly in
funnel-shaped bays and estuaries, remarkably
high ranges occur, increasing toward the
head of the inlet. Liverpool has a range of
29 ft, and the head of the Bay of Fundy,
Nova Scotia, sometimes experiences tides of
more than 50 ft.
The sun sets up tides in precisely the
same manner as the moon; however, these
solar effects are much weaker, and in fact
appear not as separate tides but simply as
modifications of the lunar tides. At times of
new moon and full moon, when the earth,
FIG. 15.4 The intervals and amounts of rise
and fall of the tide at Honolulu during a 48-hr
period. Note the alternation of smaller and larger
tidal rises.
h
12
lg h Q h 6 h 12 h 18 h 2 4 h
Feet
1
1
i MM nun
s\
1 1 1 1 II 1 1 1 1 1
A
April 22
/ \
April 23
t
/ v
i
/\
/
^ /\J
/
2
^ X-,
Afltr H A. Marm*r
Water and the seas 285
moon, and sun are nearly in line, sun and
moon tides reinforce one another, producing
unusually high tidal ranges. These tides,
referred to by the misleading term of spring
tides, recur every 2 weeks. At intervening
times, when the sun and moon tides are at
odds with one another, unusually low tidal
ranges, called neap tides, occur.
Tidal phenomena are important to the use
and development of harbors. Currents of
considerable strength are often set up by the
tides in narrow inlets and bays, and these
may cause hazards to the handling of ships
in harbor entrances and about docks. In
ports where the tidal range is large it is
sometimes necessary to construct docking
basins with lock gates to maintain water
levels high enough to keep moored vessels
afloat at low tide, as well as to decrease the
inconveniences of change of level in loading
and unloading.
OCEAN DRIFTS AND CURRENTS
The waters of the oceans, even if wave
movements are neglected, are not stationary,
but take part in a broad system of continu-
ous circulation that involves practically the
entire water mass. The pattern of movement
is three-dimensional, but the deeper parts of
the system will not be considered here.
Much the larger part of the surface move-
ment of ocean waters is in the nature of
a slow, relatively inconspicuous transfer, at
an average rate of 2^4 miles per hour, that
affects only shallow depths. The slowest
movements are more correctly spoken of as
drifts, in contrast to the deeper and more
rapidly flowing currents that sometimes attain
velocities two or three times the above
average. Such rapid currents are usually con-
fined to localities where discharge takes place
through narrow channels. An example is the
"60
50
40
20*
ulO
20
30
40 d
50
K60
O
c7O
FIG. 15.5 Generalized scheme of ocean
currents.
Florida Current, which achieves velocities of
4 to 6 miles per hour in making its exit from
the Gulf of Mexico through the narrow strait
between Florida and Cuba.
General scheme Except for the polar
seas, there is a tendency for all the great
oceans to exhibit similar general patterns of
surface currents and drifts. This is because
surface ocean currents are fundamentally re-
lated to the direction of the prevailing wind,
though temperature and salinity differences
and the shape and depth of the ocean basins
also affect the pattern.
The most conspicuous elements of the
circulation of surface waters are great, closed
286
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
elliptical whirls about the subtropical oceanic
high-pressure cells (Fig. 15.5). The trade
winds on the equatorward sides of the sub-
tropical highs in both hemispheres tend to
drift the surface waters westward before them
across the oceans in what is known as the
Equatorial Current. (There are really two
equatorial currents, separated in the eastern
part of the oceans by a minor countercurrent
setting toward the east.) Checked in its west-
ward progress by a continent, the Equatorial
Current is divided, part of it flowing north-
ward and part of it southward. Because of
the rotational deflection and the trend of the
coast line, and because of the wind direction
around the western end of the subtropical
high, the warm poleward-moving current
gradually is bent more and more to the east.
At about latitude 40 the warm surface
waters move slowly eastward across the
ocean as a west-wind drift. In the eastern
part of the sea the drift divides, a part of it
being carried by the winds equatorward along
the coast until it again joins the Equatorial
Current and thus completes the low-latitude
current circuit. In the Northern Hemisphere,
however, a considerable portion of the west-
wind drift is carried poleward by the stormy
southwesterlies, its relatively warm waters
washing the west coasts of the continents and
eventually entering the Arctic Ocean. The
Arctic, compensating for this receipt of
warm water, produces an outward flow of
cold water that passes down the western side
of the ocean into the middle latitudes. In
the Southern Hemisphere much of the west-
wind drift continues clear around the earth
in the unbroken belt of ocean that occupies
the southern middle latitudes.
It should be emphasized that this picture
of surface currents and drifts is greatly sim-
plified, for superimposed upon this general-
ized average pattern are numerous eddies and
surges, together with changes in direction
and strength of currents following the seasonal
shifts and reversals of winds.
Yet if the idealized pattern is compared with
a somewhat generalized map of actual sur-
face currents, it will be seen to correspond
fairly closely (Fig. 15.6). In the Atlantic and
Pacific Oceans the subtropical whirls, west-
wind drifts, and Arctic currents are clearly
distinguishable. In the Indian Ocean, though,
only the Southern Hemisphere pattern is well
developed.
Warm and cool currents If it is kept
in mind that poleward-drifting surface waters,
since they come from lower latitudes, are in-
clined to be warmer than the surrounding
waters, while those from higher latitudes are
likely to be cooler, the following generaliza-
tions will be understandable. In the latitudes
equatorward from about 40 warm ocean
currents tend to parallel the eastern sides of
continents, cool ocean currents the western
sides. In the latitudes poleward from about
40 the reverse is more often the case, warm
ocean currents affecting the western sides of
land masses, and cool ones the eastern sides.
Along east coasts (western sides of oceans),
therefore, there is likely to be a convergence
of contrasting currents, while along west
coasts currents tend to diverge.
A part of the cool water along west coasts
in lower latitudes those of Peru and northern
Chile, northwest and southwest Africa, and
southern California, for instance is the re-
sult of upwelling from depths of several
hundred feet along the coast. In these areas
equatorward-moving winds from the sub-
tropical whirls drive the surface waters
toward lower latitudes and away from the
land. Colder water from below, therefore,
rises to replace the surface water.
Water and the seas 287
Warm currents
Cool currents
/ ry ' / / $3 { T^ ' "*>
(WEST/ WIN!}/ DRIFT,- , ' f .
FIG. 15.6 Surface currents of the oceans. (After G. Schott.)
SEA-SURFACE TEMPERATURES
Surface temperatures of the seas range
from about 28.4, which is the approximate
freezing point of sea water, to about 86.
This is a much smaller range of values than
is experienced on the lands, very low tem-
peratures not being present in the seas (unless
temperatures of the polar ice be included).
In addition, changes in the temperature of
the sea surface during the year are remarkably
small, amounting to no more than 2 to 7 in
tropical waters and 9 to 15 in the upper
middle latitudes; and variation in the surface
temperature between day and night is no
more than a fraction of a degree. This con-
siderable variety of temperatures, combined
with the slight change from day to day and
the only modest variation during the entire
course of the year, constitutes a fact of great
importance to the earth's climate.
The heating process The chief process
by which the sea is heated is absorption of
radiation from the sun and the atmosphere;
cooling of the sea is accomplished largely by
radiation from the surface and by evapora-
tion of water from the surface. Since incom-
ing radiation decreases from the tropics
toward the poles, and since the greatest cool-
ing of the seas by radiation occurs in the
higher latitudes, especially during the winter,
it is not surprising to find that the ocean
temperatures follow essentially a latitudinal
pattern (Fig. 15.7). The tropical seas are
warm, with only small variations from place
to place. Poleward of the tropics, however,
temperatures fall off rapidly with increasing
latitude.
The fact that the sea-surface isotherms do
not strictly follow the parallels of latitude in-
dicates, of course, that radiation, which does
have a clear latitudinal pattern, is not the
only control. Prevailing air temperatures
have an effect also, particularly in lowering
288
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
MEAN ANNUAL
TMPERATURE
OF THE
SEA SURFACE
FIG. 15.7 Surface temperatures of the oceans. (After G. Schott.)
sea temperatures near the eastern coasts of
the continents in the middle latitudes during
the winter.
Much more important, however, is the cir-
culation of ocean water in the great surface-
current systems previously described, by
which warm water is brought into the middle
latitudes on the western sides of the oceans and
moved across to the eastern sides, and by
which cool water is brought equatorward on
the east sides of the subtropical oceans and
on the opposite sides in the high latitudes.
The effects of these movements of the
ocean waters may be seen in Fig. 15.7. As
there shown, the average sea temperature on
the coast of southern Japan, washed by a
warm current, is nearly 10 warmer than
that in southern California, in the same lati-
tude but washed by a cool current reinforced
by upwelling. Between Labrador, flanked by
a cold Arctic current, and the northern part
of Ireland, in the path of the warm west-wind
drift, the difference is more than 15 in Au-
gust, and during the winter it is nearly twice
that.
Ocean temperatures and climate
While the relationships between the oceans
and the various climatic elements have been
discussed earlier in the book in the chapters
dealing with specific climates, certain of the
more significant connections may conven-
iently be recalled and treated generally here.
Because of the great width of the oceans,
air masses passing across them are in contact
with the water surface for considerable
periods of time. This gives the surface layers
of the atmosphere a good opportunity to as-
sume temperatures that are approximately
those of the sea surface itself, or at least the
temperature of the air will tend toward that
goal. When air that has been so modified by
a long sea crossing passes onto an adjacent
continent, it carries the sea temperatures
along with it.
Since the ocean temperatures tend to be
relatively mild and to change but little from
winter to summer, a corresponding mildness
is a distinguishing characteristic of the tem-
peratures of those land areas into which sea
air is regularly carried. This effect is especially
noticeable on west coasts in the middle lati-
tudes, where the prevailing onshore move-
ment of marine air imparts much warmer
winter and cooler summer temperatures than
are characteristic of the interiors or eastern
sides of the continents. Thus at San Fran-
cisco the average temperature of the warmest
month is only 60, and that of the coldest
no lower than 49.
The more or less foreign temperatures
brought by ocean currents to any given
latitude are variously reflected in coastal cli-
mates. Thus in northwestern Europe, where
westerly winds carry the effects of the warm
North Atlantic Drift far into the continent,
coastal temperatures in January are 30 to 40
warmer than the average for those latitudes.
Where currents of contrasting temperatures
converge, as along the middle-latitude east
coasts of Asia and North America, the sharp
gradient in sea temperature is to a small
degree reflected in a similarly abrupt gradient
in air temperature along the coast. Cool-water
coasts in the subtropics are often foggy be-
cause warm air from over the ocean proper
is chilled to below the condensation point by
passing over the cool current near the shore.
PRI NCI PAL CLASSES
OF SEA LIFE
The myriad forms of life that exist in the
sea may, for present purposes, be divided
into three major groups according to their
mobility. Most familiar are the free-swimming
forms, which include the larger fish, Crus-
tacea, and sea mammals that are able to move
about over considerable distances in search of
food. A second group is made up of the
sessile forms, those plants, shellfish, corals,
etc., that are more or less permanently at-
tached to the bottom. The third and perhaps
Water and the seas 289
least familiar group is the plankton. These
are small, sometimes microscopic organisms,
both plant and animal, that, either because
they have no means of locomotion or because
they are so very small, are incapable of self-
determined movements of any significant
scale. Instead, they drift with the water in
which they live.
Plankton The plant plankton are the
most fundamental source of food for sea life
in general. The animal plankton and all
other forms of sea creatures feed either
directly upon these tiny plants or include in
their food other sea animals which do feed
upon the plants. Remove the plant plankton
from the sea and all other forms of life would
soon perish. Thus where the plant plankton
are concentrated, there also will be found the
greatest numbers of sea animals, feeding upon
the plants or upon one another. Any localized
conditions favoring the concentration of
plant plankton will tend to concentrate
animal plankton, fish, shellfish, and sea
mammals as well.
The needs of plant plankton are the same
as those of plants in general: light, and cer-
tain mineral and organic nutrients. The first
requirement confines them to the surface
layers of the sea, into which light can pen-
etrate. The second is manifest chiefly in a
need for constant replenishment of the
nutrients, to make up for the supply that has
been removed from the waters by the earlier-
existing generations of plankton.
This renewal of nutrients must come largely
from the waters below. Hence any process
that brings deeper water to the surface will
favor the maintenance of a dense plankton
growth. This is significantly accomplished
by (a) turbulent mixing of water by wave
action in shallow coastal waters, (b) upwelling
of cold waters along subtropical west coasts,
290
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
FIG. 15.8 The principal commercial fisheries are found in the cool seas
of the Northern Hemisphere, especially along the broader continental
shelves.
and (c) winter overturning of waters in the
higher middle latitudes. The surface waters in
areas where one or more of these processes
occur appear to be the centers of plankton
concentration and therefore of sea life
generally.
Fish and sea mammals Following the
pattern of occurrence of plankton, fish appear
to be relatively strongly concentrated on the
continental shelves, in areas of upwelling,
and in waters of the higher latitudes. The
great commercial fisheries line the margins of
the North Atlantic and North Pacific Oceans,
where fish exist in unparalleled abundance
and where there are also populous markets
on the adjacent lands (Fig. 15.8). In these
waters occur such species as the herring,
cod, haddock, halibut, mackerel, salmon,
sardine, tuna, and menhaden. Herring, the
most important of all food fish, occur here in
great numbers and sometimes in remarkable
concentration. In the same areas are found
other forms of sea life including oysters
and other shellfish, crabs and sponges that
are also valuable to man.
The corresponding waters of the Southern
Hemisphere also appear to be rich in sea life,
but the large human populations that are the
other necessary component of commercial
fisheries are wanting. Also, certain of the
most abundant fish of the Northern Hemis-
phere are not found in southern waters,
though conditions are favorable for them.
Herring have been introduced into the
Southern Hemisphere in recent years and
appear to be thriving.
It is commonly believed that the tropical
seas are generally less densely populated by
fish than are the cooler waters of the globe.
Yet while this may be true, the disparity is
not as great as once thought; at least the re-
source is sufficient to permit many peoples
of the tropical coasts to derive much of their
living from the sea. It seems clear that in the
warm waters of the low latitudes there is a
much greater variety of species than in the
higher latitudes, but that the number of indi-
viduals in any single species is likely to be
much smaller. There are, for example, no
known tropical counterparts of the tremen-
dous schools of herring and of sardines that
are found in cooler waters. Interestingly
enough, similar contrasts between tropics,
with more species, and middle latitudes,
with more individuals per species, character-
ize both the animal and plant kingdoms on
the lands.
Most of the large sea mammals, such as
the whales and seals, stay in no one latitu-
dinal area but are wanderers, ranging from
Arctic to Antarctic. Since they, like fishes,
live chiefly upon fish or plankton, they too
Water and the seas 291
seek out the areas where these are concen-
trated. Because of their value for furs, skins,
or oil, many of these animals have been ruth-
lessly hunted and their numbers greatly re-
duced, especially in the Northern Hemisphere.
The whale fishery is now largely in Antarc-
tic waters, and the Arctic fur-seal fishery is
closely regulated to prevent extinction.
The "deserts" of the sea are the interiors
of the great subtropical high-pressure whirls.
In these areas the high evaporation and low
rainfall lead to unfavorably high salinities,
and these saline surface waters converge and
sink. There is a low supply of nutrients at
the surface, and therefore the plankton pop-
ulation is low, and there is no basis for sus-
tenance of a significant population of fish.
CHAPTER 16
The waters
of the land
The occurrence and properties of
water Excepting the water that lies below
sea level beneath the continents and that
portion presently locked up in glaciers, only
about 0.3 per cent of the earth's water is on
or within the land. This tiny fraction of the
total amount of water, moreover, is the only
fresh, liquid water available to the organic
life of the continents, including the life of man.
There are variations over the earth in the
occurrence of this water of the land, and,
because water has extraordinary properties
and is, indeed, indispensable to organic life,
a great share of the differences in physical
292
environment from place to place is a direct
consequence of these variations.
Water is one of the few naturally mobile
substances on the earth, and in consequence
it functions as a great conveyor, moving
massive amounts of material from one place
to another on the land. It can do this because
(a) water can dissolve a larger number of
substances in larger amounts than any other
agent, and (b) its high specific gravity and
internal friction enable it to pick up and
transport materials easily.
Dissolving power Pure water scarcely oc-
curs in nature, for as soon as liquid water
forms in the atmosphere it dissolves carbon
dioxide and becomes weak carbonic acid.
When this acid then reaches the surface of
the earth, it instantly begins to dissolve other
compounds, and these move with the water,
either downward or across the land surface.
Because of this uniquely high solvent power
of water billions of tons of dissolved ma-
terials are constantly in transit. Also, this
quality is ultimately responsible for the chem-
ical characteristics of the sedimentary rocks
mantling most of the earth the rocks which
provide sustenance, in the form of mineral
foods, for a large share of the organic com-
plex on the land, again through the medium
of solution. The land form itself is even in
large measure a result of this solvent quality,
for a considerable share of the processes of
gradation is carried on through the medium
of solution.
Movement of solids Water is also the
primary agent moving solids over the surface:
it is estimated that roughly three times as
much material is moved in suspension in
running water as is carried in solution in it,
and atmospheric and glacial transportation
of suspended material is considerably smaller.
The high surface tension of water allows it to
be held in the regolith by capillary force, the
more finely divided solid aggregate being able
to retain more water than the coarser. This
makes the mass become "liquid" and poten-
tially mobile. Thus water, functioning with
the force of gravity, acts as an important
agent in the mass movements of the land.
An accurate summation of the total trans-
port activity that may be ascribed to the direct
and indirect action of water is beyond our abil-
ity, but the estimates that have been made of it
probably lie in the correct order of magni-
tude. It is reasonable to suppose that an
The waters of the land 293
annual average of some 250 to 300 tons of
solid sediment and 80 to 100 tons of dis-
solved material per square mile of the earth's
land surface are transported to the ocean by
water. No doubt a much larger amount is
simply moved, i.e., temporarily picked up
and deposited again on the land, shifted
laterally through mass movements, or merely
relocated vertically in the regolith. Certainly,
moving water is the most universal and sig-
nificant agent affecting the ever-changing
land surface.
The distribution over the land surface of
the earth of these activities of water varies in
somewhat direct proportion to the quantities
of water available on the land. Although
many other factors are involved at each local-
ity, a map of average annual precipitation
portrays the basic pattern of the variations in
the activity of water as an agent in the move-
ment of materials.
Water as a resource Simply as a vis-
ible and invisible constituent of his natural
environment that affects his comfort and en-
joyment water is, of course, of great interest to
man, but man's concern is greater than that. In
the first place, the very existence of life
requires water, and consequently, little if any
life can exist where only negligible quantities
of it are available. But beyond this, water,
while only one of a long list of minerals used
by man, exceeds all others in the urgency of
its need and in the quantity used.
Increasing needs Until recently a relatively
small amount of water was required to supply
the consumptive needs of man and the other
land-based organisms needs that are clearly
the most important but do not require large
quantities. Beginning with the Industrial
Revolution, however, water steadily in-
creased in significance, until today it is cer-
294
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
B.G.D.
250
200
150
100
50
Estimated total
United States water use
Estimated
1955
use
Irrigation
Steam
electric
Industrial
Domestic
1900 1910 1920 1930 1940 1950
FIG. 16.1 Estimated total United States' water
use from 1900 to 1955, in billions of gallons per
day (B.G.D). (Bated upon estimate of the U.S. Department
of Com met ce in the ff !955 Annual Report of Re\ource<i for
the Future," 1955).
tainly the most used material in our complex
modern life. Quite apart from its uses in
place, that is, for such things as outdoor rec-
reation and surface transportation, it is sought
and utilized for a large variety of home and
industrial needs. It is our major industrial
and home solvent, coolant, and waste carrier;
virtually all manufactured electricity requires
water; and almost all industrial production
requires amazing amounts of it. For example,
a steel mill uses perhaps 65,000 gallons of
water for each ton of steel it produces; each
gallon of gasoline requires some 10 gallons
of water for its preparation, and at least
2,000 gallons of water is used in producing
each pound of rayon. As the population of
the earth grows and industrial and irrigation
needs increase, the use of water is increasing
at a considerably faster rate than population.
Figure 16.1 shows, for example, that in the
United States the requirements for water
have increased more than five times since
1900, although the population has little
more than doubled.
Withdrawal and nonwithdrawal. When
water is made use of in place, the uses are
classed as nonwithdrawal; when it is diverted
from its source the uses to which it is put
fall in the category of withdrawal.
Water for withdrawal use must meet rather
stringent purity requirements, and for most
purposes it must be fresh (free of objection-
able mineral content); and only the water
precipitated from the atmosphere is generally
of that quality. Thus regions of abundant
precipitation usually, although not always,
have ample sources of fresh water close at
hand, and inhabitants may use it lavishly;
in dry regions, on the other hand, sources
FIG. 16.2 Estimated withdrawal use of water
in the United States in 1950, in billions of gallons
per day (B.G.D.). Water used for hydroelectric power
is not included. The municipal value includes water
supplied to industry from municipal water works;
the industrial value is that obtained from private
Sources Only. (From l/.S. Geolv&ical Survey Cinulat 775,
1951.)
B.G.D.
70
60
50
40
30
20
10
n-
i
Industrial Irrigation Municipal Rural
are not ample and it may not be used
lavishly, which is a fact of critical significance.
The actual amount of withdrawal water
used by man naturally varies from place to
place, depending in part upon this varying
availability and in part upon technologic
development and density of human occu-
pance. The total amount used is a meaning-
less figure, but some idea of the magnitude
may be gained from knowing that the aver-
age domestic use of water in the United
States is nearly 65 gal per person per day,
and that even the most primitive living con-
ditions anywhere on earth require at least 5
The waters of the land 295
gal per person per day. Although domestic
use always has the highest priority, it accounts
for only a minor portion of the total amount
used. Withdrawal uses for irrigation, in-
dustry, and steam-power production require
vastly more.
Withdrawal supplies are obtained from the
surface water in streams and lakes, and from
the ground water that exists below the sur-
face of the land. Surface water is by far the
more important supplier since it is usually
easier to obtain. The proportion obtained
from each source for the United States is il-
lustrated in Fig. 16.2.
SURFACE WATER
As indicated in the preceding chapter, in
the operation of the hydrologic cycle some
of the water that results from precipitation
spends part of its stay on the land as surface
water, either in temporary storage close to
the surface or as running water in the streams
that drain a watershed; the remainder departs
from the surface by evapotranspiration proc-
esses or by sinking to lower depths to be-
come incorporated in the ground- water
reservoir; and in turn, some of the water in
the ground-water reservoir drains into the
surface water. Figure 16.3 shows in a dia-
grammatic fashion these important rela-
tionships.
In considering these relationships, it should
be borne in mind that the timing of the hydro-
logic cycle is a complex matter, Thus, since
the hydrologic cycle is in continuous opera-
tion, it is difficult to estimate the volumes of
water that are in each segment of the cycle
at any one time. Also, it is known that there
are significant variations in (a) the total
amount of water involved in the operation of
the cycle at particular times, (b) the propor-
tions in each part of the cycle at specific
times, and (c) the annual regime, i.e., the
timing of the functioning of the cycle, during
a year. Yet just as the variations of the cli-
matic elements result in geographical patterns,
so are there general spatial patterns in the
functioning of the cycle. Our knowledge of
these is much scantier; nevertheless, they
constitute a fundamental part of the physical
geography of the earth.
WATER STORAGE
If a person were able to add with accuracy
the amounts of water that at a particular time
were (a) in the ocean reservoir, (b) in the
ground-water reservoir, (c) in the atmospheric
reservoir, and (d) actually in the process of
draining off the land, and if he then com-
pared this sum with the total amount of
water on the earth, he would find that there
296
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
CONTINENTAL AIR
MASSES
GROUND-WATER RESERVOIR
INFILTRATION AND MOVEMENT
BENEATH GROUND
PRECIPITATION AND
SURFACE FLOW
FIG. 16.3 Some of the major relationships of the surface-water portion
of the hydrologic cycle.
was a significant residual unaccounted for.
The difference consists of the surface water
that is temporarily held in storage on and in
the upper portion of the land, including (a)
the water in the regolith above the saturated
ground-water reservoir, (b) the water in ice
and snow on the surface, and (r) the water in
the tissues of plants and animals. Some of
the surface water is always in such storage
on and in the land, for shorter or longer
periods; this water in storage is always dis-
tributed unevenly over the earth; and at each
place the amount varies from time to time.
The study of this storage-water balance of
the earth is very complicated. There are
good records of the areal and seasonal distri-
bution of precipitation from which the stored
water is derived; but there are not good
records of the amounts in other portions of
the cycle which, if taken together and sub-
tracted from precipitation distribution, would
show how the storage remainder is distributed.
Nevertheless, enough is known to indicate
the general pattern.
Figure 16.4 shows in a generalized fashion
the average distribution by latitudinal zones
of water temporarily detained on the land.
The figure might be said to show the lati-
tudinal variation of the "average wetness" of
the land. The symmetrical distribution shown
there for the two hemispheres is to be ex-
pected, but the latitudinal variations within
a hemisphere show well the effects of (a) the
heavy precipitation of the tropics, and (b) the
marked effects in the higher middle latitudes
of lessened evaporation at all seasons and of
the storage of snow and ice in winter.
The variations during the year of the
70 60 50 40 30 20 10 10 20 30 40 50 60
North latitude South latitude
FIG. 16.4 General distribution of the amount
of water detained in temporary storage on and
near the surface of the land at various latitudes,
expressed in depth per unit area. Values are the
averages of computed monthly totals. Since the
storage water of one month may be carried over to
the next, the graph does not reveal the total
amount Of water involved. (Data from van Hylckania.)
amount of water detained on and in the land
areas at the various latitudes also are sym-
metrical; i.e., similar latitudes have similar
variations. Figure 16.5 shows diagram-
matically for each latitude the season of the
year when most water is detained. In the
higher latitudes late winter and spring are
the seasons of maximum storage, since win-
ter evaporation approaches nil because of
low temperatures, causing a large volume of
water to be locked up in snow and ice.
In the lower latitudes the maximum occurs
during and shortly after the wet season.
SURFACE RUNOFF
Sources and measurement The drain-
age in streams, called surface runoff, comes
from three immediate sources: (a) the rainfall
that remains after losses due to evapotranspi-
ration and infiltration, (b) the water released
from storage, and (c) the water that emerges
from the underground (ground-water) reser-
voir. Considerable variation from place to
The waters of the land 297
place and from time to time occurs both in
amounts of runoff and in the proportion
supplied by each source. Short-term minor
variations result from individual storms,
while seasonal differences result from varia-
tions in the regimes of annual precipitation
and the release of storage water. The propor-
tion of runoff that is discharged from the
ground-water reservoir is subject to the least
fluctuation.
The annual runoff of a watershed (a land-
form drainage area) is measured by the dis-
charge accomplished by its streams, which is
expressed as a volume per unit of time. The
total annual volume may then be divided by
the drainage area to obtain a quotient that
may be expressed as a depth of water, just as
precipitation is expressed, and annual aver-
ages of these values may be mapped. It
should be borne in mind, however, that maps
of average annual runoff provide an even
more generalized picture of variations from
place to place than maps of average annual
precipitation, since the volume of runoff for
FIG. 16.5 Highly general diagram of the
season of maximum water detention on the land at
the various latitudes in the hemispheres.
(From van Hylckama.)
LATITU
70
DE (N and S)
I/
-
60'
-
-
50
- N
-
40
x x
-
30
20
-
\
10"
-
/'
-
Winter Spring Summer Autumn Winter
solstice equinox solstice equinox solstice
298
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
an area can only be obtained from a point,
viz., a stream-gauging station, and the total
runoff is then equally apportioned to all
parts of the watershed. Consequently, a map
of runoff shows only the general pattern of
variation, not the actual amount that drains
at every point. Figure 16.6 is a map of the
average annual runoff of the United States.
The major differences shown on a map of
annual runoff are, of course, associated with
variations in annual precipitation, as a com-
parison with a precipitation map will show.
Stream flow Much of the time surface
runoff is largely fed by emerging ground
water. However, the ground-water propor-
tion of runoff is lower at the time of rainfall
or snow melt. Then, when there is a large
supply of water on the land surface, it follows
die pull of gravity and most of it quickly
collects in channels. One recognized list-
ing of these channels separates those that
contain flowing water only a portion of the
time and those that do so continuously; the
former, if they contain water with some reg-
ularity, are called intermittent streams; the
latter are termed permanent streams.
Generally, the variations in stream flow are
tied closely to variations in precipitation,
evaporation, infiltration, and storage-water
release; and the interrelations vary from place
to place and from time to time.
Unchannelized movement of water in a
thin layer over a surface, called sheet wash,
occurs when the accumulation of water on
a sloping surface is greater than the channel-
ing and water-infiltering capacity of the sur-
face forms and materials. Heavy rains and
snow melt, relatively gentle slopes, and a
surface material with a slow infiltration
capacity, such as a "tight" clay or frozen soil,
FIG. 16.6 The average annual runoff in the United States. (Generalized
from a map by the U.S. Gcokgual Survey.)
OVER40
20-40
10-20
5-10
C23 2.5-5
0.25-25
AVERAGE ANNUAL
RUNOFF
The waters of the land 299
favor sheet wash. Where there is not enough
slope to draw off the water, it simply collects
as temporary "standing water" in swamps,
marshes, and shallow lakes.
Regime of runoff As everyone knows,
creeks and rivers fill up after a heavy rain. But
there is a lag between the time of rainfall and
the rise of stream levels, resulting from the
facts that (a) it takes time for the surface flow
of water to reach the streams, and (b) a share
of the water filters downward and then moves
laterally to seep out again where runoff
channels have cut below the temporarily
water-filled upper soil layers. This water
moves more slowly because of friction.
Since all this direct surface runoff increases
after a rain, and drainage from the under-
ground reservoir fluctuates relatively little,
one might expect that the annual runoff
regime would likewise reflect seasonal varia-
tions in precipitation amounts. But the
annual variation in runoff is much more
closely regulated by the release of storage
water, and peak runoff generally is associated
with the period of peak storage. In the tropics
this period nearly coincides with the rainy
season, but for a large part of the middle and
higher latitude areas this period is around the
time of the spring equinox in each hemisphere.
Figure 16.7 is a composite graph illustrat-
ing the average regime of runoff for two areas
in Ohio. As winter wanes and spring advances,
temperatures, and consequently evaporation,
are still relatively low, and the top layer of
soil is likely to be frozen. Rainfall and
storage water from melting snow contribute
considerable direct surface runoff. As tem-
peratures rise farther and plants begin to
grow, an increasing proportion of the pre-
cipitation and storage water is subtracted
through evapo transpiration. Thus, even
though the precipitation reaches a maximum
FIG. 16.7 Twenty-five-year (1921-1945)
combined averages of the precipitation and total
runoff as measured at the Hocking and Mad Rivers,
near Athens and Springfield, Ohio, showing the
proportions of the total runoff supplied from direct
surface runoff and from ground water.
(From U.S. Geological Survey.)
in the summer, surface runoff decreases, and
ground water, an equalizing influence, pro-
vides an increasing proportion. In the autumn
the decrease of both plant growth and air
temperatures allows an increase in direct sur-
face runoff as well as a steady replenishment
of the ground-water reservoir. The concen-
tration of surface runoff in the early part of
the warming season tends to increase pole-
ward; floods and soggy ground are common
springtime phenomena in the areas of humid
climates in the middle and higher latitudes.
Factors affecting the amount of annual
runoff Figures 16.6 and 16.9 show for the
United States and the world, in a highly
generalized fashion, the annual amount of
runoff as determined in the manner outlined
on page 297.
Climatic factors It is clear from these
300
?0* 60* 0* 40^ 30* 20
FIG. 16.8 Relationship of the average annual
total runoff to average annual total precipitation on
the land per unit area according to latitude. The
vertical difference between the two represents the
loss through evapotranspi ration. North latitude is to
the left. (From L'vovich and Drozdov.)
maps that over the world as a whole, aver-
age annual runoff tends to vary directly with
precipitation, the ultimate source of surface
runofT. But some variations result from dif-
ferences in the nature of the precipitation.
Where rainfall occurs primarily in the form
of heavy or very frequent showers, a greater
proportion of the fall will immediately be-
come runoff because other factors come into
play, such as a decrease in the rate of ground
infiltration, as a consequence of saturation or
compaction of the soil.
Water held in storage near the surface, as
well as that which percolates down to the
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
ground-water reservoir, is merely delayed,
not lost as a potential source of runoff; on
the other hand, any water that is evaporated
is lost. Consequently total runoff is the
amount of precipitation minus the loss due
to evapo transpiration.
Because potential evapotranspiration is
primarily a function of temperature one would
expect, other things being equal, that the
lower latitudes would have low runoff in
proportion to their precipitation, and this
general relationship obtains to a degree. But
as Fig. 16.8 shows, actual runoff is heavy in
the tropics in spite of the high evapotranspira-
tion there, because rainfall is proportionately
even heavier. The subtropical areas, because
of their generally high temperatures and low
precipitation, are zones of low annual runoff.
The middle and higher latitudes, where
evaporation rates are relatively low, have the
highest ratios of annual runoff to annual
precipitation.
Land-surface effects The land surface, in-
cluding the bedrock on which it has devel-
oped, adds complexity to the general pattern
of annual runoff that basically results from
the climatic pattern. This is primarily a re-
sult of variations in permeability. Large areas
of the continents are covered with surface
materials that permit rainfall quickly to per-
colate to considerable depths, where the
water is beyond the reach of plant roots and
the evaporation process. This water becomes
a part of the ground-water reservoir and
eventually feeds into streams. Thus surfaces
underlain by pervious lavas and limestones
are capable of absorbing rapidly vast quanti-
ties of water, and these areas are commonly
notoriously low in surface-runoff amounts.
Also, sandy areas and the alluvial plains,
small and large, which fringe many moun-
tainous areas, particularly in dry climate
0* 10* 20* 30* 40* 50* 60*
areas, are likely to have porous surface ma-
terials, and thus low runoff. On the other
hand, there are many areas of very low
permeability in marginal sections of dry cli-
mates that have unusually low runoff amounts
for quite a different reason. The collection of
the water in surface depressions here may
allow a large proportion to fall victim to
evaporation. In general, there is likely to be
more variability in annual runoff from place
to place in the dry areas of the earth than
elsewhere.
World map of annual runoff The world
map of average annual runoff (Fig. 16.9) is
necessarily crude because stream gauging,
from which it is derived, is not widespread,
nor have such data been gathered for long
periods. Nevertheless, the pattern displayed
is not likely to be far from reality.
The map shows certain general relation-
ships that are to be expected; for example,
the areas of copious precipitation generally
have the heaviest runoff. Such areas are of
two major types: (a) the areas of heavy
tropical rainfall where runoff is high despite
the high evapotranspiration, and (b) the
areas of marine climates where relatively
heavy precipitation is combined with cool
temperatures. In many of the latter areas the
precipitation is augmented by orographic
The waters of the land 301
precipitation, but not always: the lowlands of
Europe stand out as high runoff zones in
spite of moderate amounts of precipitation.
Low runoff zones extend into humid climatic
areas well beyond the climatic dry boundaries,
for in many of the moister areas precipitation
comes in the high-sun season and much of it
is quickly lost to evapotranspiration. Notable
also are the rapid transitions from regions of
high to low runoff. Rapid gradients are
characteristic of maps of runoff at any scale
(Fig. 16.6).
The world map and the following table,
derived from the data used to prepare the
map, show that the continents differ greatly
in runoff amount. It will be seen that South
and North America are the most favored and
that Australia is the least.
Lakes, swamps, and marshes Surface
runoff, in the course of its movement in re-
sponse to gravity, is, as previously mentioned,
sometimes delayed enough for quiet, sluggish
bodies of water to result. If such water bodies
are shallow enough to allow vegetation to
grow through the thin layers of water, they
are called swamps, marshes, or boglands.
Where water in them exists in an unbroken
sheet, they are called lakes or ponds. By
definition any such body of water, as distin-
guished from a stream, must lie in a basin-
Annual Runoff by Continent
Continent
Average annual runoff, inches*
South America
North America
Europe
Africa
Asia
Australia
17.7
12.4
10.3
8.0
6.7
3.0
11 Estimate by M. I. LVovich.
like depression of the land surface. How any
particular low-lying area of this type may
have come to be is not the major interest
here, but the fact that the majority are created
by only a few kinds of processes helps to ac-
count for their location.
Regions of occurrences The majority of
lakes, swamps, and marshes occur in regions
of the world where the degradational and
aggradational processes have not for the
moment, since basins, like all other land
forms, are transitory been able either to fill
the basins with solid materials or integrate
them into "normal" stream channels. The
most obvious examples of such areas are
those that have been subject to recent glacia-
tion, especially continental. In large areas in
northern North America and Europe there
are literally tens of thousands of major and
minor basins that are perennially or inter-
mittently inundated by sluggish surface water.
Swarnps and marshes are also common in
these areas. Many lakes occur on floodplains
along the courses of sluggish meandering
streams where new surface patterns are con-
tinually being formed by gradational proc-
esses. Similar swampy areas are found in
coastal areas of unusually gentle pitch. In all
these kinds of areas the drainage of the land
is poor; that is to say, whatever the amount
of surface runoff may be, the rate is slow
enough that there is an abundance of stand-
ing water.
In some areas underlain by soluble lime-
stone (karst areas), the sinks may intersect
the saturated ground-water zone and so con-
tain lakes, or the free drainage of the sinks
may have become plugged so that water col-
lects in them. Many limestone areas, on the
other hand, show little or no surface water
even in streams, their drainage being primarily
beneath the surface.
The waters of the land 303
In dry regions most basins, whether formed
by tectonic or gradational process, are not
filled to overflowing by the meager surface
runoff; hence they are not quickly integrated
into stream drainage. These are bolsons, or
basins of interior drainage, which may con-
tain temporary lakes. Tectonic forces, espe-
cially deformation, have created numerous
lake basins, including some of the more nota-
ble of the world. Lakes Tanganyika and
Nyasa in Africa, as well as the Dead Sea, are
examples of lakes that have developed in
great down-dropped trenches (Fig. 3.6).
The occurrence of lakes in a surface-runoff
system in humid areas changes the character
of both the water and the runoff process.
Silts brought into a lake by stream flow are
deposited in the quieter water, so that water
leaving a lake is usually clear. Water flowing
out of a lake also tends to have a more uni-
form temperature, and its mineral and organic
character has usually been affected by the bio-
logical processes at work in the lake.
But the most significant effect of a lake
upon the runoff process is its regulation of
the rate of flow; a lake acts as a reservoir,
collecting and detaining water during times
of heavy surface runoff and releasing it later
at a more uniform rate. This is of great
utility to man in at least two ways: (a) it re-
duces both the incidence and severity of
downstream flooding, and (b) it raises the
volume of the stream, at the time of mini-
mum flow, above what it would otherwise
be. The maintenance of higher volume is in
many ways advantageous to water supply for
withdrawal uses, and even more so for non-
withdrawal uses. Two of the nonwithdrawal
uses, the production of hydroelectric power
and navigation, are generally limited by the
minimum flow the former by its volume,
the latter by its depth.
304
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
FIG. 16.10 Highly generalized diagram of the Tennessee River
drainage basin, showing the numerous reservoirs which have been
integrated into the scheme of natural surface runoff. Projects of this nature
are being carried out in many parts of the world where surface runoff is
Subject tO great fluctuation. (Norman J. W. Thrower.)
Man-made basins Where natural lake
basins are absent in the scheme of surface
runoff* and regulating effects are still desir-
able, man may construct artificial basins. A
steadily increasing number of these reservoirs
is being constructed in those surface runoff
areas where the regime is characterized by a
large range between maximum and minimum
flow. In the aggregate these constitute, with-
out doubt, the most notable man-made
change that is reflected in the maps of his
physical environment (Fig. 16.10).
Man-made lake basins are subject to the
same forces as natural basins. Thus, although
man can easily prevent the outlets from erod-
ing deeper and draining the reservoir, it is
difficult to control the silt content of the in-
flow in order to prevent the lake from filling
with solid material. To do this requires care-
ful planning and regulation of the land use
in an entire drainage basin, and such a com-
plex, long-range program is not easily accom-
plished except with governmental aid.
SURFACE WATER
AS A RESOURCE
The large quantities of water required for
withdrawal use by modern urban and indus-
trial centers are in some instances obtained
from wells and springs, but usually from
large lakes, large rivers, or small streams the
drainage of which is stored behind dams to
create municipal reservoirs. Only about one
out of four of the principal American cities
obtains its water supply from wells. Most of
the remainder, especially the large cities, use
surface water. Indeed, over half of the com-
munities in the United States having more
than 10,000 inhabitants are supplied from
surface water.
As the cities of the world grow in size the
problems of obtaining sufficient surface water
also grow. For example, New York City uses
about 1 billion gal of water per day, and,
because it is not located near a large lake or
a usable river, must obtain this tremendous
supply from a variety of areas. New York
City depends on seven different stream water-
sheds that gather water from a combined
area of about 2,000 square miles, an area
half again as large as Rhode Island (Fig.
16.11). The water is taken from more than
1,000 streams, small and large; it is stored in
27 artificial and natural reservoirs, some of
which are as far as 120 miles away; and it is
brought to the city by means of more than
350 miles of aqueducts and tunnels (Figs.
16.11, 16.12). 1
In many areas of the world cities have
grown up without an adequate, easily obtain-
able, supply of surface (or ground) water,
and water must be brought great distances by
aqueduct. For example, Los Angeles brings
water from the Owens River-Mono County
area on the eastern side of the Sierra Nevada,
nearly 300 miles away, and from the Colo-
rado River on the California-Arizona border.
In some parts of the world sea water is dis-
tilled, but this is costly: the unit cost of dis-
tilling is directly related to the mineralization
of the source water, and sea water has from
1 Anastasia Van Burkalow, "The Geography of New York
City's Water Supply: A Study of Interactions," Geographical
Review, vol. 49, 1959, pp. 369-386.
The waters of the land 305
32,000 to 36,000 parts per million of total
dissolved solids. In the light of present tech-
nology it appears that, even with atomic or
solar energy providing the power requirement,
it will be cheaper for some time yet to trans-
port fresh water great distances than to distill
sea water.
Surface water differs from ground water in
a number of important respects as a resource.
Generally, surface water is less mineralized
than the ground water of the same region,
because surface water is derived in part from
the runoff of rain water which has not been
so long in contact with the minerals of the
ground. However, surface water is likely to
contain larger quantities of sediment and
organic matter, including bacteria, than
ground water. For this reason many cities
find it necessary to purify and filter their
water supplies. For example, nearly half the
population of the United States uses water
that has been treated in some way. Water
used for irrigation must not have too high a
mineral content, and many industrial uses,
ranging from boilers to canning, require water
having specific mineral qualities.
The large industrial and municipal with-
drawal uses of water occasion complex prob-
lems of pollution when the effluent (the water
that has been used) is returned to surface
drainage. This affects recreation and wildlife,
as well as communities downstream that may
also use the surface water. Surface drainage
through streams and lakes is related also to
other water uses which are matters of great
public concern, such as soil erosion, flood
control, power production, and inland trans-
portation. Out of these varied uses of surface
water grow conflicts of human interest which
lie beyond the scope of physical geography.
A Middletown / New
O
Stroudsburg
-41
NEW YORK CITY'S
WATER SUPPLY
AQUEDUCTS
- OldCroton
N ew Croton
Catskill
ooooooooo Delaware
TUNNELS
Shandaken
- East Delaware
Neversink
I West Delaware
':\AT:L^mT : L C:-. OCEIA N
20 30 40
74T< - : :GiEOGR. REV,, JULY., 1959. /
FIG. 16.11 Map showing the situation of the sources of New York
City's water supply. Watershed areas are bounded by the shaded line and
reservoirs are shown in black. The Cannonsville Reservoir, the site of which
is shown in Figure 16.12, is indicated by the stippled black. (Adapted from
map in Geographical Review.)
The waters of the land 307
GROUND WATER
THE GROUND-WATER
RESERVOIR
Wherever more water is supplied to the sur-
face than immediately runs off or is evapo-
rated, the remainder sinks beneath the sur-
face of the land. Generally, water beneath
the surface is called ground water, as dis-
tinguished from surface water. Yet, as indi-
cated in the preceding discussion of surface
water, a portion of the water that seeps
downward does not go far, but instead is
stored temporarily in the upper section of
the ground. Much of this water never pene-
trates deeper, and may shortly be lost by
evapotranspiration processes or in runoff.
This water was considered, for purposes of
explaining surface water, as part of it. More
accurately, however, this water near the sur-
face, at least in humid areas, is in a sort of
transition stage between surface water, ground
water, and atmospheric water. Its ground-
water aspects will be considered now.
Water responds to the forces of gravity
and molecular attraction. The former pulls
water directly downward and acts indirectly
through atmospheric pressure. Molecular
attraction, through surface tension and capil-
larity, causes water to adhere to surfaces in a
thin film or creep into crevices and tiny
channels in the regolith, and so to remain
suspended in spite of the force of gravity. As
a volume of water moves downward from
the surface of the ground, a portion is left
behind as a consequence of molecular attrac-
tion. The upper portion of the ground is
therefore commonly damp, although by no
means saturated. On account of (a) the inter-
action of the forces of gravity and molecular
FIG. 16.12 Site ot the Cannonsville Reservoir
and dam (white line) before construction. (Board of
Water Supply of the City of New York.)
attraction, (b) the amounts of water available,
(c) the unevenness of the land surface, and
(d) the vertical variations among the surficial
materials, several zones or layers of ground-
water character are observed.
Zones Figure 16.13 illustrates the sev-
eral different recognizable sections in the
earth's reservoir of ground water. Each is a
FIG. 16.13 Zones of subsurface water
occurrence. In many places not all the zones occur;
in some places none occur.
308
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
zone into which ground water may move
during its stay in the ground-water portion
of the hydrologic cycle. Water from im-
mediate precipitation, from melting snow, or
from the surface runoff of some other area first
passes into the zone of soil water. This
upper section of the regolith usually consists
of finely divided materials having a large
admixture of organic substances. This zone
can absorb a considerable quantity of water
and retain it for a time until it passes into
another stage of the hydrologic cycle, by
evaporation directly, or through the roots
and transpiring surfaces of plants.
Water that enters the zone of soil water in
excess of the amount that can be retained
there, may percolate through and into the
intermediate zone, which is below the reach
of most plant roots. Here surface tension
causes the water to adhere to the surfaces of
rock particles and the sides of cracks and
other openings. Although these voids may
be temporarily filled during a time of heavy
ground- water recharge, usually they are not.
Air also circulates among these spaces.
The intermediate zone and zone of soil
water above it are collectively called the
zone of aeration. As a consequence of the
circulation of air in the zone of aeration,
water may be removed by evaporation, caus-
ing some fluctuation of the amount of water
there.
The downward-moving water that does
not remain in the zone of aeration enters the
zone of saturation, where all the pore spaces,
cracks, and other openings among the parti-
cles of the regolith or the bedrock are filled
with water.
The top of the saturated zone, or the con-
tact surface between the zones of aeration
and saturation, is called the water, or ground-
water, table. Immediately above the water
table and thus at the bottom of the zone of
aeration is a zone called the capillary fringe.
Here surface tension holds the water above
the saturated zone in interconnected voids or
"tubes" that are so small that water cannot
drain out of them. These extend some dis-
tance into the zone of aeration. Water may
creep upward from the water table into the
capillary fringe, but the fringe is primarily
supplied from above by the downward move-
ment of the water. The thickness of the cap-
illary fringe depends upon the sizes of the
voids; the smaller they are the thicker it will
be. Thus, in sandy areas the thickness may
be a foot or less, while in clay areas it may
be three or more feet. The capillary fringe is
significant because it often brings a source of
water within reach of deep-growing plant
roots.
The water table . If the land were
suddenly to become transparent so that the
water table at the surface of the saturated
zone could be seen, it would be noted that it
undulates in much the same way that the
land surface does. It would be apparent,
however, that its configuration is more sub-
dued than that of the land surface; that is,
the water table actually coincides or even in-
tersects with the surface in some low places
or valleys. Although it rises beneath hills
it is proportionately farther from their surfaces
at their summits.
There is no need here to go into detail
concerning the physical laws governing the
movements of ground water, but one factor
that basically accounts for much of the con-
figuration of the water table is worth con-
sidering. In order for water to flow, either
above or below ground, it requires a slope,
and the rate of flow varies with the slope.
The slope of the water table is called the
hydraulic gradient, and it is expressed as the
Hydraulic gradient=^/
= slope of the ground
water table
FIG. 16.14 The hydraulic gradient.
ratio of the head (vertical height or "fall" of
the water) to the horizontal distance between
the intake and discharge points, i.e., the points
where water enters and leaves the table (Fig.
16.14). Therefore, if a constant rate of flow is
specified, the greater the distance between the
place where the water enters the ground and
where it discharges, the higher the head must
be. Consequently, water in the saturated zone
near a discharge point can escape with little
head, while that farther away will pile up
higher until the rate of addition to the satu-
rated zone balances the rate of flow.
As the rate of addition to the saturated
zone changes from time to time the elevation
of the ground- water table changes significantly
being higher during periods of net ground-
water recharge and lower during periods of
net discharge (Fig. 16.15). The regime of
surface runoff considered on page 299 is
likely to be strongly correlated with the ele-
vation of the water table but with somewhat
of a lag, since it takes time for the water to
move through the ground.
Factors which affect the occurrence of
ground water Ground water near the land
surface is vitally significant just in the fact
that almost all land vegetation derives its
sustenance from this water and the minerals
dissolved in it. In many rural areas and in
The waters of the land 309
some municipalities it is a primary source of
water for domestic use; and in some places,
where surface-water supplies are scarce, it
even provides much if not all the water used
for irrigation and stock raising. Consequently
the character of the water and the variations
of the depths and thicknesses of its several
zones are matters of great importance in
physical geography. The number of variables
involved is very large, and only a few of the
more important relationships can be suggested
here. The elements of first rank that affect
the character of the ground-water reservoir
in any place are (a) the precipitation and
evaporation in an area, (b) the porosity of the
water-bearing materials, and (c) their
permeability.
FIG. 16.15 Fluctuations in the level of the
ground-water table and their effects upon streams
and vegetation.
NO WILT
MAXIMUM HEIGHT OF WATER TABLE
NO WILT
HEAVY OROUNO~WATR RECHAftQE
WILT
AVERAGE HEIGHT OF WATER TABLE
\NO WILT
*ROUND~WATK DEPtETlQH
MINIMUM HEIGHT OF WATER TABLE
WILT
310
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
Precipitation and Evaporation The major
influences on the occurrence of ground water
are generally precipitation and evaporation.
The heavier the precipitation and the less
the evaporation, the more water is likely to
percolate downward to enter the saturated
zone, and consequently the higher the ground-
water table is likely to be. It is these two
factors that, in combination, maintain the
ground-water table in some areas at such a
level that it commonly intersects the undula-
tions of the land surface, creating numerous
lakes, swamps, and streams. In such areas
the zone of aeration is likely to be thin even
in the higher and drier land. Of course, as
local relief becomes higher, the zone of aera-
tion becomes thicker beneath the hilly areas
because of the greater lateral drainage .of the
water of the saturated zone. In dry climates
and areas where meager precipitation occurs
at a time of high temperatures, the water
table is likely to be buried deeply every-
where except in the deepest valleys.
Porosity The term porosity refers to the
ability of the regolith and bedrock to absorb
variable quantities of water in response to
the force of gravity. This depends upon
many factors, but primarily upon the abun-
dance and the size and shape of the spaces
within the materials. Unconsolidated or
loosely cemented sands and gravels, some
limestones and lavas, and greatly fractured
bedrock are porous because the sum of the
spaces between the particles is large.
Permeability The term permeability in-
cludes porosity but refers to the ease with
which water can move. Permeability, or con-
versely impermeability, depends primarily on
molecular attraction, which, acting mainly
through surface tension, tends to retard the
movement of water. The average size of the
particles in a mass affects the amount of sur-
face area inversely; namely, the smaller the
particles, the larger the total surface area.
The larger the surface area, the greater the
volume of water that can be held by surface
tension; hence, fine-textured materials tend
to be relatively impermeable. For example,
the surface area of the particles in a given
volume of clay may be five thousand times
that of the same volume of gravel, making
the permeability of the gravel much the
greater, while the porosity of the two ma-
terials is the same.
The permeability of the upper surface ma-
terial (the soil) is in general independent of
the average size of the particles in the regolith
of which it is composed. Instead, the perme-
ability of the soil is greatly affected by its
content of organic material, and in general,
the more organic material there is, the more
water the zone of soil water can absorb and
transmit.
Rock formations and deposits which hold
large supplies of water and allow it to move
easily are called aquifers. These may range
from fractured but otherwise solid rocks to
those with many pore spaces, such as sand-
stone, that allow water to move. The most
permeable formations are beds of gravels and
loose sands. These are relatively widespread
since they are normal products of the grada-
tional processes.
DISCHARGE AND RECHARGE
OF GROUND WATER
Source and timing of recharge The
ground-water reservoir in any area is re-
charged primarily from two sources: (a)
direct infiltration from precipitation, and (b)
infiltration from streams and other bodies of
water that receive drainage from other areas.
There are important regional contrasts in the
relative dominance of these suppliers and in
the time when most recharge occurs.
Since precipitation is the ultimate supplier
of almost all the water on the land, it might
be surmised that recharge from that source
would ordinarily occur at the time of highest
amounts of precipitation; but it is more com-
plicated than that. Over much of the earth
the maximum precipitation occurs during the
time of high sun when air temperatures, in-
cident sun energy, and vegetation growth are
all at a maximum; therefore, much of the
precipitation that occurs during this period
is los,t through evapotranspiration. Conse-
quently maximum recharge of the ground
water occurs, rather, when the balance of
these factors is favorable for it. This depends
primarily upon the timing of the climatic
elements, particularly precipitation and
temperature.
Regional variations In humid areas re-
charge usually begins and reaches a maxi-
mum in the early part of the rainy season.
Here, where the rainy season generally coin-
cides with high sun, the increasing temper-
atures soon turn the tide in favor of losses
by evapotranspiration, so that even where
amounts of rainfall increase to a maximum
during summer, the recharge of the ground
water tends to decrease. Only in the humid
tropics where potential evapotranspiration
rates are relatively constant and in the humid
mesothermal areas with low-sun precipitation
does maximum recharge ordinarily coincide
with the time of maximum precipitation.
The recharge of ground water in arid
regions follows a similar regime, closely tied
to the annual variations in precipitation
amounts. But there is one major difference
between arid and humid regions with respect
to recharge: recharge in humid areas usually
occurs locally, that is, where the precipita-
The waters of the land 311
tion occurs, while the recharge area in arid
regions is commonly displaced from the area
of precipitation. The scanty precipitation at
low elevations of arid regions is quickly lost
by evaporation, but that which falls on the
cooler, higher areas is not so subject to loss
by evaporation and there is usually more of
it. Thus it collects in stream channels and
flows to the edges of the uplands, where it is
likely to encounter marginal alluvial deposits
made up mostly of highly permeable gravels.
The water quickly sinks and while doing so
also moves laterally beneath the surface,
sometimes for considerable distances.
Effluent and influent streams Water
that enters the ground and moves beneath
the surface as ground water must ultimately
leave the ground water. It may do so in two
ways: by emergence into the surface-water
supply or by evapotranspiration. In humid
areas the loss of water from the saturated
zone is usually by outflow to streams; in dry
areas the loss is more often by direct
evaporation.
Streams may be classed as either effluent
or influent in their relationship to ground
water. Effluent streams are those that are fed
by a water table above their level; here the
stream channel intersects the water table,
and water drains into it from the ground-
water reservoir. Influent streams are those
from which water feeds into the ground; here
the water table lies beneath the bottom of
the stream channel (Fig. 16.16). The streams
of humid climatic regions tend to be effluent
streams, and because of the steady addition
of outflowing ground water, tend to increase
in volume downstream; dry-land streams
tend to have opposite characteristics. A small
stream that is situated where its valley is
in a variable relationship with the fluctuating
water table, changes from time to time, being
312
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
INFLUENT STREAM
EFFLUENT STREAM
WATER
FIG. 16.16 Cross sections of an influent and
an effluent stream.
effluent when the water table is high and
being influent, or even drying up entirely,
when the water table is low.
Springs Where ground water discharges
onto the surface instead of at or below the
level of a flowing stream, a spring exists. A
spring may flow either continuously or inter-
mittently, and its water may be either cold or
warm, hard or soft (page 313).
Springs result from a variety of conditions
involving the position of the water table, the
configuration of the land surface, and the
nature and structure of the rock materials.
Figure I6.\7a illustrates the occurrence of
springs on the sides of a valley which has
eroded below the usual level of the water
table. Springs of this type are common, and
often they are the main water sources for
small brooks at the headwaters of rivers. If
the level of the water table is lowered after a
period of protracted drought, such a spring
will cease to flow until the water table is
again raised by the downward seepage from
further rains.
Figure 16.176 illustrates the site of a spring
caused by the movement of water downward
through porous formations and then hori-
zontally along the top of an impervious layer.
Permeable formations of gravels, sandstones,
or other porous materials underlain by com-
pact shales often produce many such springs
in an area. Figure 16.17r illustrates how
water from a wide area of rocks, even rocks
of low water- holding capacity, may converge
upon a spring site that results from a fault.
GROUND WATER
AS A RESOURCE
Ground waters are not used as a source of
water supply to the extent that surface waters
are. Nevertheless, the ground-water reservoir
is extensively utilized, especially (a) where
surface supplies are limited or have unde-
F I G . 16.17 Some of the many possible
conditions of surface, material, and structure that
are related to the occurrence of springs.
SPRING
SITE
sirable qualities, and (b) in smaller cities and
villages, in suburban areas, and on farms.
The approximate proportions of ground
waters and surface waters taken for the major
withdrawal uses in the United States are
shown in Fig. 16.2.
Almost no water is free from dissolved or
suspended material, but the nature and
quantity of such materials varies widely from
region to region. Ground water ordinarily
has been filtered through the earth, sometimes
for many years, before it again comes or is
brought to the surface; it is therefore rela-
tively free from mud and other suspended
materials. On the other hand, ground water
commonly contains dissolved minerals, and
some of these, such as sulphur or iron, may
impart to water a disagreeable taste or render
it unfit for certain industrial processes. Some
minerals give tonic, laxative, or other me-
dicinal qualities to the water.
Among the most abundant of the soluble
salts found especially often in ground water
are compounds of calcium (lime), sodium,
and magnesium. In desert regions, for in-
stance, seepage waters are commonly charged
with compounds of these elements and other
salts to a degree that retards or prevents their
use. In the United States these are known as
alkali waters. In humid regions most of the
readily soluble sodium compounds have long
since been removed from the upper portion
of the ground. However, limestones and
lime-cemented sediments furnish calcium and
magnesium compounds which, although they
do not much affect the taste of water, give it
a quality which does affect its domestic and
industrial utility.
The amount of mineral in solution usually
is expressed in terms of the parts of dissolved
mineral per million parts of water (ppm).
Regions underlain mainly by crystalline
The waters of the land 313
rocks or by highly siliceous sands or sand-
stones may contain as little as 5 or 10 ppm.
These are the naturally soft waters. Water
containing as much as 60 ppm still is con-
sidered soft, but if water contains more than
120 to 180 ppm it is considered hard water.
In regions of lime-containing sedimentary
rocks, well waters in common use contain
300 to 500 ppm and, in a few places, as
much as 700 to 800 ppm. Hard waters, if
not "softened," may cause serious problems
in the home and in certain industrial proc-
esses. This is because of their chemical re-
actions and especially the formation of un-
desirable precipitates. It is estimated that
use of hard water for municipal supply costs
the homeowner well over $100 per year
directly and indirectly. The ground waters
that supply some 10 per cent of the urban
population in the United States are twice as
hard as the surface waters used for that pur-
pose (160 ppm compared with 80 ppm,
approximately).
The use of wells and spring waters
Throughout the world there are no doubt
thousands of farmhouses and not a few vil-
lages that are located upon sites originally
chosen, long ago, because spring water was
found there. Large numbers of these springs,
most of them on valley slopes, still are flow-
ing and still supply water. Yet the substitu-
tion of tilled crops for forest and grassland
has tended to increase the rate of runoff and
so to decrease the proportion of the precipi-
tation that infiltrates. The consequent lower-
ing of the water table has had the effect of
rendering the supply of spring water less de-
pendable, and at the same time the growth
of population has tended to make this source
of supply less adequate and more subject to
pollution. In the more heavily occupied areas
the withdrawal of ground water by artificial
314
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
means (wells) has also greatly depleted the
ground-water supply.
Kinds of wells Wells are holes that pene-
trate below the water table where ground
water may drain into them from an aquifer
and then be brought to the surface. Formerly,
many wells were made by simply digging a
hole below the level of the water table, and
they seldom were many feet deep. Millions
of such dug wells still are in daily use in
nearly all parts of the world, although this
shallow and open construction makes them
particularly subject to surface pollution.
Since most dug wells cannot easily be ex-
tended far below the water table, the normal
rising and falling of the water table with
periods of recharge and discharge makes
them also a relatively undependable source
of water supply.
A driven well is obtained by forcing a
point (a pointed length of pipe with screened
holes in it) into the ground, adding succes-
sive sections of pipe in the process. If the
point enters an aquifer, the water will enter
the holes and may be pumped out. Since the
pipe forms a casing all the way to the aquifer,
there is less likelihood of pollution than in
the case of a dug well. But driven wells are
usually shallow also, and can only be put
down in unconsolidated materials.
A drilled well is made by boring a hole
into the bedrock until a bedrock aquifer is
pierced. The hole is cased, at least in its
upper portion. The hole may be drilled
until the rate of flow of water into it provides
the supply desired. Ordinarily only drilled
wells extend very far beneath the surface and
merit the term deep wells.
The quantity and the quality of water
from deep wells depend upon the nature of
the aquifer and its structural relationships. If
the wellhole terminates in a thick, porous
aquifer such as a porous sandstone, it may
provide an abundant and continuous supply
of water. Where the rock beneath a locality
is the massive crystalline type, the water yield
may be continuous but not abundant, since
such bedrock has little pore space. The rate
of flow into a well in dense rock is sometimes
increased by using explosives at the bottom
of the hole to shatter the surrounding rock
and thus make numerous crevices through
which the water may flow. However, some
hard crystalline rocks are so low in water
content that no operation can bring about
enough flow to justify the very high cost of
drilling deep wells in them. Shale rocks,
although not hard, are commonly compact,
impervious, and dry, but they are usually
closely associated with other sedimentary
rocks which are porous.
Wells in regions of fractured limestones
draw water primarily through fissures, such
as joints and other fractures that have been
enlarged by solution, and they may yield
abundant supplies. On the other hand, since
the water may enter the system of fissures
directly from the surface drainage, some of it
through sinkholes, it may not have the bene-
fit of much natural filtering, and therefore
possibly be polluted when withdrawn. It may
be little safer than the surface waters of the
region, which have at least been exposed to
the bacteria-destroying power of sunlight.
Effects of wells on water table The surface
of the ground-water reservoir (the water
table) is in a natural state of equilibrium
with respect to its discharge and recharge.
These rarely remain constant, and the water
table rises and falls in response to their natu-
ral fluctuations, but the rises and falls offset
each other, over a period of time. When man
extracts water from the reservoir, he upsets
this natural balance by adding an unnatural
pRI.6l.NAL;. WATER. JA^LE*.':-
FIG. 16.18 Development of a cone of depres-
sion around a well. When withdrawal exceeds the
rate of movement, the cone will ultimately reach
the bottom of the well.
discharge. The water table therefore falls
until a new balance is reached, either by an
increase in recharge or a decrease in natural
discharge. When water is withdrawn from a
well at a rate greater than lateral movement
of ground water can supply it, a cone of
depression forms around the well, and as
long as the withdrawal exceeds the rate that
water can be supplied, the cone steadily
deepens and grows laterally (Fig. 16.18).
Obviously, if discharge continues to be faster
than the lateral movement of water, the cone
will deepen until it reaches the bottom of the
well. The cone may even intercept the cones
of other wells, reducing their yield; but in
any case the natural discharge will be de-
creased somewhere. The gradual lowering of
the water table involved will, of course, in-
crease the costs of pumpage. If the well is
located near the sea, the development of the
cone may reverse the direction of ground-
water flow, causing salt water to move toward
the well, as has happened in numerous places.
Artesian wells Any well in which water
rises above the level of the tapped aquifer is
commonly referred to as an artesian well.
Formerly, the term was restricted to wells
that flowed freely without pumping. Artesian
wells are possible under any one of several
sets of conditions of underground structure,
The waters of the land 315
two of which are illustrated in Fig. 16.19.
But the favorable situation must include the
following conditions: (a) the aquifer must be
of some permeable material; (/;) the aquifer
must outcrop, or be exposed at the surface,
in a region of sufficient precipitation to fill it
with water; (c) the formation must dip beneath
a capping layer of some impermeable material
such as shale; (d) it must lead toward a
region where the land surface is lower than
it is at the exposed end of the pervious for-
mation; and (e) there must be partial con-
striction (or total blockage) of exit sufficient
for the water that collects in the higher por-
tion of the aquifer to be placed under pres-
sure. Water will then rise in a well, or even
flow from the opening, as long as the rate of
recharge exceeds the rate of loss through
withdrawal from the well and natural seep-
age. In a few regions saucerlike structural
basins contain aquifers which outcrop at the
edges of the basin and incline from all sides,
underneath other rocks, toward its center,
where artesian water may be had in abun-
dance. Artesian conditions also occur on a
smaller scale in constricted layers of gravel
FIG. 16.19 (Above) A structural artesian
condition such as that described in the northern
Great Plains of the United States. (Below) A local
artesian condition that might occur in an area of
glacial deposition.
Saturation level in aquifer^
WELL-
c Elevation of swamp
SWAMP
r v .v. .r...-L . .
i < "f *F~'V$&*iW afo * 1 ffi?' ' ?; v G RAti E L- ;< *'l*T??^:i
316
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
FIG. 16.20 The great artesian basin of
Australia. (From Jame\ E. Collier.)
or sand, called lenses, in the otherwise
clayey drift of glacial deposition.
Notable artesian structures underlie some
areas of truly great extent. One is the northern
Great Plains region of the United States.
There porous formations, especially sand-
stones, outcrop at considerable elevation
near the Rocky Mountains and incline east-
ward, under suitable capping layers, toward
the lower plains. They yield artesian waters
far out in the eastern part of the Dakotas.
Parts of the dry lands of Australia also are
blessed with artesian structures. One of these
structures is deservedly called the great
artesian basin (Fig. 16.20).
The effect of excessive artificial discharge
through wells drilled into artesian structures
is ultimately similar to the effect of with-
drawal upon the level of the water table.
Thousands of flowing wells in the Dakotas
and hundreds in Australia, because of care-
less waste of water through them, have de-
creased the pressures in both regions until
many wells now require pumping, and the
flow of others is much reduced.
SUGGESTED READING
Kuenen, P. H.: Realms of Water, translated by May Hollander, John Wiley & Sons, Inc., New
York, 1955.
Langbein, Walter B., et al.: Annual Runoff in the United States, U. S. Geological Survey Cir-
cular 52, 1949.
McGuiness, C. L.: The Water Situation in the United States with Special Reference to Ground
Water, U. S. Geological Survey Circular 114, 1951.
Thomas, H. E.: The Conservation of Ground Water, McGraw-Hill Book Company, Inc., New
York, 1951.
Thornthwaite, C. W., and J. R. Mather: "The Water Balance," Publications in Climatology,
Drexel Institute of Technology, Philadelphia, vol. 8, no. 1, 1955.
U.S. Department of Agriculture: "Water," Yearbook of Agriculture, 1955.
van Hylckama, T. E. A.: "The Water Balance of the Earth," Publications in Climatology,
Drexel Institute of Technolocrv. Philadelnhia. vol. Q. no. 2. 1.Q5fi.
CHAPTER 17
Natural
vegetation
NATURAL VEGETATION AS A GEOGRAPHICAL ELEMENT
The plant cover which varies greatly in
kind and in density from region to region
over the earth is one of the most striking
features of the land surfaces, for the visual
landscape is to a significant degree the prod-
uct of the vegetation mantle. Forested areas
stand in marked contrast to grasslands; the
green woodland in leaf gives a totally dif-
ferent scenic effect to that provided by the
somber grove which has shed its leaves; and
some woods of middle latitudes which are
spectacular in their beauty during the period
of rich and varied autumn colors are beauti-
ful in an entirely different way in spring.
Indeed, natural vegetation takes high rank
among those elements which serve to differ-
entiate regions in appearance and attractive-
ness, and for this aesthetic contribution
alone is a feature of great geographic
importance.
But in addition to its aesthetic qualities,
native vegetation has important resource
value. In the preagricultural stage of human
development it was the only source, besides
animal life, of the essentials for food and
clothing. Since then, as civilization has ad-
318
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
vanced and population multiplied and spread,
the original vegetation over huge areas of the
continents has been consumed or destroyed,
leaving behind a greatly altered landscape
consisting of a modified vegetation cover, as
well as human settlements, tilled fields, and
other signs of land utilization. Nevertheless,
even in this twentieth century forest products,
in the form of lumber, fuel, and pulp, and
natural pasture lands for animal grazing con-
tinue to make an important contribution to
economic well-being. As a resource in
another form, the enduring attraction of
woods and forests with their wildlife gives
many of the most popular summer and winter
playgrounds their special lure.
Since the natural vegetation is an expres-
sion of the whole physical environment, and
reflects the integration of all environmental
conditions, past as well as present, it likewise
serves as an indicator of the potentialities of
an environment for human use; thus the
suitability of a soil for certain types of farm-
ing is often clearly shown by the density and
composition of the vegetation cover. More-
over, even in those parts of the earth where
the original vegetation has long since disap-
peared and agriculture is of long standing,
the soil continues to bear the imprint of the
type of plant cover under which it developed.
CAUSES OF REGIONAL
VARIATIONS IN THE
PLANT COVER
The character of the earth's present cover
of natural, or wild, vegetation reflects chiefly
the present physical environment, whose
principal conditioning elements are climate,
soils, and organisms. Unlike animals, plants
do not have the power of locomotion, cannot
construct shelters, and do not generate heat;
so they are unable to escape the effects of
the surrounding environment to the same
degree. All the factors of the environment act
collectively and simultaneously upon plants,
and the action of any one element is condi-
tioned by all the others.
In part, however, the present cover of veg-
etation is the product of time processes and
not only the previously mentioned occur-
rences of human history, but also evolutionary
process involving modifications and regional
shiftings of plants which result from past
environmental changes.
Climatic effects In its broader pattern,
the distribution of natural vegetation over
the earth reflects present climatic conditions
more strikingly than the effects of any
other single factor. Plant geographers have
long recognized this fundamental relation-
ship, which is evidenced by the approximate
agreement between general climatic character-
istics, on the one hand, and the general
characteristics of vegetation, on the other.
This relationship, moreover, extends deep
into the past. Vegetation, being basically
dynamic, not static, has responded to long-
term changes in world climate: when climate
has changed in the past so has the world
pattern of vegetation.
But because world climates have been
stabilized now for at least several millenniums,
the present arrangement of the great vegeta-
tion grouping represents a relatively enduring
situation. In this grouping the areally extensive
vegetation types, corresponding to the major
types of climate, are called climaxes or plant
formations. Illustrations of important climaxes
are the tropical rainforest of the constantly
wet low-latitude climates and the coniferous
(needle-leaf) forests of the subarctic climates.
"The climax communities are considered to
be the highest types of vegetation that can
develop under the different aspects of climate,
and are in dynamic equilibrium with the
climate." l
Heat and water are the two climatic ele-
ments that most importantly affect plant
growth and vegetation characteristics. No
plants can live entirely without water, and
for every species of plant there appear to be
three critical temperatures: (a) lower and (b)
upper limits beyond which it cannot exist,
and (c) an optimum temperature in which it
grows most vigorously.
Different species resist cold in different
ways. Some adjust by halting certain func-
tions during the period of low temperatures.
This may be evidenced by a marked external
change, such as occurs in fall when certain
trees and shrubs shed their leaves to remain
bare in winter. These are the previously
mentioned deciduous (seasonally leaf-shed-
ding) plants, so called to distinguish them
from the evergreens, those plants that retain
some foliage through the year. In the tropics,
lacking a cold season, seasonal leaf fall where
it occurs is induced by a dry season. Most
coniferous trees belong to the evergreen
group, and lapse into a dormant period
without an apparent outward change. In
some species of plants the vegetative parts
are caused to die by the cold season, and the
plant is perpetuated only by a seed which is
resistant to cold. These are the annuals, and
they stand in contrast to perennials, whose
vegetative parts live on year after year.
Water, taken in at the roots of plants, is
the principal ingredient of sap, in which
mineral matter in solution is carried to all
parts of the plant. Transpiration of water
takes place through the leaves, the process
being associated with chemical changes by
1 Stanley A. Cain, Foundations of Plant Geography, Harper
& Brothers, New York, 1944, p. 11.
Natural vegetation 319
which the sap ingredients are prepared for
assimilation by plant tissues.
Plants that are at home in wet climates or
in wet, swampy locations are called hygro-
phytes. These plants usually have long and
relatively fragile stems containing a minimum
of woody fiber, and leaves are large and usu-
ally thin. Roots are likely to be shallow. The
banana tree, characteristic of the wet tropics,
is a hygrophyte. At the opposite extreme are
the xerophytes, which are adapted to drought
conditions. The roots of xerophytes are deep
or widespread, which increases the depth or
area from which water can be obtained, while
stems are likely to be short and strong.
Leaves are smaller and thicker, their stomata
(openings for transpiration) fewer to protect
them against rapid transpiration; leaves may
even be replaced by thorns. A hairy under-
cover is common. A thick, corky bark or a
coating of wax may further protect against
transpiration. Certain desert species one
being the fleshy-stemmed cactus adapt
themselves in a different way, viz., by accum-
ulating supplies of water within their vegeta-
tive structures.
Soil and organisms Although climate,
especially through temperature and precipita-
tion, sets the broader outlines of the earth's
vegetation groupings, modifications and
variety within the larger plant formations are
usually the result of secondary factors, chiefly
soil and organisms.
Soil is not a completely independent element
of the environment, for general soil character
is greatly influenced by climate as well as by
the vegetation cover. But nonclimatic factors
such as the nature of the bedrock, quality
and depth of parent soil materials, drainage
conditions, angle of slope, and exposure
cause many regional and local variations in
soils; and it is these variations which cause
320
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
much of the variety within the larger climaxes
of vegetation. The soil environment specifi-
cally modifies plant life chiefly through its
temperature, chemical composition, and water
retentiveness.
Further variety is added to the vegetation
mantle through the effects of organisms
effects achieved in a variety of ways. Man, one
of the more important organisms, has left a
strong mark. Thus overgrazing, involving other
animals but also fostered by man, may change
the native vegetation of a region, as it has,
in all probability, on the North American
Great Plains; and man-set fires have been a
major modifier of vegetation over extensive
areas. Indeed, over extensive parts of the
earth, man, as previously stated, has so
greatly modified the original vegetation
through his use of the land that at present it
bears little resemblance to what it was in its
native state. Other kinds of organism influence
are associated with the work of pollinating
insects and the relations between hosts and
parasites.
Classification of the earth's vegetation
The classification and brief description of the
earth's native vegetation which follows is
plant geography in its broadest aspects an
attempt to describe the principal plant as-
sociations, show their relationships to the
environmental complex, and indicate their
world distribution (Plate 5).
The broad outline of plant geography here
presented is possible because of the fact that
in spite of the incursions of man over
extensive areas covering scores and even
hundreds of thousands of square miles the
vegetation cover maintains a considerable
degree of similarity, provided the climatic
environment is fairly homogeneous. Even
more impressive, similar environments on
widely separated continents appear to have
plant formations which are much alike in
general aspect, even though they are not
composed of identical species. Thus the
plants found in the tropical rainforests of the
Amazon River Basin in South America and
in the Congo River Basin in Africa are fairly
similar in general appearance and type; and
the same plant similarity exists in the grass-
lands of Argentina, the United States, and
Hungary. Thus, on a basis of common phys-
ical needs, certain plants genetically unrelated
to one another repeatedly grow in inter-
mingled fashion in similar environments.
It is these plant associations, prevailing
over extensive areas and occupying char-
acteristic physical environments, that are the
topic to be emphasized in this chapter.
THE GREAT PLANT ASSOCIATIONS
No widely accepted geographical classifica-
tion of the earth's plant cover has as yet been
evolved, partly because of a lack of reliable
information on the nature of the vegetation
mantle over extensive areas. The classifica-
tion that follows of the great plant asso-
ciations into classes and types of natural
vegetation is therefore tentative in char-
acter.
1. Forest associations
a. Tropical forests
(1) Tropical rainforest
(2) Lighter tropical forest (including
semideciduous, deciduous, scrub-
and-thorn forest)
b. Middle-latitude forests
(1) Mediterranean woodland and
shrub
(2) Broadleaf forest
(a) Deciduous
(b) Evergreen
(3) Needle-leaf or coniferous forest
(4) Mixed broadleaf-needle-leaf forest
2. Grassland associations
a. Tropical grasslands (wooded savanna
and savanna)
b. Middle-latitude grasslands
3. Desert shrub
4. Tundra
Plant geographers commonly recognize
four principal classes of natural vegetation:
(a) forests, (b) grasslands, (c) desert shrub,
and (d) tundra, which is composed chiefly of
herbaceous plants other than grass. Without
doubt the distribution of these major classes
of vegetation over the earth's land areas is
environmentally controlled, largely the result
of climate in fact, but it is by no means
easy to make broad generalizations about the
specific qualities of the environment of each
class.
As a general rule forests are characteristic
of relatively humid climates. Because of its
deep or extensive root system, the tree is
better able to tap deep-lying supplies of
water than grass, so that how precipitation is
distributed through the year is not as impor-
tant to trees as to grass. Normally trees
do not thrive in climates with cold, dry
winters where there is a prevalence of strong
or continuous winds, with resulting excessive
transpiration.
Grassland associations dispute the posses-
sion of the land with woodland and are likely
to prevail where the environment discourages
luxuriant tree growth. Thus, in the middle
latitudes grasslands occupy drier climates
than do forests. A winter climate that is cold,
dry, and windy does not adversely affect
grasses as it does trees. But in the tropics
Natural vegetation 321
grasslands and woodlands appear to occupy
a wide range of climates, and trees and grass
are frequently intermingled. Admittedly, the
origin of tropical grasslands is a highly con-
troversial question.
TYPES OF FORESTS
AND THEIR D I ST R I B U T I N 2
LOW-IJVTITUDE FORESTS
Tropical rainforest The most luxuriant
type of woodland community, the tropical
rainforest, is the climax vegetation of trop-
ical lowlands and slopes where rainfall is
heavy and well distributed throughout the
year, there being no marked dry season. Dis-
tribution of this type of forest is imperfectly
known. Certainly the Amazon River Basin,
in northern South America, and west cen-
tral Africa are the two largest areas of tropical
rainforest, although it is found along many
rainy coasts and on numerous islands in the
tropics as well (Plate 5).
The tropical rainforest has three principal
characteristics: (a) A great variety of different
species of trees is present, in contrast to most
middle-latitude forests, where one or at most a
few species may form almost solid stands. But
rainforest trees, although species are numerous,
are rather similar in appearance, (b) There
exists a strong vertical stratification in the
forest, the many species arranging themselves
in several groups, each having a particular
height limit (Fig. 17.1). The result is a forest
with a number of tree tiers, each with its
own height level and each lower one reflect-
ing an increasing tolerance for the shade
imposed by the canopy above, (c) The num-
2 In addition to their previously stated classification as
deciduous and evergreen (p. 319), trees are classified as either
(a) broadleaf or (b) needle-leaf (coniferous). Some broadleaf
trees are evergreen, some deciduous; coniferous trees (coni-
fers) are predominantly evergreen.
322
FIG. 17.1 Tropical rainforest in the Amazon
River Basin of Brazil. Note the density of the stand
and the variability in the size and height of
individual trees. (Hamilton Rice Expedition of
1924-1925.)
her of lianas, other kinds of climbers, and
epiphytes, is unusually large. The giant
lianas have the appearance of great cables
interlacing the branches of the forest crown
and binding the individual trees together.
External and internal appearance In ex-
ternal appearance the rainforest presents a
richly varied mosaic of many shades. The
mature leaves are a deep green, but young
leaves are highly colored, resembling autumn
foliage in middle latitudes. The result is a
forest in which the fresh green of middle-
latitude woodlands is absent. Just as the
climate exhibits little seasonal change, so
does the vegetation: there is no general
dormant period when the forest is bare of
foliage. Different species drop their leaves at
different times, and trees without leaves may
be observed at any time.
Viewed internally, the tropical rainforest is
seen to be composed of trees which vary
greatly in height and diameter growing close
together. Trunks are rather slender and have
branches only near the top. The bark is thin
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
and smooth. The mass of vines and creepers
appears almost to suffocate the trees that
support it. Because of the heavy shade pro-
duced by the almost impenetrable canopy of
tree foliage, the undergrowth is usually not
dense. The typical jungle with its thick un-
dergrowth generally occurs just in areas
where light reaches the forest floor, as along
rivers and coasts or in abandoned clearings.
Lighter tropical forest The lighter
tropical forest includes a variety of woodland
vegetation types semideciduous, deciduous,
scrub, and thorn whose distinctive char-
acteristics and precise distributions are
not well enough known to permit local-
izing them individually on a small-scale,
generalized vegetation map such as Plate 5.
As a rule, where soils and drainage do not
interfere, tropical rainforest gives way at its
climatic limits to semideciduous and decidu-
ous forest, and as rainfall continues to decline
this in turn passes over to wooded grassland
and finally to scrub-and-thorn woodland
and desert shrub.
Compared with the rainforest, the lighter
tropical forest is composed of small trees
widely spaced, with a dense undergrowth of
shrubs or grass (Fig. 17.2). Also, more of the
trees are deciduous in character. Not all
species are leafless during the drier dormant
season, but enough are to make the season of
drought the time when contrast with the rain-
forest is most marked.
The scrub-and-thorn forest found in parts
of the lighter tropical forest varies in density
from an open, parklike growth of low stunted
trees and thorny plants to dense thickets of
the same. The trees composing this dry forest
are small in diameter, rarely exceeding 1 ft.
No other type of tropical forest equals this
forest in tolerance of physical conditions.
Utilization of tropical forests Although
tropical forests occupy nearly 50 per cent of
the earth's total forest area, they at present
only supply the limited needs of local popu-
lations and furnish to world commerce small
quantities of special-quality woods, such as
dyewoods and cabinet woods, Nevertheless
these low-latitude forests, especially the trop-
ical rainforest, represent one of the world's
great potential timber sources. The problems
involved in their utilization labor supply,
sanitation, the need for new logging tech-
nologies, how to utilize the great variety of
species composing the tropical forestare
serious, but none appears to be insur-
mountable,
Natural vegetation 323
MIDDLE-LATITUDE FORESTS
Mediterranean broadleaf evergreen
woodland and shrub The Mediterranean
broadleaf evergreen woodland and shrub is
characteristic of subtropical locations with
mild rainy winters and long, dry, and usually
hot summers. The largest region of this
forest is the Mediterranean borderlands;
smaller areas exist in California, middle Chile,
southern Australia, and the Cape Town
region in southernmost Africa. It is an un-
usual woodland, for seldom are trees broad-
leaf and evergreen and at the same time
adapted to serious summer drought. Instead
FIG. 17.2 Lighter tropical forest (semideciduous) in central Africa. The
trees are sufficiently far apart that they do not create a dense shade. Coarse
grasses mantle the forest floor. (Aawican Geographical Society.)
324
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
FIG. 17.3 Mediterranean woodland In California. An open stand of
dwarf oak and shrub. (U.S. Forest Service.)
of dropping their foliage in the dry season,
as trees of many tropical deciduous forests
do, the trees in this woodland adjust to
drought through protective devices thick
bark and small, thick leaves with hard shiny
surfaces which reduce the loss of water by
transpiration.
Where climatic and soil conditions are
most favorable, the Mediterranean woodland
consists of low, or even stunted, widely
spaced trees with thick trunks and gnarled
branches (Fig. 17.3). Between the trees the
ground is partly covered by bush vegetation
or bunch (tufted) grass. Seemingly more
widespread is a vegetation cover consisting
largely of shrubs and bushes in which there
may be stunted trees. The chief economic
importance of this bush thicket lies in its
protection of slope lands from the injurious
effects of rapid runoff.
Temperate broadleaf forest Within
the more humid parts of the middle latitudes
are found two great groups of forests: (a)
forests of broadleaf trees, and (b) forests of
needle-leaf trees, or conifers. Over large
areas these trees are mixed in conifer-broad-
leaf forests. As a general rule, but with im-
portant exceptions, the coniferous forests
occupy the colder locations and thus are
usually on the poleward side of the broadleaf
woodlands. In regions of porous, sandy soils
where water is deficient, and on steep moun-
tain slopes where soils are thin or rocky and
temperatures lower, conifers may supplant
broadleaves even in the lower middle latitudes.
Temperate broadleaf forests vary widely in
composition, the dominant tree species dif-
fering from one region to another. In some
areas, especially along their poleward
margins, there are numerous conifers among
them so many, in fact, that some plant
geographers designate such forests as mixed
rather than broadleaf (Fig. 17.4). In the
eastern United States two general broadleaf-
forest areas are distinguished: (a) a north-
eastern area including northern Wisconsin
and Michigan, New York, and southern New
England, where birch, beech, and maple pre-
dominate but there is a large infusion of
hemlock and other conifers; and (b) a central
and southern area lying south of the first
and terminating at the northern and western
boundary of the sandy Atlantic and Gulf
Coastal Plain (Fig. 17.5). In this latter forest,
which was originally the finest and most ex-
tensive area of broadleaves anywhere in the
world, the broadleaves oak, chestnut, hickory,
and poplar predominate, but the coniferous
pines become prominent toward the Coastal
Plain margins. The greater part of the original
American broadleaf-forest belt, lying as it
does in an environment eminently suited for
agriculture, has now been cleared and turned
into farm land.
Natural vegetation 325
By far the greater part of the temperate
broadleaf forest is deciduous in character,
the trees dropping their leaves in fall and re-
maining without foliage during the winter
season (Fig. 17.4). Except in the dormant
season, this forest is rather uniformly bright
green in color. Along the humid subtropical
margins of the middle latitudes evergreen
broadleaf forests are to be found, but these
are not nearly so extensive in the middle
latitudes as the deciduous variety. These sub-
tropical forests, in many respects akin to
those of the wet tropics, occur principally in
southern Japan and in southern and south-
eastern Australia.
Needle-leaf forest Coniferous trees are
predominantly evergreen, the addition and
fall of the needles being continuous rather
FIG. 17.4 Broadleaf deciduous forest (oak and hickory) in northern
Indiana. Much of this type of woodland occupied potentially good agricultural
land, and as a consequence was destroyed in the process of settlement.
flf.S. Deharfment of Agriculture.}
326
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
Wettern larch and
western white pina
FIG. 17.5 Natural-forest types of the United States. (U.S. Forest Service.)
than confined to any particular period or
season. Unlike broadleaves, the needles of
conifers are xerophytic in character, so that
shedding is not necessary to protect against
a cold or a drought season. On the whole,
the crowns of coniferous forests do not in-
tercept so much sunlight as the crowns of
broadleaf woodlands, and yet less sun actually
reaches the earth in coniferous forests, be-
cause (a) they lie predominantly in higher
latitudes where there are longer periods of
low sun, and (b) they are never without
foliage. As a result there are usually less
surficial vegetation, a minimum of bacterial
activity, and smaller accumulations of humus
in the soil.
Subarctic conifers Conifers are most ex-
tensively developed in the severe subarctic
regions of North America and Eurasia, where
they form wide and continuous east-west
forest belts stretching from coast to coast
(Fig. 17.6). The name taiga has been given
to the subarctic coniferous forests. On their
northern frontiers they make contact with the
tundra, a region thoroughly hostile to trees.
The Eurasian taiga forms the largest single
continuous forest area on the earth (Plate 5).
Conifers (larch, spruce, fir, pine) predom-
inate in the taiga, although broadleaf trees
(alder, willow, aspen, birch, mountain ash)
are scattered throughout, individually as well
as in thickets or clusters. Species are few in
number. Trees are small in size, usually not
over 1 \k ft in diameter, and growth is slow
(Fig. 17.7). On the shaded forest floor vege-
tation is meager, mosses and lichens being
the most common plant forms, and some-
times even these are stifled by the thick
blanket of slowly decomposing needles. Little
organic matter is made available to the soil,
for needle leaves are a poor source of humus
to begin with, and the low temperatures and
deep shade act to retard decomposition and
discourage the activity of soil fauna.
Natural vegetation 327
F I G . 1 7 . 6 In many parts of ice-scoured subarctic Canada the coniferous-
forest COVer is thin and patchy. (Royal Canadian Aii Force.)
Conifers in lower middle latitudes South
of the great belts of subarctic conifers are
other, less extensive areas of needle trees
which are more valuable forest regions (Plate
5). This is because they are composed of
larger trees and superior timber species, and
also are more accessible.
In western North America broken belts of
conifers extend southward from the taiga
following the rainier highland chains the
Pacific Coast mountains and the Rocky
Mountains to beyond the Mexican border
(Fig. 17.5). These forests of the American
Pacific Coast states, western Canada, and
Alaska constitute the most extensive area of
fine coniferous forest anywhere in the world.
Large trees, dense stand, good-quality timber
all contribute to this high rank (Fig. 17.8).
East of the Rockies conifers extend south-
ward from the taiga into southeastern Canada
FIG. 17.7 Side view of subarctic coniferous
forest in Canada. Note the small diameter of many
Of the trees. (U.S. Forest Sen>ice.)
328
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
seems somewhat out of place, for rainfall is
abundant, and the growing season long.
However, the poor, sandy, droughty soil and
the high evaporation are offsetting factors,
creating an environment that is generally
hostile to broadleaf varieties. Open, parklike
character, with the ground covered by a
mantle of coarse grasses or low shrubs, is
typical. During the last few decades this
southern pine forest has been one of the
principal sources for American lumber, al-
though the peak of its production has been
passed (Fig. 17.10). Extensive areas of low-
grade cutover land are now one of the most
conspicuous features of the southern pine
region. On the poorly drained floodplains of
the Coastal Plain, pines give way to a con-
F I G . 17.8 Interior view of Douglas fir forest in
the Pacific Northwest of the United States. Trees
are tall, of large diameter, and form a dense stand.
(U.S. F&rest Service.)
and the northern portions of the northeastern
tier of American states Minnesota, Wis-
consin, Michigan, New York (the Adiron-
dacks), and much of Maine. The most valu-
able timber trees from this eastern forest have
been removed, leaving behind extensive areas
of cutover waste-land of little value. South of
the taiga in Eurasia valuable coniferous forests
occupy the slopes of the Alps, the Carpathians,
and other highland regions, as well as certain
sandy areas of coastal and outwash plains.
The southern pine forest of the Atlantic
and Gulf Coastal Plain in the United States,
separated from the northern conifers by an
extensive broadleaf forest, is composed of 10
different species of pine, of which the long-
leaf pine is most abundant (Plate 5; Fig. 17.9).
Climatically this subtropical needle-tree forest
FIG. 17.9 Southern pine forest of the United
States, composed of longleaf, loblolly, and slash
pines, typical of the Atlantic and Gulf Coastal Plain.
(U.S. Forest Service.)
Natural vegetation 329
MATURE TIMBER LAND
Each dot represents
IO.OOO acre-s
FIG. 17.10 Distribution of land with timber large enough to be
currently merchantable. It does not include second growth cut primarily for
chemical distillation, firewood, posts, ties, mine props, etc. (U.S. Forest Service.)
trasting type of forest composed of such trees
as the tupelo, red gum, and cypress.
TYPES OF GRASSLAND
AND THEIR DISTRIBUTION
Tropical grasslands: wooded savanna
and savanna There is at present too little
reliable information on the various types and
gradations of tropical grasslands and their
distribution to permit a satisfactory classifica-
tion and mapping. Therefore the various
low-latitude grasslands, or savannas, are here
grouped together under one general heading.
The reader should be forewarned, however,
that this grouping must not be interpreted to
indicate uniformity of species and appearance.
There are open savannas, with only occasional
trees, and there are others in which trees or
shrubs are so numerous that it is difficult to
decide whether the vegetation is more prop-
erly classified as grassland or woodland.
There is also regional variety in the height
and spacing of the grasses.
Until recently tropical grasslands were be-
lieved to be definable as the climax, or plant
formation, of areas where precipitation was
intermediate between the heavy year-round
rainfall of the tropical rainforest and the con-
stant drought of the desert. But this climatic
explanation has been seriously questioned in
recent years. It now seems doubtful whether
tropical grasslands are a plant formation in
equilibrium with a specific type of climate at
all. Since with declining rainfall tropical rain-
forest frequently gives way to semideciduous
330
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
FIG. 17.11 Tall, coarse grass, studded with low trees, in the wooded
Savanna Of Africa north Of the equator. (American Geogtaphical Society.)
and deciduous forest, and finally to scrub-and-
thorn forest, perhaps a tropical-grassland
climate is nonexistent. Thus it has been
fairly well established that the tropical grass-
lands of Latin America are fairly coincident
with areas of flattish or slightly rolling terrain
where, because of the mild relief and an im-
permeable subsoil, drainage is poor. In such
areas the vegetation may be subjected to
standing water in the rainy season and a
parched soil at other times, and repeated
burnings may help in establishing and ex-
panding the grasslands.
Character of vegetation Savannas of both
the wooded and the open variety are com-
mon. As previously stated, the trees are few
and widely scattered in some places, while in
others they are so numerous they form
thickets (Fig. 17.11). Clumps of trees inter-
mingled with patches of open grassland also
are a very common arrangement. Throughout
the tropical grasslands dense forests usually
occupy the river floodplains. Tall trees are
not absent from tropical grasslands, to be
sure, but low and dwarf varieties are more
common. Usually they have twisted and
gnarled trunks, a thick corky bark, and
leathery leaves.
Savanna grasses are variable in height. In
Africa there are high savannas where the
grasses range from 5 to 15 ft in height, but
these do not seem to exist in Latin America.
Much more common are the tall bunch grass
(1 to 2 ft high) and the short bunch grass
(under 1 ft high). Both grow in slender tufts
with much hare earth between individual
tufts, so that the grass may occupy not more
than 60 per cent of the soil surface. The
blades of mature savanna grasses are stiff and
leathery, and only the fresh young shoots are
palatable to most grazing animals. Among
the natives of tropical grasslands it is a com-
mon practice to burn off the grasses in the
dry season in order to make room for new
growth at the beginning of the rains.
Middle-latitude grasslands The grass-
lands of middle latitudes appear to be the
climax vegetation of subhumid and semiarid
regions, and in this they stand in contrast to
the savannas of the tropics. An additional
contrast is the general absence of trees in the
grasslands of middle latitudes, except along
their contact zones with forest and in the
vicinity of streams.
FIG. 17.12 (From map by ./. Richard Carpenter.)
Natural vegetation 331
In the grasslands of interior North America
east of the Rocky Mountains three large sub-
divisions are recognized: (a) the short grass
occupying much of the semiarid Great Plains,
(b) the transitional mixed-grass prairie coin-
ciding with a somewhat more humid belt to
the east of the short grass, and (c) the tall-
grass prairie, or true prairie, typical of the
better-watered lands between the mixed
prairie and the eastern forest (Fig. 17.12).
This tall-grass prairie originally extended
eastward as far as western Indiana in the
form of a wedge driven into the forest. West
of the Rockies, there still exist extensive areas
of short bunch grass in parts of Washington,
Oregon, Idaho, and California. Almost all of
the original prairie of interior North America
east of the Rockies has been converted into
agricultural land of high quality. Originally,
the native wild grasses in this region attained
TYPES OF NATURAL
GRASSLANDS EAST OF THE
ROCKY MOUNTAINS
Snort -grass plains
0:1 Mixed -grass prairie
Tall -grass prairie
332
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
heights of 3 to 8 ft, and were intermingled
with many colorful flowering plants.
Although North American grasslands have
been subdivided into several groups and
their distributions shown on Fig. 17.12, this
same degree of detail has not been possible
for the remainder of the earth's middle-
latitude grasslands. For this reason, on
Plate 5 all middle-latitude grasslands have
the same legend. This should be interpreted
to mean that insufficient information is avail-
able to permit a more detailed system of
classification and of distribution similar to
that for North America.
PLANT COVER OF
DESERTS AND TUNDRA
Deserts A few deserts or parts of deserts
are extensive areas of rocky plain or sand
and almost wholly without plant life. But
most arid regions of both low and middle
latitudes have some vegetation, even though
FIG. 17.13 Desert shrub, chiefly sagebrush,
in Nevada. This type of vegetation cover is of little
Value for grazing. (John C. Weaver.)
it is sparse. It may be low bunch grass, with
widely spaced bushes, or fleshy water-storing
plants such as cacti; but much more com-
monly it is the perennial xerophytic shrub.
In the United States, for instance, the latter
type of vegetation predominates over a large
part of the area west of the Rockies, inter-
rupted here and there by bunch grass, or, at
higher elevations, by forests. Rainfall over
much of this region is under 12 in. per year.
The perennial shrubs of desert areas grow
far apart, with much bare soil showing be-
tween them a response to low rainfall (Fig.
17.13). Growth is very slow. Desert shrubs,
exemplified by such American types as sage-
brush and creosote bush, are physiologically
equipped through special forms of roots,
stems, and leaves to withstand drought. Some
are deciduous, others evergreen in character.
Unlike the perennial shrub, the desert
annuals do not show drought-resistant char-
acteristics. Their stems and leaves are del-
icate, roots are thin and relatively shallow,
and flowers are conspicuous. They adapt to
drought through a very short life cycle: the
dormant seeds germinate after a shower, the
plant grows and makes its seed rapidly, the
plant dies forthwith.
The vegetation of the desert proper is
scanty and pale green, and so stands in con-
trast to the rich verdant color and luxuriance
of vegetation around oases, where water is
abundant a contrast that is striking when,
as frequently, almost knife-edge boundaries
separate the two.
Tundra Genuine tundra is composed
largely of such lowly forms as mosses, lichens,
and sedges, the whole of this vegetation in-
completely covering the ground. In places
there is much bare, stony soil with only the
most meager plant life. On the southern
margins of the tundra, where it merges with
the taiga or coniferous forest, the vegetation
cover is more complete, and stunted and
creeping forms of trees and bushes are con-
spicuous. Grasslands exist on the marine
margins of the tundra.
The coldness and acid character of tundra
soil retards water absorption, and in the long
winter period of physiological drought the
soil moisture is locked up in solid form; so
most tundra plants appear xerophytic, having
stiff, hard, leathery leaves with thick cuticle.
The short period between frosts makes the
vegetative period in the tundra as short as 2
months, or less, and for this reason plants are
Natural vegetation 333
compelled to hurry through their vegetative
cycle. Even so, many of them are frozen while
still in flower or fruit.
Dry tundra is composed principally of
lichens interspersed with coarse, grasslike
sedges the predominance of lichens result-
ing in a dull, gray landscape tone. Wetter
flooded areas along streams and shallow
basins on higher ground are characteristically
moss swamps. The southward-facing drier
slopes are flower oases, where in summer
brilliant colors are in evidence in a great
variety.
SELECTED REFERENCES
Borchert, John: "The Climate of the Central North American Grassland," Annals of the Associ-
ation of American Geograpturs* vol. 40, 1950, pp. 1-39.
James, Preston E., and Clarence F. Jones (eds.): "Plant Geography," in American Geography:
Inventory and Prospect, Syracuse University Press, Syracuse, N. Y., 1954.
Kiichler, A. W.: "A Geographic System of Vegetation," Geographical Review, vol. 37, 1947,
pp. 233-240.
Rand McNally & Company: "World Natural Vegetation," colored map in Goode's World Atlas,
Chicago, 1960, pp. 16-17.
Richards, P. W.: The Tropical Rainforest: An Ecological Study, Cambridge University Press,
New York, 1952.
Schimper, A. F. W.: Plant Geography upon a Physiological Basis, English translation, Oxford
University Press, New York, 1903.
U.S. Department of Agriculture: "Climate and Man," Yearbook of Agriculture, 1941.
U.S. Department of Agriculture: "Grass," Yearbook of Agriculture, 1948.
U.S. Department of Agriculture: "Trees," Yearbook of Agriculture, 1949.
U.S. Government Printing Office: "Natural Vegetation," Atlas of American Agriculture, sec. E,
1924.
Weaver, John E.: The North American Prairie, Johnsen Publishing Company, Lincoln, Neb.,
1954.
, and Frederic E. Clements: Plant Ecology, 2d ed, McGraw-Hill Book Company, Inc.,
New York, 1938.
CHAPTER 18
Soils
The nature and significance of soil
The outermost rocks of the earth's crust are
continuously being decomposed by mechanical
and chemical weathering processes, and as a
consequence, most areas are covered by the
thin layer of more or less finely divided material
called the regolith. Some of this is residual,
that is, it accumulates on top of the bedrock
from which it forms; while some of it is
moved by agents and deposited in other areas,
for example as loess, glacial drift, or alluvium.
In any case, most of the earth's land surface
is covered with exposed, loose rock debris;
only in limited localities does the bare bed-
rock protrude or ice cover the surface.
334
Except when frozen, this superficial mantle
of regolith is relatively porous, so that air
and water circulate between the mineral par-
ticles. The top of the layer, being exposed to
sunlight, is regularly bathed with energy,
and the interaction of all the components
supports various forms of life, adding yet
another element to its character. This
exceedingly thin layer where the solid, liquid,
and gaseous inorganic and organic ingredients
are integrated is called "soil." Only when all
of these components are present is the mix-
ture true soil.
In most places this complex, life-supporting
veneer extends downward from the surface
only a few feet. In this narrow zone where
inorganic and organic materials interact, in-
numerable processes are continually at work
developing layers, or horizons, with different
chemical and mechanical characteristics. A
vertical slice through a particular soil cutting
through the soil horizons is called the profik
of that soil.
It would be difficult to overemphasize the
significance of soil. The variety of chemical
elements on which human life depends are
primarily needed in the form of organic com-
Soils 335
pounds, such as proteins, fats, carbohydrates,
and vitamins; and these come from the soil
either by way of plants man consumes directly
or as animal products derived from plants.
Some areas are covered with soils that under
natural conditions can support a large and
healthy human population, but the reverse
seems to be true of even larger areas. If the
qualities of a soil are nutritionally deficient,
so will be the food derived from it, and con-
sequently the health of people who attempt
to subsist upon that food.
THE CONTROLS OF SOIL FORMATION
The soil anywhere represents a stage in a
continuing evolutionary process, and its
characteristics develop as a result of the in-
teraction of many controls. The individual
controls vary in their effects and relative im-
portance from place to place, and some of
the variations are quite systematically arranged
over the earth, such as those deriving from
climatic factors, while others are not, such as
those primarily dependent upon the character
of the bedrock or the land form. The general
world pattern of soil regions is more subject
to internal variation than the pattern of cli-
matic variation because some of the important
soil controls do not exhibit a patterned dis-
tribution. The jnajor controls that combine
to produce a soil are parent material, climate,
living organisms^- land form, and time.
Parent material Each kind of rcgolith
and there are many kinds, such as the sed-
iments of old lake bottoms, the accumulation
of glacial drift, the new alluvium on flood-
plains and deltas, aeolian deposits of volcanic
ash and loess, and, especially, the residual
mantle weathered in place from bedrock
contains a combination of mineral ffrains^ff
particular chemical character which have
weathered to a particular array of jfragmcnt
sizes. Although theprocesses of dfjY f> ^p nM>nf
may impart ne\vjcharactcristics to the soil,
they arc not likely to erase complctelYjhe
distinctive effects deriving from this parent
materiairSome regoliths may modify rapidly,
whereas others may be highly resistant to
change Some are highly complex aggrega-
tions of mineral compounds; others arc
The fragment sizes derived from the
weathering of the parent material ^aie Q
great importance because they affect the
degree to which water and air^canj:irculate
in the sofT layer. The importance of the min-
eral content lies in the. fact that the cKemicai
character of the soil is largely the source of
the soil's fertility. A mineral element not in
the parent material will be missing from the
336
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
soil. Thus the parent material may be
thought of as a limiting factor with respect
to some of the soil's mechanical and most of
its nutrient characteristics.
Climatic factors Soil formation is in-
fluenced both directly and indirectly by the
climate. Climate directly affects the rate of
weathering of rocks and the amount of water
percolating through or evaporating from the
soil. Prevailingly high temperatures promote
rapid weathering and other chemical changes
in the soil, while cold temperatures slow
them; and alternating seasons of rain and
drought develop soil compositions and colors
differing from those of continuously rainy
regions. Water more prevalent, of course,
in humid regions than in dry lands acts to
remove the lime and other soluble salts from
the soil.
Climate also affects the soil character in-
directly through its influence upon the
organic content of the soil. Most organisms
can flourish only within certain temperature
and moisture ranges, and partly as a conse-
quence of such restrictions, there are definite
zones of plant life on the earth. The vegeta-
tion has, in turn, a marked effect on, soil
character, and as a result, soil forms vary
with the different organic forms they support.
Because the vegetation of an area is depend-
ent to some degree upon the climate for the
form it takes, the soil of the area also is con-
trolled by the climate.
Plants and animals Various kinds of
organisms and their tissues affect the soil
character in a variety of ways. For example,
when plants and animals die their remains
become a part of the soil complex, and the
microorganisms (bacteria, protozoa, fungi,
etc.) in the soil are primary agents affecting
the manner in which the decomposition of
plant and animal remains takes place. Some
microorganisms also can change atmospheric
nitrogen into a form that can be utilized by
plants. The soil is modified by burrowing
organisms and plant roots because by pen-
etrating the soil they add to its porosity.
Deep-rooted plants too, bring minerals up
from the subsoil and hold them in their
tissues; when the plants die and decay, these
minerals are returned to the upper soil layers.
Land form Slope characteristics are im-
portant factors in soil formation because
slope differences affect, sometimes greatly,
the moisture and air conditions within the
soil and, even more significantly, the rate of
its surface erosion. Maximum soil develop-
ment will most likely take place on undulat-
ing but well-drained uplands with free under-
drainage and only slight surface erosion. On.
such sites surface materials are removed at a
rate slow enough to allow a relatively deep
penetration of the effects of the soil-forming
processes. Soils formed under such circum-
stances become fully developed and possess
well-defined profiles. The soils of steep
slopes, on the contrary, generally fail to de-
velop these characteristics because acceler-
ated surface erosion restricts the profile
development in several ways, such as thin-
ning the horizons and restricting the vegetative
cover and consequently lessening the organic
content. Soils of poorly drained or marshy
areas develop quite different profiles, but
in these cases it is primarily because they re-
main waterlogged and air cannot penetrate
them.
Time Because the other soil-forming
processes do not proceed at the same rates
in different environments, time is not a con-
stant control either. Thus there is no specific
length of time in which a soil develops its
own particular characteristics. Some may
reach a condition of relative balance in a
comparatively short period, possibly in a few
hundred years or even much less; others
may require thousands of years.
If the usual soil-forming processes of an
area have been locally restricted in any way,
the profiles will show the effects of these
modifications. Since there are a great many
ways in which this can happen, it is to be
expected that some soils of a region will not
have the typical mature profile. In fact, in
many regions there is little, if any, mature
Soils 337
soil. Because farming modifies a soil, it is
also likely that nearly mature soils are re-
stricted to untouched soils. Many regions of
high agricultural development have remain-
ing only a few scattered remnants of the
virgin soils. On the other hand, it is impor-
tant to keep in mind that the immature soils
of an area commonly have distinctive qual-
ities closely related to those of the actual or
hypothetical mature soil of that region.
THE ELEMENTS OF SOIL CHARACTER:
COMPONENTS OF SOIL
The soil cover varies from one place on
the earth to another, so that generalizations
regarding the geographical occurrence of soils
on the basis of similarities in their qualities
and profile characteristics may be made in
much the same way atmospheric phenomena
are categorized, leading to a system of cli-
matic types, groups, and regions. Because
the factors affecting the formation of soil
vary systematically from place to place just
as the climatic controls do, the soils of the
earth also vary systematically. In order to un-
derstand the world pattern of soil regions, it
is first necessary to define and describe the
essential characteristics that make one soil
similar to another but different from a third.
The more important of these are (a) fertility,
that is, the chemical characteristics affecting
nutritional quality, (b) texture and structure,
or the sizes and arrangements of the in-
organic particles, (c) organic components,
both plant and animal, included as integral
parts of the soil, (d) water and air relation-
ships, (e) color, and (/) soil profile.
Fertility All plants and animals living
on land, including man, ultimately obtain
their sustenance from the soil. Animals are
nourished by plants or by other animals
which feed on plants, and plants obtain from
the soil the elements required for their photo-
synthetic construction of carbohydrates and
their biosynthetic production of protein and
other essential foods. 1
Many chemical elements are required to
sustain life; but some are needed in relatively
large amounts, such as oxygen, carbon,
hydrogen, nitrogen, sodium, calcium, potas-
sium, phosphorus, sulphur, magnesium,
and iron, while others are required only in
very small amounts, such as manganese,
copper, zinc, iodine, and boron. Although
some are supplied directly from the air as
gases, others are obtained through the water
in the soil, others, such as nitrogen, are
supplied through the organic material in the
soil, and still others, such as the metallic
elements calcium, potash, and phosphorus,
are derived from the soil's inorganic matter.
1 W. A. Albrecht, "Soil Fertility and Biotic Geography,"
Geographical Review, vol. 47, 1957, pp. 87-105.
338
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
The available supply of the critical chemical
elements is referred to as a soil's fertility.
The earth's crust, and consequently the
soil, is in large measure composed of only a
few elements, with the other life-giving ele-
ments occurring relatively rarely. The most
abundant components, which constitute the
bulk of the soil, are the elements oxygen,
silicon, aluminum, and iron as they are com-
bined in the common minerals and their
weathered derivatives. The major fertility
elements, excepting carbon and nitrogen, are
provided from the same source.
Reduction of fertility and remedies
The supply of essential minerals in a soil
may be reduced in several ways: by erosion;
by excessive use, or overcropping; and,
especially, by leaching, the dissolving of
elements by water percolating downward. In
arid regions the rate of removal of the fertility
elements by leaching is low. In fact, the ac-
cumulation of soluble minerals in the soil
may be large enough to be even harmful. In
humid regions, however, the loss by leaching
is generally heavy. The slowness with which
chemical elements are supplied by natural
processes often results in soils of humid
lands being deficient in one or more of the
critical elements. The deficiency may be
partially remedied in several ways: by ap-
propriate mineral fertilizing; by fallowing,
leaving the land idle so that natural decom-
position provides an additional supply; and
by manuring, returning a major proportion
of the plant growth to the land in the form
of animal excreta and plant refuse.
Factors affecting fertility Plants obtain
their mineral nutrients only from the supply
of minerals dissolved in the water in the soil.
The transfer is made by means of an ion ex-
change between the plant roots and the solu-
tion. 2 Each plant species has a certain set of
nutrient requirements, and a plant's ability^to
obtain these nutrients as well as the soil's
capacity to make them available assuming
that the elements are in the soil is greatly
affected by the soil solution.
One of the more important indicators of
the availability of nutrients, but by no means
a complete index of a soil's potential, is the
acidity or alkalinity of the soil. With all
other aspects being favorable, this does pro-
vide an indication of the kinds of plants that
will grow in a particular soil and the food
elements the plants will contain. In some
localities organically derived acids and
abundant rainfall tend to reduce the avail-
ability of the basic compounds as well as to
remove those compounds from the soil by
leaching. The soil solution then is likely to
have an acid reaction favorable to the pro-
duction of excessive amounts of bulk carbo-
hydrates, such as starches, sugars, fats, and
cellulose, as compared to the nutritionally
more significant proteins. In less humid
regions, where there is less leaching, soils
normally have a neutral or somewhat alka-
line reaction, and there is likely to be a
greater supply of calcium, magnesium, and
the other essential elements conducive to the
production of proteins as well as car-
bohydrates.
It is apparent that the most universal factor
affecting the fertility of a soil is the degree of
leaching to which the soil is subjected. This,
2 The processes by which a plant obtains its nourishment
are very complex and as yet not completely understood. A
root takes in water from the soil by capillary pressures and
the osmotic pressures of the solution. The soil's nutrient
value is dependent upon the exchange of the plant's ions
(electrically charged particles in solution) for the nutrient ions
available on the surfaces of the soil particles in contact w ; ' K
the root.
Soih 339
in turn, depends largely upon the rainfall,
though also, to some extent, on temperature,
vegetation, and other factors. Thus humid
regions tend to have less fertile and more
acid soils than do dry regions.
Texture and structure Assuming favor-
able climatic conditions, of course, the produc-
tivity of a soil depends upon several physical
characteristics in addition to its inherent fer-
tility. Among the more important are its tex-
ture, or the proportional sizes of the various
inorganic particles, and its structure, the
manner in which these particles tend to
clump or aggregate.
Texture The inorganic particles of a soil
commonly occupy nearly one-half the volume
of the upper part of the profile, and most of
the chemical reactions within a soil, such as
those that make nutrients available to plants,
take place on the surfaces of the particles.
The smaller the soil particles, the more
specific surface, or total surface per unit
mass, there will be. Consequently, the re-
activity of a soil, which is the soil's ability to
react chemically and thus provide an ionic
food supply, varies in direct proportion with
the specific surface (Fig. 18.1). The sizes of
the particles also affect markedly the move-
ments of water and air within a soil. Very
large particles, which allow free drainage,
have poor water retention, making soils dry
out quickly after rains. Conversely, very
small particles inhibit drainage and air
movement.
Particle sizes are grouped in classes rang-
ing from sands (the largest) to clays. The
table shows the class limits assigned by soil
scientists.
Since the inorganic mass of a soil usually
contains fractions from more than one class,
various combinations of percentages are given
MORE
LESS
/
/
/
/
/
SPECIFIC SURFACE OF A SOIL
(Sands Silts Clays)
FIG. 18.1 The general relationship between
the specific surface of a soil and its chemical
reactivity.
specific names, loam being the general term
assigned to combinations that include moder-
ate amounts of all three (Fig. 18.2). The most
significant fraction of a soil as far as reactivity
is concerned is the clay because clay particles
are the smallest. In addition to the sand, silt,
and clay particles, however, there are very
much smaller particles of either organic or
inorganic origin called colloids. The role of
colloids in the physical and chemical proc-
esses of the soil is known only imperfectly,
but it is suspected that one of their functions
is the formation of gelatinous films on soil
particles that affect the clumping of the
particles.
The soil's texture usually varies from
Texture Classes
Name
Diameter, mm
Sand
2.0 to 0.05
Silt
0.05 to 0.002
Clay
Less than 0.002
340
IOO 9O dO 7O 6O SO 4O 3O 2O IO O
PER CENT SAND (2.O to O.O5 mm)
F I G . 1 8 . 2 The textura! triangle used by the
soil scientists of the U.S. Department of Agri-
culture. In order to find the textural class, the
percentages of sand, silt, and clay are entered on
the appropriate scales and the hatch lines are
followed to the intersection of the three lines.
horizon to horizon. Downward-moving water
may wash with it the smaller particles, mostly
the clay and colloidal fractions, and thus
reduce their proportion in the upper part.
This mechanical removal is called eluviation.
The load removed by eluviation may be de-
posited lower in the profile. Such charging of
a layer with fine particles from above is called
illuviation. Because this shifting of particles
is accomplished by moving water, the soils
of humid lands tend to be eluviated as well
as leached.
Structure Not all the important physical
characteristics of a soil depend only upon its
texture. For example, soils that consist largely
of clay particles are not necessarily compact
and impervious to water and air, as might be
expected. Instead, in many clayey and silty
soils the individual particles are arranged
together in tiny clumps, which, as a result,
have some of the physical characteristics as-
sociated with larger particles. This property
of a soil, its structure, is beneficial for the
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
soil's productivity because it may permit
considerable pore space to develop. Thus an
internal structure can develop in which the
pore space among the structural units avail-
able for air, water, and root penetration is
much greater than in a soil of the same tex-
ture but with a less favorable structure (Fig.
18.3). In some soils the amount of pore
space may be less than 20 per cent of the
soil volume, whereas in highly structured
clays it may exceed 60 per cent. Most agri-
cultural soils include amounts of pore space
comprising from 35 to 50 per cent of the soil
volume. *
Good structural arrangements including a
high percentage of pore space commonly are
found in soils of fine texture that have con-
siderable organic content, but sandy soils are
essentially without structure, with each sand
particle acting as an individual unit. A
favorable structure is promoted by the pres-
ence of lime, colloids, and organic material
that form gluey films which help the soil
particles to stick together. On the other
hand, a good soil structure may be destroyed
by improper treatment.
Organic components The organisms
and partially decomposed organic matter
FIG. 18.3 Individual particles in a soil with a
structure may be arranged in clumps, thus
affecting the pore space.
make the soil complex essentially different
from raw regolith. Dead organic matter de-
rived from plant and animal tissues provides
the major food- for bacteria, fungi, and proto-
zoa. These soil microorganisms, which may
constitute so large a part as to total 1,000 Ib
per acre, perform many useful functions:
they rot organic matter; they promote good
soil structure; they make nitrogen available
to the plants; and they produce antibiotics
that promote the quality and health of plants.
The organic fraction of the soil is con-
stantly being used by the plants, but under
natural conditions a fresh supply of raw
organic matter is added each year to the soil.
Partially decomposed plant remains are
called humus. In the natural course of soil
variation from place to place some soils
have relatively small amounts of humus and
others are richly supplied. Some, such as
peat soils, are made up largely of slightly de-
composed organic matter which has not yet
reached the condition of humus.
The part played by the humus and the
living microorganic population within the
soil in maintaining soil quality includes the
following: (a) the organic material, when
dissolved, directly supplies food for plants,
including nitrogen and some quantities of
the essential mineral elements, such as cal-
cium, magnesium, and phosphorus; (b) the
dead organic tissues are the major food source
for the living microorganisms of the soil,
which in turn affect the health and quality of
the higher forms of organic life supported by
the soil; (c) the process of organic decompo-
sition yields complex organic acids which
contribute to further weathering of mineral
matter; (d) the humus has a high water-hold-
ing capacity, which helps the soil retain a
supply of water for the soil solution and
at the same time retards the leaching of dis-
Soils 341
solved minerals until the plants can use
them; (e) the humus promotes the develop-
ment of a structure favorable to water and air
circulation, plant cultivation, and root
development.
Nitrogen is essential to plant growth and
protein production. The supply in the air
is not directly available to plants, but is
transformed, largely through the work of
microorganisms, into the soluble form of
nitrates which the plants can use.
The activity of the higher forms of life,
such as many kinds of insects and, especially,
earthworms, is extremely favorable for soils.
Insects are responsible for a considerable
portion of the processing of plant residue
into humus; together with worms they affect
the porosity of soil with their burrows and
galleries; and they do extensive transporting
and overturning of the soil. The several
million earthworms which may inhabit an
acre of soil can bring as much as 20 tons of
material to the surface in a year. This is an
important aid in the vertical mixing of the
soil materials.
Water and air relationships Although
the proportions vary from soil to soil, per-
haps 50 per cent of the volume of an average,
good-quality surface soil consists of inorganic
particles, organic materials, and living organ-
isms; water and air circulating within the pore
spaces make up the remainder of the complex.
Water and the gases of the air take part in the
inorganic and organic chemical reactions that
occur in the soil, and hence are just as much in-
tegral constituents of soil as the solids. Al-
though plants derive their food from the soil
solutions, only a few types of plants are able to
thrive in soils in which the pore space is al-
ways filled with water; most of them require
soils containing both air and water.
Forms of occurrence of water The water
342
Hygroscopic
Capillary
Gravitational
FIG. 18.4 Forms of soil water. Stippled areas
indicate individual soil particles or structural units;
blackened margins, water; and white areas, air
spaces.
in the soil is supplied by the atmosphere.
Even in regions that are nearly rainless, there
is a molecular film of water on the surfaces
of the soil particles that is called hygroscopic
water (Fig. 18.4). It adheres firmly, does not
move from one place to another, and is very
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
resistant to both evaporation and the absorb-
ing power of plant roots.
Soil particles that are moistened more fre-
quently have thicker films of water, called
capillary water (Fig. 18.4). This water, which
also tends to be held upon the soil particles
by surface tension, is absorbed by the soil
colloids, causing them to swell and giving
them their jellylike consistency. The capillary
film does not fill the pore spaces, thus allow-
ing the soil air to circulate. Capillary water,
with its dissolved materials, is readily ab-
sorbed by plant roots.
When the supply of capillary water is
abundant, it moves slowly downward under
the pull of gravity. When the supply is
diminished by plant use or direct evaporation,
it may move horizontally, or even creep up-
ward, under the pull of its own surface ten-
sion. In fine-textured soil, water may move
in this fashion with relative ease, although in
periods of extreme drought it may not do so
fast enough to furnish plants with a sufficient
water supply. In soils of coarse texture, both
the usual supply and the movement of capil-
lary water are limited.
Immediately following a rain the pore
spaces of a soil may be filled with water dis-
placing the air. In this condition there is
water in excess of that which can be held to
the soil particles by surface tension, and the
surplus will move downward to the saturated
zone of ground water. This is called free or
gravitational water.
Variations hi soil-water supply Other
things being equal, the regional variations in
the supply of soil water depend directly upon
the ratio of precipitation to evapotranspira-
tion. Where precipitation is relatively high in
proportion to evapotranspiration, there will
be more gravitational water and hence more
leaching. Less gravitational water may be ex-
pected where precipitation is low and
evapotranspiration is high. There may be
other complicating factors, however, such as
the intensity of precipitation, or the water-
retention ability of the ground cover. Never-
theless, in general, the humid areas of the
earth are regions of net downward water
movement.
In sites where the ground-water table coin-
cides with the land surface, or in localities
where there is an impervious layer in the
subsoil, there may be a more or less perma-
nent supply of gravitational water near the
surface. This creates a waterlogged or
swampy soil in which most cultivated plants
will not grow. Where free drainage condi-
tions exist, the gravitational water continues
downward, quickly in soils of coarse texture
or open structure and slowly in those fine
and compact. In arid regions gravitational
water may move downward only for a few
feet, carrying with it dissolved salts, and the
water may then be lost through evapotranspi-
ration. In this way lime and other salts may
accumulate in definite horizons of dry-land
soils, while in humid-land soils they are
leached out and carried away in the under-
drainage.
Soil color Because the soil color is sig-
nificant as an indicator of physical or chem-
ical conditions, many soils have color terms
as parts of their names. Among the commonest
colors found in soil horizons are shades of
red, brown, and yellow caused by the different
forms, degrees of hydration, and concentra-
tions of the oxides of iron and aluminum
usually forming a considerable proportion of
the inorganic fraction of soils. Black and
dark-brown colors in soils usually, but not
always, denote considerable organic content.
Gray layers in an otherwise dark soil indi-
cate poor drainage and waterlogging. While
Soils 343
in some humid regions a whitish color may
show a lack of the iron oxides and organic
matter, the same color in arid regions may
denote a harmful concentration of soluble
salts. In many soils two or more color-forming
ingredients are present, giving rise to inter-
mediate colors, such as yellowish-brown and
grayish-brown. It is commonly assumed
with good reason that the darker soils are
more productive than the light-colored ones
(red to white).
Soil profile It was previously noted that
soils are characterized by an internal vertical
arrangement of layers, or horizons, with dif-
ferent thicknesses and different chemical and
physical properties. These are designated as
A, B, and C horizons, reading from the top
down. The thicknesses of the horizons vary
greatly, so that in some types of soil the
horizons are thin and in others so thick and
irregular that, for purposes of better descrip-
tion, each horizon is further subdivided as
A!, A 2 , A 3 , etc. (Fig. 18.5).
The horizons within a profile are distin-
guished from one another in texture, struc-
ture, and so on. In the A horizon organic life
and debris is most abundant, but some soils
have only a thin surface layer of the organic
material. In humid regions the A horizon is
characterized generally by leaching and
eluviation and is left poorer in soluble sub-
stances and coarser in texture as a result.
The B horizon may be, in contrast, one of
illuviation and also a zone of nutrient enrich-
ment. In it may be deposited some of the
materials carried in solution from the layer
above. The C horizon is the little-changed
parent material from which the solid fraction
of the soil derived.
Pan layers One of the commoner and
less desirable features of a soil profile is that
known as a pan layer. This is a dense, im-
344
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
CONTRASTING SOIL PROFILES
A N.DAKOTA CHERNOZEM
Black to dark brown,
mellow silt loam, 10 to
20 inches deep, high
in organic matter
Yellowish brown, silt
loam, low m lime
Grayish yellow
silt loam
Horizon of lime
accumulation
Parent material-
glacial drift high
in lime
A MICHIGAN PODZOL
Loose forest litter
Black peaty leaf mold
Gray sandy humus
Loose whitish gray
sand, sometimes of a
fluffy appearance
Lower limit irregular
Brown sand, irregularly
cemented into sandy
hardpan or a stony
layer of coffee brown
color
Loose yellowish sand
with spots and streaks
of brown
Grayish yellow
loose sand
FIG. 18.5 The characteristic elements within
the profiles of two different soils.
penetrable zone that interferes with root
penetration and water movement and is un-
favorable from the point of view of soil use
and management. Pan layers develop as a
result of a variety of factors, but the usual
causes are excessive gravitational water in
humid regions and precipitation of carbonates
in dry regions, both of which result in a
strongly illuviated or even cemented horizon.
There are many kinds of pan layers grouped
into several general types; only the most im-
portant can be mentioned here: 3
1. Claypan is a compact layer of unce-
mented clay resulting either from a high con-
centration of clay in the original subsoil or
from illuviation. Claypans are relatively wide-
spread, occurring in very smooth or flat lands
of arid regions and of the humid middle lati-
tudes. Because the nutrients in ciaypans do
not differ much from those of the soils in
which the ciaypans occur, the aspects which
limit crop productivity are mostly poor
permeability and other unfavorable physical
properties.
2. Hardpan is a layer of chemical cementa-
tion, the commonest kind being the iron
oxide crust, or laterite, which occurs widely
in humid tropical soils in a variety of phys-
ical forms. Other hardpans, which are not so
widespread as those in the tropics, occur in
humid areas of the middle and high latitudes
and also in several warmer regions. The
latter, sometimes called caliche, are the result
of calcareous cementation. Generally, hard-
pans are unfavorable to plant growth, some-
times because of their relatively low nutrient
qualities, but primarily because their density
prevents root and water penetration.
SOIL CLASSES AND REGIONS
SOIL CLASSIFICATION
As the foregoing discussion of the com-
ponents of soils and the factors involved in
their development has indicated, a soil is
similar to a living organism: some of its char-
acteristics are, in a sense, hereditary, such as
those derived from its inorganic ancestry
(parent material), while others have devel-
oped more as a result of environmental
factors. Although the soil-forming controls of
climate and organisms (especially vegetation)
3 Eric Winters and Roy W. Simonson, "The Subsoil,"
Advances in Agronomy, vol. 3, 1951, pp. 31-45.
generally tend to produce dominant soil
characteristics, the numbers of different com-
binations of important qualities that can occur
is very large. Consequently, there are a great
many kinds of soils, and in order to consider
how they vary from place to place it is nec-
essary to classify them. Fortunately, the soil
scientist has devised a system that may be
used to study their geography, or their syste-
matic variation over the earth.
The basic unit in the classification is the
soil series. Each series includes all soil bodies
that have closely similar horizons and pro-
files developed from similar parent material. 4
Several soil series may differ in detail but
may yet have the same number and arrange-
ments of horizons in their profile. Such a
collection of related soil series is called a
great soil group. There are less than 100
great soil groups, and each may be assigned
to one of three soil orders, the highest cate-
gory in soil classification.
The three orders are zonal, intrazonal, and
azarial. Zonal soils include all those soils that
have well-developed and mature profile char-
acteristics largely resulting from the dominance
of climate and vegetation among the soil-
forming controls. Intrazonal soils also have
clearly developed profiles, but unlike those
of zonal soils their profiles reflect the domi-
nance of some other more localized soil-
forming factor, such as poor drainage or a
particular parent material. Azonal soils, such
as are found in dune sand or recent alluvium,
do not have much profile development. Fig-
ure 18.6 illustrates for portions of the United
States how the various categories in the
classification system can be mapped.
4 There are hundreds of soil series, and each can be fur-
ther subdivided into types, usually on the basis of surface
textural differences, and the types into phases, on the basis
of some quality significant in their utilization.
Soils 345
REGIONALIZATION OF SOILS
In order to study the general pattern of soil
variation over the earth, it is desirable to
recognize regional soil associations that
show primarily the broad effects of the major
soil- forming controls. Some of these controls
vary over the earth in a relatively systematic
fashion, such as climate and vegetation, but
others do not, such as parent material and
land form. Consequently, the characteristics
of the zonal soils are used to describe the
major soil regions. It is to be expected that
azonal and intrazonal soils may commonly
occur within such regions, and to the degree
they do, the broad generalizations are less
applicable than one would wish.
Basic categories The many great soil
groups of the zonal order have here been
sorted into eight general categories to which
have been added the categories of alluvial
soils (azonal), because alluvial areas are sig-
nificant regions of human use, and the moun-
tain soils, because mountainous areas show
tremendous internal variety in soils, as in
climate. The following outline shows the 10
basic categories:
1 . Soils of the humid lands
a. Latosolic soils
b. Podzolic-latosolic soils
c. Podzolic soils
d. Podzol soils
e. Tundra soils
2. Soils of the subhumid lands
a. Chernozemic soils
b. Chernozemic-desertic soils
c. Desertic soils
3. Other soils
a. Alluvial soils
b. Complex soils of areas of high local
relief
346
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
DISTRIBUTION
OF THE
DOMINANT SOIL
ORDERS
IN THE UNITED STATES
SOILS OF A PORTION OF
LANGLADE CO., WIS.
Gray brown
podzohc soils
Red and yellow
podzolic soils
Prairie soils
Chernozem soils
Chestnut soils
Dry sands
Lithosols
Planosols
Bog and half bog soils
Alluvial soils
Antigo silt
oam, slop-
ing phase
Kennan stony
fine sandy loam
Onamia fine
sandy loam
Kennan fine
sandy loam
DOMINANT SOIL
ASSOCIATIONS IN THE
UPPER MIDWEST U.S.
FIG. 18.6 Soils maps of different scales show different degrees of
detail in the classification of soils. The lower left-hand map shows the soil
series (identified with names such as "Antigo") and some of their types and
phases. The right-hand map shows the great soil groups; the top map shows
the soil orders. Note the area covered on one map in relation to the scale on
the adjacent map. (Generalized from map\ of the U.S. Department of Agriculture
Wisconsin Sail Survey.)
The eight classes of zonal soils are shown on
a hypothetical continent in Fig. 18.7 in order
to clarify their typical positions and arrange-
ments. Similarly, Fig. 18.7 is intended to
show in a highly schematic manner the rela-
tionship between climate (with its commonly
associated vegetative cover) and these broad
regional soil categories. The diagram repre-
sents a land mass extending northward from
the equator to the high latitudes and grading
from a humid east to an arid west.
Differences between humid and dry
land soils Fundamental differences exist
between zonal soils found in humid lands
COLD
DRY
DRY
TUNDRA
SUBARCTIC
Soils 347
COLD
WET
COOL
DESERT STEPPE
WARM
COOL
WARM
CONTINENTAL
MESOTHERMAL
WET AND DRY
CONTINUOUSLY WET
WEI
HOT
HOT
COLD
DRY
DRY
CHERNOZEMIC
DESERTIC
GRASSLAND
LATOSOLIC TROPICAL
' FOREST AND """"
GRASSLAND SOILS
WET
WET
HOT
FIG. 18.7 Highly generalized relationship between major climatic zones
and broad descriptive soil categories.
HOT
and those found in dry lands. The soils of
humid lands have generally developed under
natural vegetations of forest or woodland in
areas of generous precipitation. In such
regions organic matter is incorporated only
slowly and there is a net downward move-
ment of soil water. Therefore, the mature
soils of humid regions as a whole are much
348
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
leached and relatively infertile, prevailingly
light-colored, usually acid, and characterized
by a comparatively low content of organic
matter. From a fertility standpoint they tend to
support bulky plant life that has a high con-
tent of carbohydrates in comparison with the
protein and mineral content.
The soil-forming processes and the pro-
files that result in warm humid regions are
notably different from those in cool humid
regions. The dominant factors affecting the
development of soils in the humid tropical
and subtropical regions are abundant pre-
cipitation and warm temperatures, which
combine to produce a latosol. 5 In these soils
the basic soil minerals and even silica are
dissolved and leached away, leaving a con-
centration of reddish iron and aluminum
oxides in both the A and B horizons. In the
higher latitudes, on the contrary, the dom-
inant factors are abundant moisture associ-
ated with cool temperatures, which combine
to produce a podzol. In podzols iron and
aluminum are removed from the A horizon
and deposited in the B horizon. Silica, left
behind in the A, forms a relatively light gray
horizon beneath a dark surface layer of
humus and above the brown B horizon. In-
termediate between these lower and higher
latitudinal areas the soils show some of the
effects of both the latosolic and podzolic
processes.
Soils of the drier lands are not greatly
leached, because of the lack of water, and
consequently have a considerable, and in
some places excessive, content of soluble
alkaline compounds. Calcium and magnes-
5 The term latosol is preferred by the soil scientist to the
term laterite, which is now more often reserved for certain
extreme clayey concentrations rich in iron and aluminum
oxides that are commonly associated with latosols.
ium may be plentiful in these soils, and they
are likely to have a neutral or basic reaction.
Where there is enough moisture, they are
likely to support a less woody, grassy vegeta-
tion, with a high mineral and protein ratio in
proportion to the carbohydrates. Because of
the high evaporation characteristic of these
areas, an actual horizon of lime concentra-
tion often develops. Although there are some
significant differences between the soils of
the warm dry regions and those of the cool
dry regions, these differences are not so
marked as are those between the soils of the
very arid or desert regions and the subhumid
margins of the dry regions. The typical
desertic soil of the very dry region is light-
colored, high in saline or alkaline minerals,
and low in organic matter, while the cherno-
zemic soil of the barely subhumid region is
black or dark brown in color, neutral or
moderately alkaline, and high in organic
content.
The world pattern of soil distribution is
shown in a generalized way on Plate 6.
Several circumstances act to exclude detailed
charting on a map such as this, including the
following: (a) There is no abrupt change
from one soil category to another, as there
usually is not from one climatic type to
another, but rather there tends to be a gradual
transition. (/;) The small scale of the map
necessitates drawing only a general picture of
the soil variations, in spite of the*fact that
the details of soil distribution are highly com-
plex, (c) Large areas of some of the conti-
nents, especially in the lower latitudes, are
not completely surveyed, (d) The categories
are made up largely of great soil groups in
the zonal order, but in many areas intrazonal
and azonal soils abound and are intimately
linked with the zonal soils.
SOILS OF THE HUMID LANDS
Podzol and tundra soils The typical
zonal soils of the higher-latitude regions that
have humid climates are the podzol and
tundra soils. Podzols occur in areas covered
largely by forest vegetation, while tundra
soils lie in the more poleward treeless areas.
Tundra soils are generally associated closely
with areas of tundra climate, while podzols
are found mostly in the cool-summer micro-
thermal and subarctic climatic regions. Like
the associated climatic areas, these soil cate-
gories are essentially confined to the Northern
Hemisphere.
The mature podzol develops under a needle-
leaf or mixed broadleaf-needle-leaf forest.
The relatively low temperatures, combined
with the substantial forest litter, retard bac-
terial action. The spongy, often soggy, dark
layer of raw humus, or half-decomposed
organic remains, that accumulates becomes
highly acid as the result of fermentation,
similarly affecting the downward-moving soil
solutions. The strong acidity is unfavorable
to earthworms and other small, burrowing,
soil-mixing organisms. Partly for this reason
there tends to be a comparatively sharp sepa-
ration between the layer of raw surface humus
and the horizon beneath it (Fig. 18.8). More-
over, the strongly acid solutions remove
much of the iron and aluminum from the A
horizon, making it appear as if it had been
bleached to a grayish-white color. Conse-
quently, beneath the layer of raw humus, the
A horizon is strongly leached and through
eluviation has lost most of its clay and finer
constituents. It is, therefore, generally infertile
and nearly structureless (Fig. 18.8). The B
horizon is strongly illuviated, typically brown,
and may contain a pan layer. In large sections
Soih 349
of Europe and North America, the C horizon
is composed of sandy glacial drift.
Unimproved podzols are poor soils for
most farm crops. As a consequence of culti-
vation and cropping, the supply of organic
matter is soon lost, and the grayish surface
soil requires lime, fertilizer, and good manage-
ment to keep it productive. Although some
food plants for man can be grown on typical
podzols, much of their nutrient productivity
for humans must be obtained by way of
animals that are better able to subsist on the
FIG. 18.8 Profile of a podzol soil in Ontario.
Note the typical heavy leaching of the lower A
horizon and the strongly illuviated B horizon.
(G. A. Hills, Ontario Department of Lands and Forests.)
350
FIG. 18.9 Profile of a podzolic forest soil.
Compare this with Fig. 18.8. Note that in this
gray-brown podzolic soil the organic matter is
better mixed and the effect of bleaching in the A
horizon is not so marked. (G. A. Hills. Ontario
Department of JMndt and Forests.)
vegetative production of these soils. Good
yields of some grasses, potatoes, oats, rye,
and numerous vegetables are obtained from
finer-textured podzols after lime and fertilizers
are applied.
In the treeless region of the tundra soil
profiles show evidence of excessive moisture,
because of the low rate of surface evaporation
in the cold climate and the permanently
frozen subsoil. A brown peaty surface layer
is commonly underlain by a grayish horizon
that is characteristically plastic or even fluid.
A large part of the tundra is poorly drained
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
and consists of bog and hummocky marsh-
land, making the soils similar in some re-
spects to the glacial marsh and bog soils
found in middle latitudes, many of which are
now drained and cultivated. Large areas of
the tundra cannot be drained, however, and
thus are unsuited to tillage, supporting but a
sparse natural vegetation useful only as nat-
ural pasture.
Podzolic soils The group of podzolic
soils is characteristically found in those
regions of the world that have broadleaf
deciduous forests and humid microthermal
climates. Thus they usually lie equatorward
of the true podzols. As in the podzols, there
is a dark surface layer of organic material 1
to 3 in. deep in the virgin podzolic soil, but
because the warmer temperatures permit
more bacterial action, this layer is neither so
matted and soggy nor so acid as that of the
podzols. Moreover, the organic material
derived from a broadleaf forest contains more
lime, potash, and other basic elements than
does that from a needle-leaf forest, making
these soils more fertile. The A horizon of the
podzolic forest soil is leached but is neither
so impoverished nor does it appear so
bleached as in the podzol soil (Fig. 18.9). It
generally is grayish-brown because of the
presence of a brown hydroxide of iron and
some organic matter. The quantity of organic
material decreases downward, and the B
horizon is commonly a lighter yellowish-
brown and denser than the A horizon be-
cause the B horizon has been illuviated. As
in the podzols, the C horizon is the little-
changed parent material of the soil.
The podzolic forest soils generally have
better structures than other forest-land soils,
keep their structures better under cultivation,
and respond more readily to the application
of lime and organic fertilizers. The humus is
better distributed in the upper soil horizons
than in the podzols, because earthworms and
other soil organisms thrive under the less
acid conditions.
Podzolic soils occur in some of the in-
tensively cultivated agricultural lands of the
world, such as the northeastern United States,
northwestern Europe, and several other re-
gions of smaller size. These are areas of great
agricultural diversity, and one of the more
distinctive characteristics of the podzolic
soils is their suitability for a wide variety of
crops: hay and pasture, small grains and
corn, vegetables, root crops, and many others.
Soils associated with podzol and pod-
zolic soils In the areas of podzol and
podzolic soils are numerous other soils that
are not zonal. Some are azonal soils such as
fertile river alluvium or more infertile sands
and gravels, the latter resulting from accumu-
lations such as sandy glacial outwash or from
the abandoned shore deposits of temporary
glacial lakes. Even more widespread are intra-
zonal soils resulting from poor drainage, such
as the soils formed in the depressions of
glaciated plains or in other marshy or boggy
places.
The poorly drained soils (bog soils) are
high in organic matter derived from the re-
mains of grasses, sedges, and other marsh
plants. Where underlain by clays and loams,
these soils may be made productive with
artificial drainage. Other poorly drained in-
trazonal soils (planosols) develop on extensive
flat or gently sloping uplands (Fig. 18.6).
Here poor drainage causes the formation of
an eluviated and acid A horizon underlain
by a dense pan layer.
There are also many soils with immature
or imperfectly developed profiles found in
areas subject to podzolic processes, especially
in glaciated areas. Recent alluvial deposits
Soils 351
and ice-scoured and stream-eroded slopes
comprise large total areas of soils with ab-
normal or immature podzolic profiles. In the
glaciated regions of North America and
northern Europe there are also considerable
areas of rocky ground containing what is
called a lithosol, a stony, azonal soil.
Latosolic soils Not as much is known
about the character and distribution of the
various soils of the tropical areas as about
the soils of the middle latitudes. It is ap-
parent that the vegetative cover has more
internal variety; the soils also show great
variety, ranging from darker grassland soils
to forest-land soils showing latosolic
development.
The term latosol is applied to those soils
in which high temperatures and abundant
precipitation are the dominant soil-forming
controls. Weathering commonly extends to
considerable depths, and the chemical activity
is intense. This sometimes so modifies the
parent materials that there may be little sim-
ilarity between the chemical nature of the
soil and that of the inorganic mass within
which it has formed (Fig. 18.10). The pro-
files are unusually deep, sometimes extend-
ing downward more than 10 ft, and the
horizons are poorly differentiated. The soils
are low in silica, have a relatively high clay
content, and are high in the oxides of iron
and aluminum, making most of them reddish
or yellowish. In some there has developed a
material called laterite, a claylike material
especially rich in the oxides of iron and
aluminum that develops into hardpans or
crusts. Being highly leached, latosols are
generally infertile, and they are not usually
capable of sustained cropping without heavy
fertilization. Many, however, are granular
and very porous and are, therefore, capable
of being tilled immediately after heavy rains;
352
FIG. 18.10 Profile of yellowish red latosol
formed from a coarse-grained metamorphic bed-
rock parent material (gneiss) northwest of Rio de
Janeiro, Brazil. Latosol profiles are typically deep
and commonly do not have as much horizon
differentiation as podzols. There is some darkening
of the thick A horizon by organic matter. Plant
roots extend to depths below 5 ft in this soil.
(Roy W. Simonson, U.S. Soil Conservation Service.)
some are highly resistant to erosion but sub-
ject to drought.
At first glance it seems remarkable that the
generally infertile latosols should be able to
support such abundant natural vegetation as
tropical rainforest and yet be so unproductive
of other kinds of plants. This no doubt re-
sults from the close interrelation between the
natural forest vegetation and the soil. Woody
vegetation is mostly carbohydrate (cellulose)
and the deep roots of the trees bring to the
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
surface at least small amounts of mineral
nutrient elements from underlying sources.
When the portion above ground dies and
decays, some of these elements are returned
to the surface soils, thus providing a small
continuous supply so long as the forest
exists. When this cyclic movement of mineral
nutrients is interrupted by clearing and culti-
vation, the nutrients are quickly depleted.
Latosols are not well suited to shallow-
rooted protein crops that draw heavily on
soil fertility in the surface layers. They sup-
port crops that utilize the intense tropical
sunlight and abundant rains for the produc-
tion and storage of cellulose, starches, sugars,
fats, and other carbohydrates. Those tropical
latosolic soils that support grass are commonly
more deeply weathered and less fertile than
the corresponding middle-latitude soils. Like
many other tropical soils they are likely to be
reddish.
There arc many intrazonal and azonal
soils included in the tropical areas. For ex-
ample, there are those intrazonal soils result-
ing from poor drainage and those with an
unusually limy parent material, In the azonal
category belong the thin, stony, immature
soils of steep slopes, the porous sands with-
out profile development, the recent deposits
of volcanic ash, and, especially, the recent
deposits of floodplains and deltas. The rate
of alluvial accumulation is often too rapid to
permit the development of mature profile
characteristics, yet in many instances tropical
alluvial soils are, where adequately drained,
more productive agriculturally than the more
mature latosols with which they are associated.
Podzolic-latosolic soils Even though
the podzolic-latosolic soils are the dominant
zonal soils in only a few general areas (Plate
6), they do occur in areas of considerable
agricultural importance, such as the south-
eastern United States and China. As a result
of latosolic processes the upper horizons are
composed of considerably weathered, brown-
ish clays and loams, while on the other hand,
as a result of podzolic processes the thin
upper horizon of moderate organic content
is underlain by a leached zone above the
thick, acid, latosolic B horizon of red or
yellow material.
The fertility rating of these soils is not
high, but with fertilization and careful
management they are productive. Under
cultivation the colors of the red and yellow
subsoils usually predominate, because crop-
ping quickly uses the small reserve of organic
matter, and tillage tends to intermingle the
A and B horizons. Podzolic-latosolic soils
are especially subject to the loss of the upper
horizon through rapid erosion.
SOILS OF THE SUBHUMID LANDS
Chernozemic soils Previously it was
pointed out that there is considerable cor-
relation between the occurrence of humid
climates and forests, on the one hand, and
between subhumid climates and grasslands,
on the other. It is not possible, however, to
draw a clearly defined boundary line dividing
the dry from the humid lands that will also
coincide with a line separating forest and
grasslands. Nevertheless, limited soil moisture
and grass vegetation produces soils that are
very different from those that develop under
conditions of more soil moisture and forest
vegetation.
In subhumid regions, where there is
enough moisture to support luxuriant grass,
the periodic growth and death of part of the
root system introduces organic matter into
the soil and hence results in a regular auto-
matic incorporation of a large organic frac-
tion at depths of several inches to three
Soils 353
or four feet. Because these chernozemic soils
are less leached than any of the soils pre-
viously considered, they are correspondingly
higher in available calcium, magnesium, and
other soil bases and nutrient elements; thus
they are more fertile than soils developed
under more humid conditions. The horizon
characteristics in the profiles of the soils of
subhumid areas are not so sharp as in the
soils of humid lands in the middle and higher
latitudes.
The zonal soil known as prairie is found in
the more humid margins of the chernozemic
areas, occurring widely in the United States,
Russia, and South America. This soil has
formed in a sufficiently humid climate so that
leaching has lowered the supply of available
bases to the point where the soil reaction is
neutral or even slightly acid. The upper
horizons have a fine granular structure and
are very dark brown. Both these qualities are
derived from abundant and deep accumula-
tions of organic matter originating in the
thick-grass sod. The typical mature soil is
found on rolling interfluves where the nat-
ural vegetation of prairie grasses was best
established. Because of its quality and cli-
matic location, prairie soil is among the most
productive of soils. In the United States it
developed mainly in regions where the parent
material is older glacial drift along with con-
siderable quantities of loess to add to its fer-
tility. The rich Corn Belt chernozemic soils
of central Illinois, Iowa, and Missouri are
mostly prairie soils. On steep slopes, espe-
cially along the margins of streams, fingers of
woodland originally projected into the grass
prairies. On such sites podzolic soils
developed.
The zonal soil known as chernozem (a
Russian word meaning black earth), from
which the general category is named, is par-
354
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
ticularly fertile. In the middle latitudes, it
formed in association with shorter prairie
and steppe grasses and under sufficiently low
precipitation (about 20 in. in the United
States) that an abundance of lime and alka-
line minerals remains. In fact, in chernozems
and the soils of drier regions, calcium car-
bonate accumulates in a definite zone; the
drier the region the nearer to the surface the
horizon of lime accumulation will be. The A
horizon has a high organic content, is black
or very dark brown (Fig. 18.11), and has a
FIG. 18.11 Profile of a chernozem formed
from glacial till in South Dakota. The A horizon
extends to a depth of a little over a foot, while the
B horizon extends to a depth of nearly 2% feet. The
white spots in the B and C horizons are carbonate
accumulations that commonly occur in dry-land
SOilS. (Roy W. Sinumson, U.S. Scil Conservation Service.)
granular, porous structure. Upon tillage it
crumbles into a fine seedbed with a large
capacity for retaining capillary water. Both
organic and fertility reserves are high. In gen-
eral there is no better soil than chernozem
for grains and other extensive high-protein
field crops that draw heavily upon soil
fertility.
In regions of subhumid tropical grasslands
there are also some dark-colored soils classed
as chernozemic, but these contain less abun-
dant organic matter because long-continued
high temperatures hasten decomposition,
even under subhumid conditions. Thus the
tropical chernozemics are clayey, plastic, and
less favorable for tillage. Some chernozemic
soils, such as the black soils of central India,
are derived from the weathering of basic igne-
ous rocks; they owe their color and fertility to
the unusual mineral content of the parent
material.
Chernozemic-desertic soils In a tra-
verse from the less arid to the more arid sec-
tions of subhumid lands a change occurs in
the character of the soils. The chernozemic-
desertic soils of the drier sections are affected
by the decreased precipitation in several
ways. The soils have developed under a
grass vegetation less luxuriant and deep-
rooted than that in the chernozemic zone,
and the grass cover provides a less abundant
supply of organic material. The humus is in-
termingled with a powdery surface soil that
lies above a subsoil of a somewhat coarse
and lumpy structure. The slight precipitation
and the high rate of evaporation of soil
moisture also results in a horizon of accumu-
lated lime or other alkaline substances rela-
tively near the surface (1 to 2 ft), and in
some localities the lime is so abundant that it
forms a pan layer in the soil. Brown or
reddish-brown is the prevailing color.
In general, chernozemic-desertic soils are
easily tilled, and are well adapted to culti-
vation, if irrigated. The fact that these soils
are predominantly used for livestock grazing
rather than cultivation is caused by the
deficiency of precipitation rather than by the
deficiencies of the soils.
Desertic soils Because they develop
under sparse vegetation, usually widely
spaced shrubs, desertic soils are low in organic
content, making the lighter colors predom-
inate, with the reds, browns, yellows, and
grays of weathered rock minerals widely ex-
posed (Fig. 18.12). The characteristic colors
are occasionally lightened by the accumula-
tion of whitish alkaline substances near or
upon the soil surface. Although desertic soils
are characteristically low in nitrogen, they
are likely to contain considerable supplies of
other nutrient elements.
It is to be expected that the larger parts of
the great deserts contain no mature soils. In-
stead, there are patches of bare rock, ex-
panses of desert gravels covered with the
pebbles remaining after deflation, tracts of
dune sand, and areas of immature soil result-
ing from the recent and rapid growth of
alluvial fans. Alluvial soils are the most
widely cultivated in arid lands partly because
their situations allow them to be more easily
irrigated. But parent materials in deserts often
contain so many decomposed rock fragments
that they may be well supplied with soluble
minerals, and if abundant water is available
for irrigation they may be made agriculturally
productive.
MOUNTAIN AND ALLUVIAL SOILS
In the discussion of the plan of classifying
and regionalizing the diversity of the earth's
soil cover, two basically nonzonal categories
were included: alluvial soils and the soils of
Soils 355
areas of high local relief. Little characteriza-
tion can be applied to the soils of high-local-
relief areas because of the complexities intro-
duced by variations in slope and climate
which in turn induce great variety in vegeta-
tive cover. In any case, the soils of these
areas are not widely used. From the point
of view of human use alluvial soils are far
more important.
As a class, alluvial soils probably support
a larger proportion of the world's population
than any other single kind of soil. However,
it is difficult or impossible to generalize to
any great extent about these azonal soils be-
FIG. 18.12 Profile of a desertic soil. A
sierozem (near-desert soil) formed on alluvial
deposits in Nevada. Although the regolith is deep,
horizon differentiation is low. (Roy W. Simomon, U.S.
Soil Conservation Service.)
i %*A*'
356
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
cause the specific characteristics of an alluvial
soil body are largely those typical of the
regolith in the area where the specific alluvial
parent material originated. Consequently, the
textures may range from sands through silts
to clays; the colors may range from the light
hues of the desertic soils to the dark shades
of the chernozemic soils; and the soils may
be more or less rich in plant nutrients. How-
ever, all are generally free of stones and are
easily cultivated.
Most of the great soil categories previously
described also include alluvial soils associ-
ated with the mature zonal soils, and in
many instances the alluvium constitutes the
most prized lands. Not all alluvial soils are
productive, however, because they may be
too wet or too dry, they may occur where the
growing season is too short, or they may be
subjected to frequent flooding. In those Far
Eastern areas where rice is grown in alluvial
paddy lands, the alluvial soils are probably
more generally utilized than elsewhere in the
world. Most of Japan's productive land, for
example, is alluvial but in units too small to
be shown on the world map. Almost all of
Egypt's dense agricultural population sub-
sists on the production from alluvial soils.
On the other hand, the tropical alluvial soils
of some parts of central Africa are at present
little used, but in this case one reason for not
using the soils is that the lands are infested
with the tsetse fly.
SOIL EROSION
Even though erosion is a normal process,
it can be greatly accelerated when unnatural
soil conditions are created by tilling and
grazing the land. Erosion results in the loss
of the finer fractions and the upper horizons
of mature soils at a much faster rate than
normal development processes can replace
them, thus destroying the natural relationship
among the soil components (fertility, texture
and structure, organic content, water, and
air). Because the application of ever-increasing
amounts of bulk-producing, artificially de-
rived chemicals to stimulate plant growth is
a costly method of countering the effects of
erosion, the most modern practice is to culti-
vate the land in ways that reduce the losses
by erosion to a minimum.
Not all kinds of soil are equally subject to
destructive erosion. Some soils, such as some
latosols or eluviated sandy soils, might per-
haps benefit by a faster removal of surface
soil, thus exposing the less weathered minerals
and less leached layers underneath. But
these soils, because of their high porosity, are
among those least subject to rapid erosion.
On the other hand, some of the dark-colored
soils that have considerable organic accumu-
lations in the upper horizons, are highly sub-
ject to erosion, as are certain of the forest
soils, such as many of those in the podzolic-
latosolic category.
Causes and kinds of erosion Soil
erosion occurs primarily as a result of rain-
drop impact, washing by running water, and
blowing winds. The principal means whereby
tillage and grazing may accelerate the normal
rate of erosion are the loosening of the soil
by cultivation and the removal of the protec-
tive cover of vegetation. The rate at which a
soil may be eroded depends upon the textural
and structural conditions of the soil, the con-
ditions of climate, particularly the number of
intense rainstorms per year, and the degree
of land slope. The extent to which soil erosion
has progressed in the United States is shown
in Fig. 18.13.
One of the most widespread and least
noticed kinds of erosion on tilled land is the
Soils 357
LEGEND
OR
(25 to 75 per cent of lost,
gylliesj
SEVERE
than 75 per ctnt of topsoil lost, may
numerous or gullies.
in of low
fMany small no! be shown at this
on from 1934 reconnaissanct trosion survey of the United
and other soil conservation surveys by the Soil
FIG. 18.13 Generalized distribution of the extent of soil erosion by
Wind and Water in the United States. (From a map by U.S. Soil Conservation
Service.)
sheet wash that occurs during rainstorms and
results in the removal of a uniform thin frac-
tion of the soil. This is particularly harmful
because it removes the finer and more nutri-
tionally useful of the soil particles first, some
in solution and some in suspension, resulting
in fertility erosion. In some kinds of soil,
especially in compact clays and silts under-
lain by looser materials, gullying may become
deep. This process, if left unchecked, can
destroy both topsoil and subsoil beyond all
hope of repair. The damage may spread from
the eroded upland soil to the adjacent low-
land soils, which can be ruined by being
buried under accumulations of the coarser
and less fertile alluvial products of the
erosion.
In subhumid plains great damage may re-
sult from wind erosion on surfaces laid bare
by plowing or by overgrazing of livestock.
The powdery soil exposed on the bare sur-
faces may be removed to a depth of several
inches by a single windstorm.
Conservation It certainly is not pos-
sible to stop losses by wind and water
erosion entirely; these have gone on since
the world began. However, it is possible to
reduce the rate of destructive erosion brought
about by careless human disturbances of the
natural balance of the soil components. A
program of planned soil conservation would
include (1) the return to permanent forest or
permanent grass of those lands in which
erosion has progressed so far as to destroy
the value of the land for tillage, and (2) the
protection, through proper tillage and
crop production so that they may continue to
be productive. Among the recommended
methods of protection are (a) the construc-
tion of dams or obstructions to erosion in
gullies already formed; (b) the plowing and
tilling of land in strips along contour levels
so that furrows will be arranged at right
angles to the land slope, thus reducing the
cropping practices that provide the most
nearly continuous protective vegetative cover.
Above all, the consciousness of everyone
must be awakened to the need for soil pro-
tection and the disastrous consequences to
the world's rapidly increasing population
that may arise from the needless waste of this
fundamental source of nutrition.
SUGGESTED READING
Albrecht, W. A.: "Soil Fertility and Biotic Geography," Geographical Review, vol. 47, 1957,
pp. 87-105.
Hole, F. D.: "Suggested Terminology for Describing Soils as Three-dimensional Bodies,"
Proceedings of the Soil Science Society of America, vol. 17, 1953, pp. 131-135.
Simonson, Roy W.: "Changing Place of Soils in Agricultural Production," Scientific Monthly,
vol. 81, 1955, pp. 173-182.
Stallings, J. H.: Soil Conservation, Prentice- Hall, Inc., Englewood Cliffs, N. J., 1957.
U.S. Department of Agriculture: "Soils and Men," Yearbook of Agriculture, 1938.
U.S. Department of Agriculture: "Soil," Yearbook of Agriculture, 1957.
Simonson, R. W., "What Soils Are," pp. 17-29.
Russell, M. B., "Physical Properties," pp. 31-38.
Richards, L. A., and S. J. Richards, "Soil Moisture," pp. 49-60.
Allaway, W. H., "pH, Soil Acidity, and Plant Growth," pp. 67-79.
Dean, L. A., "Plant Nutrition and Soil Fertility," pp. 81-85.
Broadbent, F. E., "Organic Matter," pp. 151-157.
Clark, F. E. "Living Organisms in the Soil," pp. 157-165.
Winters, E., and Roy W. Simonson; "The Subsoil," Advances in Agronomy, vol. 3, 1951, pp.
31-45.
CHAPTER 19
Mineral
resources
Minerals as resources In addition to
the many phenomena at the surface of the
earth that directly contribute to man's exist-
ence as a living organism, there occur, deeper
in the shell of the earth, many substances
that man has learned to use to his material
advantage. These are those elements and
their compounds, either inorganic or organic
in origin, that are loosely classed as mineral
resources, or more simply, minerals. They
are employed in a variety of ways: as sub-
stances from which to fashion tools and other
useful objects, as sources of energy, as ma-
terials for road building, and so on, almost
without end. At man's present stage of devel-
opment the list of mineral resources is long,
and it continues to grow each year at an in-
creasing rate as he learns at an increasing
rate to use these substances he finds in the
earth.
To divide the mineral resources according
to the use to which each is put would not be
completely satisfactory, for many of them
serve a variety of needs. Thus the entire class
of minerals consists of three major categories
only partly based on use: (a) those used pri-
marily as fuels, i.e., as sources of energy, (b)
those used primarily because they are metallic,
and (r) nonmetallic minerals not used pri-
marily as fuels.
359
360
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
Many minerals are relatively abundant but
widely dispersed in the earth's crust and
regolith, occurring in relatively small quanti-
ties in any one locality. To collect a mineral
from this dispersed state is costly, and it is
justified only when the specific value of the
mineral is extraordinarily high. Consequendy,
man is usually interested only in those places
of occurrence where the mineral content has
somehow become sufficiently concentrated to
form what is called a deposit. Thus the
geography of mineral deposits, that is, the
facts of their distribution and of related phe-
nomena is an item of considerable concern.
Few people realize how rapidly the de-
pendence of civilization upon mineral re-
sources has grown in recent decades how
different the present is, in this respect, from
even the recent past. Since the beginning of
the twentieth century more minerals have
been extracted from the earth's outer crust
than in all previous history. Yet it may be
asserted that the known supply is actually in-
creasing. This apparent paradox lies in two
facts: (a) As already suggested, a resource
does not just exist, but in a very real sense is
created by man. For example, coal and the
radioactive minerals have existed much longer
than man has; yet they became resources
only when man learned to use them, (b) Man's
scientific understanding and technical skill,
like his capacity to utilize minerals, are in-
creasing at an increasing rate. Consequently,
he is both continually finding new supplies
and able to use more efficiently those of
which he now knows.
THE MINERAL FUELS
Mineral sources of energy Of all the
things that make life today different from life
in the past, perhaps none is so significant as
man's use of the large supplies of energy
available to him in the solid earth. The
power sources he has tapped, up to the pres-
ent, are primarily coal, petroleum, and nat-
ural gas, and all three come from the solid
earth. Of course, man uses many other
sources of power, ranging from falling water
to wood, but he depends primarily upon the
mineral fuels. Although the time when solar
and other forms of nuclear energy will sup-
plant the mineral fuels seems to be coming
steadily closer, the use of mineral fuels is
still increasing and will, no doubt, for some
time to come.
Through growth processes, some of the
sun's energy is "built into" the tissues of all
living plants and animals. Then, in the
normal course of events, this energy is lib-
erated after death as a consequence of de-
composition processes. For example, heat is
often liberated when oxygen combines chem-
ically with decomposing substances. But if
the complete decomposition of an organic
material is prevented, it becomes a potential
source of usable quantities of heat that can
be obtained by inducing combustion later.
The use of firewood is an example of the use
of such a source of energy involving only a
short delay. Coal, petroleum, and natural
gas are sources of such energy that have
been stored a long time. At certain times
in the geologic past, conditions seem to have
retarded decomposition of organic forms, and
simultaneously to have allowed vast amounts
of them to accumulate and be transformed
into coal, petroleum, and gas.
The occurrence of deposits of these
materials, called "fossil" fuels, required the
interaction of a large number of physical
phenomena, and as a result considerable
parts of the earth are deficient in deposits of
these substances, and other regions have
large supplies of one or even all three.
COAL
Origin of coal Coal is sedimentary rock
derived largely from the unoxidized remains
of plant tissues. The carbon-bearing tissues
were preserved from ordinary decomposition
by their submergence in swamp waters and
their subsequent burial and compaction by
layers of clays, sands, and limes; and the
burial ultimately made these beds of organic
material members of a series of horizontal
sedimentary rocks. From this sedimentary
origin, two points of significance about coal
beds and their coal may be inferred. First,
the original attitude of swamp accumulations
being nearly horizontal, as may be observed
in modern swamps, many coal beds still are
essentially horizontal, a condition that sim-
plifies the problem of mining. Second, since
individual swamps seldom have covered vast
areas, single beds of coal are not of great
extent.
In addition, some coal beds show evidence
of diastrophic disturbance after their creation,
and this commonly has involved a more
complete metamorphism that has changed
the ratios among the various constituents
(carbon, gases, water, ash, etc.), a change that
markedly affects their utility.
Although most coal beds are relatively
small, the same is not necessarily true of coal
Mineral resources 361
fields, or areas of coal beds. In some areas
conditions favorable to coal formation must
have existed widely and for long periods. In
these regions large and small swamps
flourished, dried up, and their organic ac-
cumulations were buried by earthy sediments
at the same time that other swamps were
coming into existence nearby. If subsidence
of an entire area took place, newer swamps
may have formed above the remains of the
older but separated from them by layers of
inorganic sediments. In some coal fields the
beds are widely distributed in area and in
vertical sequence.
Since coal occurs among other rock layers,
it is possible, by studying the general rock
structure, to determine the probable extent of
a coal field or region and, by means of test
borings, to discover the number and relative
thicknesses of the coal beds in its various
parts. Thus geologists can approximate with
fair accuracy the potential supply (reserve) in
a given field, a given country, and even in the
world.
Classes of coal Classes of coal differ
greatly from region to region and sometimes
even within the same field, and only four of
the more significant will be mentioned here.
All coal was initially similar to the first class,
which is peat, the partially preserved,
crumbled, and blackened organic remains
that may be seen in present-day swamps and
bogs. The other forms of coal represent suc-
cessive stages in the transformation of peat
that results from compression and the loss of
water and gases (Fig. 19.1). Thus a second
class, somewhat more compact than peat, is
the crumbly brown coal called lignite.
Further transformation additional losses of
volatile constituents and corresponding in-
creases in the relative content of fixed carbon
' produces a "soft" black class of coal called
362
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
Volatile
Hydrocarbons
H r O,N
Fixed
carbons
Mineral
Matter or Ash
FIG. 19.1 Stages in the metamorphosis of vegetable material into coal
Of various types. (After Ntwbern.)
Evolved gases
C0 2f CO, H 2 0,
CH 4l etc.
bituminous. There is an almost endless list of
slightly different grades of bituminous coal,
the most widely used class. Further compres-
sion still, often associated with warping and
faulting, and sometimes heating produces the
class of "hard" coal called anthracite, which
is mostly carbon.
One of the most significant distinctions
among the various bituminous coals is based
upon suitability for the manufacture of coke
to be used in the blast-furnace extraction of
the metal iron from its ore, the only means
of recovering iron in large quantities used at
present. Coke for use in the blast furnace is
mainly the hard carbon that remains after the
volatile constituents have been driven off and
is prepared by roasting bituminous coal in
special ovens. But the metallurgical require-
ments of coke are so stringent that only a
small proportion of the world's bituminous
deposits can be used to produce it. Conse-
quently, the areas where iron can be pro-
duced economically are seriously restricted.
Coal is mined in many ways, depending
upon the structural and situational relation
of the beds to the earth's surface to which
the coal must be brought. In some areas the coal
seams (beds) occur so close to the surface that
they may be mined in open pits after the re-
moval of only a few feet of the covering
earth or rock known as overburden (Fig.
19.2). In others the beds may be reached
only by mine shafts of great depth. In some
localities the seams have become easily ac-
cessible as a result of the exposure, by degra-
dation, of outcrops of coal among the other
rocks of valley walls (Fig. 19.3). In regions of
more complicated rock structure, coal beds
once horizontal may have been variously
deformed, and therefore present different
degrees of accessibility (Fig. 19.4).
COAL REGIONS OF THE WORLD
There is a vast amount of still-unnamed
coal in the world: present estimates of the
total minable reserve, which are probably of
the right order of magnitude, total several
trillion tons, enough for many hundreds of
years at the present rate of use. But the re-
serve is very unevenly distributed among the
land areas (Fig. 19.5); and because coal is
the principal source of power in manufactural
Mineral
FIG. 19.2 Giant furrows turned by power shovels in the process of strip
mining in southern Illinois. The 4-ft-thick bed of coal exposed in the bottom
of the trench will be mined out before the next furrow is turned.
industry and in the production of electricity,
as well as necessary for the smelting of iron,
the location of these major deposits in rela-
tion to the world centers of heavy manufac-
ture, present and future, is a matter of critical
importance.
Three general features of its distribution
are geographically significant: (a) almost all
the known minable reserve lies in the North-
ern Hemisphere; (b) this reserve is about
evenly divided between North America and
Eurasia; and (c) more than four-fifths of the
total is estimated to lie within the boundaries
of only three nations: the United States (ap-
proximately two-fifths of the total), the
U.S.S.R. (approximately one-third), and
Communist China (approximately one-tenth). 1
A considerable proportion of the rest of the
Northern Hemisphere reserve (approximately
one-eighth of the total) occurs in the several
countries of western and central Europe. It
should be noted that the continent of Europe,
not including the U.S.S.R., ranks first in coal
production, slightly ahead of both the United
States and the U.S.S.R.
Thus the known coal reserves are relatively
concentrated within a few national areas; but
1 E. Willarcl Miller, "World Patterns and Trends in Energy
Consumption," Journal of Grography, vol. 58, 1959, pp.
269-279.
ISO 160 140 120 100 80 60 40 20
Mineral resources 365
40 20 20 40 60 60 100 120 140 160 I8O
ESTIMATED
COAL RESERVES
FIG. 19.5 General representation of the world distribution of estimated
coal reserves. The sizes of the circles are in proportion to the reserves
believed to lie within the boundaries of national areas. Countries possessing
less than one-half of one per cent of the world total are not included.
(A ft tt Miller and other*.)
posits, namely the ancient rocks of the
Canadian Shield and the Appalachian Pied-
mont, the western sections of the cordilleran
region, and the relatively recent sedimentary
rocks of the Atlantic and Gulf Coastal mar-
gins. The two major producing areas are the
Appalachian Province and the eastern region
of the Interior Province.
Tht Appalachian Province Much the most
important among the coal fields of the conti-
nent is that of the Appalachian hill region. It
is comprised of two principal subdivisions:
(a) a large section of little-folded rocks with
numerous beds of bituminous coal that ex-
tends from northwestern Pennsylvania through
Ohio, West Virginia, Kentucky, and Ten-
nessee, into northwestern Alabama, and (b) a
small, highly folded section containing an-
thracite in the Ridge and Valley region of
northeastern Pennsylvania.
The bituminous section includes numerous
workable beds of coal of good quality, some
of which are of the character required for the
manufacture of blast-furnace coke. The de-
posits are largely found within the limits of
the dissected Appalachian hill country, and
they are noted for the ease with which they
are mined. The coal beds, traversed by in-
numerable deeply incised stream valleys, are
often exposed along the valley walls (Fig.
19.4). The abundance, accessibility, and high
quality of these bituminous coals give the
Appalachian field first importance in America
and perhaps in the world. More than three-
fourths of the coal output of the continent is
obtained from this field, including most of
the coal used in the eastern and northeastern
industrial districts, as well as most of the
American export coal.
The anthracite occurs on the eastern
Mineral resources 365
ISO 160 140 120 100 60 40 20
*0 20 tO 4 SQ S0 100 120 WO !O 1*0
100 80 40 SO
40 " " G~~20 40 6Q 100 (20 i*0 160" ISO
FIG. 19.5 General representation of the world distribution of estimated
coal reserves. The sizes of the circles are in proportion to the reserves
believed to lie within the boundaries of national areas. Countries possessing
less than one-half of one per cent of the world total are not included.
(Aftft Miller and ofhen.)
posits, namely the ancient rocks of the
Canadian Shield and the Appalachian Pied-
mont, the western sections of the cordilleran
region, and the relatively recent sedimentary
rocks of the Atlantic and Gulf Coastal mar-
gins. The two major producing areas are the
Appalachian Province and the eastern region
of the Interior Province.
The Appalachian Province Much the most
important among the coal fields of the conti-
nent is that of the Appalachian hill region. It
is comprised of two principal subdivisions:
(a) a large section of little-folded rocks with
numerous beds of bituminous coal that ex-
tends from northwestern Pennsylvania through
Ohio, West Virginia, Kentucky, and Ten-
nessee, into northwestern Alabama, and (b) a
small, highly folded section containing an-
thracite in the Ridge and Valley region of
northeastern Pennsylvania.
The bituminous section includes numerous
workable beds of coal of good quality, some
of which are of the character required for the
manufacture of blast-furnace coke. The de-
posits are largely found within the limits of
the dissected Appalachian hill country, and
they are noted for the ease with which they
are mined. The coal beds, traversed by in-
numerable deeply incised stream valleys, are
often exposed along the valley walls (Fig.
19.4). The abundance, accessibility, and high
quality of these bituminous coals give the
Appalachian field first importance in America
and perhaps in the world. More than three-
fourths of the coal output of the continent is
obtained from this field, including most of
the coal used in the eastern and northeastern
industrial districts, as well as most of the
American export coal.
The anthracite occurs on the eastern
366
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
BITUMINOUS
LOW -GRADE
BITUMINOUS
LIGNITE
FIG. 19.6 The principal coal deposits of North America, showing the
distribution of major classes of coals.
margins of the Appalachian hill region in an
area that has been subjected to extreme fold-
ing, which means much greater cost and dif-
ficulty of mining. Anthracite mining has been
steadily declining, and now provides less
than 5 per cent of the nation's coal production.
The Interior Province Like the Appalach-
ian bituminous section, the Interior Province
is abundantly provided with coal deposits,
and again the coal generally is bituminous,
but not of coking quality. The several areas
of the Province are known, respectively, as
(a) the eastern region (Illinois, Indiana, and
Kentucky), (b) the northern region (Michi-
gan), (c) the western region (Iowa, Missouri,
Kansas, Oklahoma, and Arkansas), and (a)
the southwestern region (Texas). Of these,
the eastern region is by far the greatest pro-
ducer. In the eastern and northern regions
the rocks have broad synclinal (saucerlike)
structures, and in the former, the coal beds
of the middle portion are so deeply buried
under younger rocks that they are difficult to
reach. Therefore, mining is practiced mainly
about the margins of the field.
Other areas The Rocky Mountain prov-
ince is made up of many fields spread from
southern Montana to New Mexico, and in-
cludes abundant deposits in Wyoming, Col-
orado, and Utah. Coal of all grades occurs,
but most of it is bituminous quality.
The Great Plains province includes areas
extending from Wyoming and the Dakotas
into southern Alberta and Saskatchewan. In
its eastern sections the coal is lignite, but in
the western sections the coal is higher quality.
The Great Plains province ranks far below the
eastern fields in production. Even the small
fields in the Canadian Maritime Provinces
produce more coal than the Canadian Great
Plains province.
The Pacific Coast fields are made up of a
few deposits, namely, those of Alaska, Van-
couver Island, and the Puget Sound region.
The Alaskan deposits have more future than
present value.
Coal is not abundant in eastern Canada
either. It is a matter of great concern to Can-
ada that its most populous and industrially
developed region, which lies between Lake
Huron and the city of Quebec, is practically
devoid of coal. The best and most used
Mineral resources 367
deposits, including some coking coal, are
found near Sydney, on the northern coast of
Nova Scotia. These supply a local iron and
steel industry.
Central and western Europe The
area comprising central and western Europe
contains numerous coal deposits that extend
in a relatively narrow zone from Great
Britain eastward across the Low Countries
and Germany into Poland and Czechoslovakia
(Fig. 19.7). As previously stated, central and
western Europe ranks first in the world in
coal production if the output of all its coun-
tries is totaled and compared with the out-
puts of other major coal-producing areas. In
total coal reserves this area ranks third
FIG. 19.7 The major coal deposits of central and western Europe.
*
COAL FIELDS
BITUMINOUS
LIGNITE
IOO 2OO 3OO 4OO
> HUNGARY /
' V
YUGOSLAVIA
368
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
among the continents; but even though most
of its supply is bituminous and thus gen-
erally high-grade, it is likely that the total
European bituminous resource is little more
than one-half as great as that available in
North America. In addition, North America
has nearly seventy-five times as much lower-
grade coal.
The principal European coal deposits are
so distributed that they fall within the bound-
aries of several European countries, the in-
dustrial advancement of which may be attrib-
uted in part to the availability of these sources
of fuel. The leading nations in both produc-
tion and proved reserves of good-quality coal
are Germany, the United Kingdom, and
Poland. Czechoslovakia, the Low Countries,
France, and Spain are all second-rank pro-
ducers, far below the first three named. An
indication of the wide availability of coal in
Europe is the fact that coal is produced in
more than 20 countries. Yet only the United
Kingdom and Germany have large deposits
of the coking-quality coal required for the
smelting of iron ore.
Great Britain British coal fields occur in
numerous regions in England, Scotland, and
Wales, and only two parts of the island are
more than a few miles removed from one or
more of these fields, which contain mostly
bituminous coal (Fig. 19.7). Associated with
each of the major British coal fields is an in-
dustrial district; and some of the fields,
especially those of south Wales, are close to
the sea and well situated for the export of
coal. Also, the quality of British coal is gen-
erally high. However, the remaining beds are
becoming increasingly difficult to mine,
greatly increasing the cost of production.
Continental fields The coal fields of
western continental Europe are numerous,
but none covers as much area as the greater
of the fields in North America. The more im-
portant fields and most of the better grades
of coal lie in an east-west belt through the
center of the continent. The ancient crystal-
line rocks of Scandinavia and Finland to the
north of that belt and the much disturbed
rocks of the mountain systems and of the
Mediterranean Sea Basin to the south of it
include either no coal or only small and un-
important fields. The various fields include
coals of many types, among them the low
grades of coal and even peat which are much
more used in continental Europe than in
Great Britain or the United States.
The western end of this important and
productive zone extends from northern
France across central Belgium and into
Germany, its most productive portion lying
in the Ruhr River Valley, just east of the
Rhine. This field is of particular importance
because it has long been the center of the
heavy iron and steel industries of Germany
and because it contains a reserve of coking
coal reputed to be larger than any other in
continental Europe. Nearby is the coal field
of the politically famous Saar region. The
eastern end of the central European coal belt,
the major deposits of which lie in East
Germany and especially in Poland and ad-
jacent portions of Czechoslovakia, is also
highly productive.
The leading individual coal producers are
West Germany in the western section and
East Germany and Poland in the eastern sec-
tion of this European area. Their combined
output rivals those of the United States and
the U.S.S.R.
The U.S.S.R. The coal fields of Soviet
Russia are numerous and widely distributed,
but most of the large reserve of good-grade
Mineral resources 369
COAL FIELDS
B ITUM INOU S
LIGNITE
FIG. 19.8 The major coal deposits of the U.S.S.R., eastern and
southern Asia.
coal is in Siberia (Fig. 19.8). The total pro-
duction is approximately equal to that of the
United States.
Neither of the great industrial regions in
and about Moscow and Leningrad is adjacent
to local supplies of good coal, although there
is lignite near Leningrad, and the Moscow
region includes a large area with coals of
subbituminous and lignite grades. Rather, the
Donets River Basin in southern European
Russia is first in industrial importance: it
supplies the heavy industry of the southern
region, and some is shipped to the Moscow
industrial center. This greatly folded area
yields some anthracite and much bituminous
coal, being valued especially for its coking
coal, which is not generally abundant in the
U.S.S.R.
The Kuznetsk Basin of southern central
Siberia is second in importance at present. It
is the source of fuel for a growing industrial
district, and some of its coal (supplemented
from Karaganda) moves more than 1,400
miles west to the iron and steel center of
Magnitogorsk in the southern Ural Mountain
region. The Kuznetsk region is estimated to
be tremendously rich in high-quality reserves;
indeed, it is thought to be second only to the
Appalachian coal fields in the United States.
The Karaganda field, located midway between
the Kuznetsk and Ural areas, is third in
importance.
There are smaller coal fields on the flanks
of the Ural Mountains, in the region west of
Lake Baikal, and far to the east in Siberia. In
the isolated forest areas of northern Siberia
370
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
After L Dudley Stamp
FIG. 19.9 The principal coal fields of southern
Africa and Australia.
are extensive coal deposits whose boundaries
and reserves are imperfectly known.
Eastern and southern Asia In eastern
and southern Asia there are widely distrib-
uted deposits of coal, with Communist China
seeming to have by far the greatest amount,
although estimates are perhaps based upon
insufficient evidence (Fig. 19.8). Coal is
found in many parts of China, but the greatest
deposits are in north China and Manchuria.
The provinces of Shansi and Shensi in north
China seem to have the largest reserves of
good quality. The coal fields of Manchuria
support a considerable industrial development
and are especially valuable because they con-
tain good coking coal. As Communist China
continues to grow industrially, coal produc-
tion will also rise. Even currently, China is
no doubt the fifth largest producer after the
United States, the U.S.S.R., Germany, and
Great Britain.
Other Asiatic countries that have important
coal supplies are India and Japan. Those of
India, located in the northeastern part of the
Deccan Plateau about 150 miles inland west
of Calcutta, are now being much used in the
iron and steel industries of the same region.
The reserve of coal is large, but the supply
of coking coal is limited. Unfortunately for
industrial Japan, the reserve of coal in that
country is relatively small and scattered, and
many of the beds are badly faulted. The
most productive field is that in northern
Kyushu.
South America, Africa, and Australia
Hardly 3 per cent of the world's coal reserves
is contained in South America, Africa, and
Australia together. This small amount is un-
evenly distributed with Africa and Australia
having the most and being about equally
endowed; South America has very little.
The major field in Australia is near the
east coast of New South Wales, a principal
center of population (Fig. 19.9). Because of
its abundance, good quality, and accessibil-
ity, Australian coal is the leading reserve in
the Southern Hemisphere, but the total re-
serve is not comparable with that of the larger
fields of the Northern Hemisphere.
The African coal reserve is not quite so
large as that of Australia, but the present
production meets the requirements of South
Africa. The deposits are located in the south-
eastern part of the continent, mostly in the
Union of South Africa (Fig. 19.9).
South America has the misfortune to be,
of all the continents, least well endowed with
coal. But there are a few small bituminous
deposits in the Andes of Colombia and Peru
and on the coast of central Chile, and there
is some low-grade coal in southern Brazil.
PETROLEUM AND NATURAL GAS
The mineral fuels petroleum and natural
gas, though relatively recent additions to
man's energy resources, are important ones.
Petroleum is a liquid that can be transported
easily, in or out of pipes; the energy avail-
able from it is greater than that available
from coal; and a large variety of lubricants
and other useful compounds is available
from it as well. Just the cleanliness, compact-
ness, and convenience of petroleum as a fuel
and the fact that new machines are continually
being devised for using the products de-
rived from it have made petroleum a critical
item in the resource inventories of modern
nations. Natural gas, a lighter hydrocarbon
usually associated with petroleum, is fast be-
coming a major source of energy. It, too, is
easily transported in pipes.
Structural associations Petroleum, nat-
ural gas, and asphalt, another substance re-
lated to petroleum, are presumably of organic
origin, but they have been so long included
in the rocks they are found in that no trace
of any organic antecedents is clearly discern-
ible. These hydrocarbons, which probably
originated from small marine organisms whose
remains were somehow prevented from com-
plete decomposition, are only found in sedi-
mentary rocks. Oil-and-gas-bearing rocks
occur in a considerable variety of structural
associations of different geologic ages. Like
coal, however, the rocks are not found among
ancient crystalline rocks of pre-Paleozoic
age.
Easily obtainable oil and gas saturate the
pore spaces of permeable rocks, especially
sandstones and limestones, just as the pore
spaces are filled elsewhere by ground water.
Structures containing petroleum are com-
monly overlain by others saturated with
water. This tends to place the oil under
pressure and concentrates it in limited de-
posits, called pools, in some form of structural
pocket or trap from which the oil and gas
cannot escape. The most numerous of these
are the tops of anticlines that are capped by
shales or other impervious rocks which pre-
vent the oil and gas from floating upward
and escaping. Many other kinds of traps
occur into which the petroleum has migrated
Mineral resources 371
from surrounding areas because of the pres-
sure exerted by the denser ground water.
Oil and gas are obtained by drilling through
the impervious capping rocks (Fig. 19.10).
When the petroleum is under considerable
pressure it may be forced upward violently,
but in other cases, and even eventually from
such gushers, the oil must be raised by pump-
ing. Moreover, since the oil is contained in
the small pore spaces in the pervious rock,
much of it exists as a film of oil clinging to
the rock particles, and not all of it can be re-
covered by pumping. Even with the most im-
proved methods a considerable portion of the
original oil remains in the ground when the
expenditure for pumping becomes un-
profitable.
There is less significant difference among
varieties of petroleum than among coals.
Most petroleum contains a variety of hydro-
carbons that may be partially separated by
distillation and then compounded in almost
any combination desired.
PETROLEUM REGIONS OF THE WORLD
Because of the nature of its occurrence, it
is more difficult to estimate the quantity of
producible reserves of oil that still exist than
reserves of coal. Recent estimates indicate the
amount remaining in the earth as perhaps
1,250 billion barrels, not counting large
amounts in shales and tar sands. 2 As with
coal, almost all of this is concentrated in the
Northern Hemisphere, and the United States
and the U.S.S.R. are well endowed, but there
the similarity ends. The Middle East has per-
haps half the world's reserve, while Europe
has very little. Also, concentration of oil is
much greater than that of coal. Even though
2 E. Willard Miller, "World Patterns and Trends in Energy
Consumption," Journal of Geography, vol. 58, 1959, pp.
269-279.
372
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
FIG. 19.10 One of many types of geologic structures in which
petroleum is entrapped. Note the relation between the locations of several
wells and the nature of their products. Some such anticlinal structures are
concealed by overlying strata.
the major reserves of coal are concentrated in
a few areas, there are numerous deposits,
throughout the world, that can more or less
adequately supply nonindustrial needs, as
previously noted. This is not nearly so true
of petroleum. Indeed, two circles with radii
of a little less than 2,000 miles and with
centers approximately at 35 N Lat and 50 E
Long in the Eastern Hemisphere and at
20 N Lat and 85 W Long in the Western
Hemisphere would outline areas that prob-
ably contain some three-quarters of the
world's known reserves.
As a consequence, trade in petroleum is far
more important than trade in coal, and the
relative locations of producing areas and con-
suming areas are items of extreme geograph-
ical significance.
Western hemisphere The United States
is endowed with several regions in which
petroleum and gas occur in great volume,
and the production and consumption of these
fuels in the United States far exceed those of
any other country. In recent times productive
deposits have also been discovered in Canada,
and the regions bordering the Caribbean are
likewise important producers of petroleum,
especially for export to less favored areas.
Each of these regions includes a number of
subdivisions. Some of the included structures
contain both oil and gas, some yield oil but
not much gas, and others yield gas alone.
The several regions are shown in Fig. 19.11.
Mid-continent, Gulf Coast, and California
The mid-continent region includes several
widely scattered fields including hundreds of
pools in Kansas, Oklahoma, central and
western Texas, southeastern New Mexico,
southern Arkansas, and northern Louisiana.
This region has been producing for many
years. Many of its deposits have been ex-
hausted, but new ones have been discovered,
and the practice of deeper drilling has reached
oil in lower and older rocks. Gas is abundant
in this region also, and pipelines now carry it
to industrial consumers far to the north and
east.
The Gulf Coast region includes numerous
pools found in many locations in the rocks
of eastern Texas, Louisiana, and Missis-
sippi. In some areas the deposits are associ-
ated with a large number of salt domes, or
mounds underlain by rock salt. The deposits
also extend out into the continental shelf,
and considerable offshore development is
taking place, although the cost of such re-
covery is, of course, much greater than on
land.
Mineral resources 373
The mid-continent and Gulf Coast regions
have long been the most productive in the
United States and contain its greatest proven
reserves, with the Gulf Coast region estimated
to have the largest proportion of the nation's
total reserves.
They are followed by the California region,
which includes oil and gas fields in a belt
that extends from the environs of Los Angeles
northward. The California region is also
highly productive; as a state California ranks
second only to Texas in importance.
Other regions of the United States The
Rocky Mountain region is comprised of many
FIG. 19.11 The principal oil-and-gas-producing regions of the Western Hemisphere.
374
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
fields distributed over a large area which is
mainly in Wyoming, although it extends
northward into Montana, south into Colo-
rado, and eastward into North Dakota. Pro-
duction is increasing in this area, but it is far
behind that of the above regions. The eastern
interior region includes several fields of
minor importance located in Ohio, Indiana,
Illinois, and Michigan. The greatest pro-
ducer here is the field located in southeastern
Illinois and adjacent Indiana.
The first oil and gas field to be developed
on a modern scale was in the Allegheny
region of Pennsylvania, and it was for many
years the most productive area in the world;
but it has now declined to minor significance.
The petroleum has long been noted for its
superior quality as a source of lubricating oils.
The question of how long the United States
can maintain its present abundant petroleum
production is not capable of assured answer.
A feverish search continues for new pools
and for new structures in all areas where the
occurrence of petroleum is a possibility. Tens
of thousands of new wells are drilled an-
nually, and the proven reserves of the coun-
try have been increasing a little each year.
But for several years the United States' im-
ports of petroleum and its products have ex-
ceeded exports, and the demand continues
to increase rapidly. The need for conservative
practices in the production and use of these
essential products is evident.
Canada. Until recently oil production in
Canada was restricted to small amounts pro-
duced in Ontario in the section north of
Lake Erie, in southern Alberta, and in the
Mackenzie River Basin west of Great Bear
Lake. But with discoveries in the vicinity of
Edmonton in central Alberta, Canadian pro-
duction and reserves have increased. Recent
estimates indicate that Canadian reserves and
production are ahead of Indonesia and
Mexico and exceeded only by the United
States, Venezuela, the U.S.S.R., and the
Middle Eastern countries. There is, more-
over, every reason to believe that Canadian
status in the world oil situation will continue
to rise. Most of the production occurs in
Alberta, with some in British Columbia to
the northwest and in Saskatchewan and
Manitoba to the southeast (Fig. 19.11).
The Caribbean Included in the Caribbean
region are several areas of considerable im-
portance. Chief among them is that located
in northern South America, mainly in the
Maracaibo and Orinoco River Basins of
Venezuela but including smaller areas in
Colombia and the island of Trinidad (Fig.
19.11). Venezuela has perhaps 6 to 8 per
cent of the world's known reserves, and has
regularly ranked next after the United States
among the countries of the world in produc-
tion. A second productive area, located in
eastern Mexico, near Tampico and Tuxpan,
yielded abundantly early in the present cen-
tury, but it has now passed the peak of
its productivity.
South America beyond the Caribbean
borders gives some evidence of widespread
occurrence of petroleum, but only Argentina
and Peru have significant production.
Eastern Hemisphere Because of the
extreme fragmentation of the oil-producing
areas in the Eastern Hemisphere by national
boundaries, the casual observer may not
realize that the major producing deposits
there are as localized as those of the Western
Hemisphere. Although some oil and gas is
known in many localities in Europe, Asia,
Africa, and Australia, the region of large
present output is confined to an area centered
in the Middle East and extending north into
the U.S.S.R. (Fig. 19.12). The only other
Mineral resources 375
PRINCIPAL
PRODUCING
AREAS
FIG. 19.12 The principal oil-and-gas-producing regions of the Eastern Hemisphere.
area that produces a significant amount is great importance of oil in the Middle East con-
Indonesia, and its production is less than 5 tribute to the explanation of why so many
per cent of the hemisphere's total.
problems of world politics and economic
Middle East Several facts related to the strategy originate in or near this part of Asia.
376
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
The petroleum deposits there are located in
several political subdivisions near the Persian
Gulf, mainly in Saudi Arabia, Iran, Iraq, and
Kuwait (Fig. 19.12). Moreover these areas,
taken together, produce about two-thirds as
much oil as the United States, most of which
is destined for export to the great consuming
area of Europe. Finally, the proven reserves
of the region are perhaps half the world's
known supply, as previously stated.
U.S.S.R., Europe, and others The U.S.S.R.
has been rapidly increasing its petroleum and
natural gas production. It now rivals Vene-
zuela as a petroleum producer second to
the United States; and although the out-
put of the U.S.S.R. is perhaps one-third
that of the United States, its potential
reserve seems to be larger. 3 The major pro-
ducing areas of the U.S.S.R. are adjacent to
the Ural and Caucasus Mountains and the
Caspian Sea. Of several fields in this region
those near the Middle Volga River and west
of the Ural Mountains, Baku (at the eastern
extremity of the Caucasus Mountains),
Grozny (north of the Caucasus), and Turk-
istan are the most productive.
Europe, outside the U.S.S.R., is one of the
great petroleum-consuming areas, but within
its area it produces less than 3 per cent of
the world's total. The leading production is
still in the Ploesti area of Romania in south-
eastern Europe; there is much smaller, but
locally significant, production in West Ger-
many, Austria, France, and the Netherlands.
Eastern and southeastern Asia contain
widely scattered deposits from Sakhalin
southward, the most important being in the
Indonesian region, especially Sumatra, British
Borneo, and New Guinea.
Petroleum has long been produced in
Egypt, and is now being produced in the
3 ibid.
western part of north Africa in Algeria and
the Sahara, but southern Africa, like Australia,
has no known deposits of significance.
OTHER MINERAL FUELS
Although liquid petroleum, coal, and nat-
ural gas are the most widely used mineral
fuels today, they are not the only mineral
sources of energy upon which man may draw.
For example, the petroleum that exists in oil
shales and tar sands is already being used.
Yet these sources are expensive because the
material must be quarried or mined and then
treated before the crude oil it holds can be
obtained. Moreover, no very reliable esti-
mate of the total potential energy available
from these sources has yet been made. It is
known that the supplies are large, perhaps
considerably more than those of liquid petro-
leum. Nonetheless, nearly three-fourths of the
remaining supply of energy obtainable from
fossil fuels is in the form of coal.
Because of the rapidly increasing produc-
tion and consumption rates of the fossil fuels,
other mineral sources, such as the nuclear
energy from uranium and thorium are already
being developed; and the total energy poten-
tially available from these sources is enor-
mously greater than that from the remaining
fossil fuels. Consequently, it appears more
accurate to predict that the future of mineral
sources of energy will be different from the
past than to say it will be dim.
THE METALLIC MINERALS
Metals and modern civilization
Among the many elements available in the
earth's crust are some such as iron, copper,
or aluminum that are called metallic. Man
learned early that the use of these metallic
elements was greatly advantageous in many
ways, and the development of civilization
and the course of human events have been
strongly influenced by variations in the
occurrence of metallic resources from place
to place and by the differing abilities of
people to put these endowments to use.
The early use of metals was largely con-
fined to such forms as utensils, weapons,
tools, and sewers. Since the Industrial Rev-
olution, however, the employment of metals
has increased a thousandfold. Modern blast
furnaces produce in a day now more iron
than was produced in a year 200 years ago;
and the power-driven machines that are the
major use of metals today made man a mobile
being and have increased his productivity
and efficiency beyond the wildest expecta-
tions of even 100 years ago. The changes
wrought by the use of metals are indeed
staggering.
Metals are sometimes used in the pure
state, as, for example, copper or gold, but
usually they are mixed to produce an alloy
that has more desirable characteristics. Bronze,
a mixture of copper and tin, and brass, a
mixture of copper and zinc, are examples of
alloys with which man has long been familiar.
But by far the most important alloys are the
steels, made by mixing other metals with iron,
without which modern high-speed metal-
working machines and efficient technologic
processes would be impossible.
A few metals that are used for a variety of
purposes and in large quantities, such as
copper, aluminum, and, especially, iron, may
be thought of as fundamental resources. Thus,
so much iron is required, and it is of such
comparatively low specific value, that the pos-
session of a domestic supply of iron ore is con-
sidered along with a supply of coal or
petroleum a matter of major economic im-
Mineral resources 377
portance by the great nations. Other metals,
such as chromium or tungsten, are used in
relatively small quantities, and although they
may be economically important to a partic-
ular region, they can hardly be called basic
mineral resources. The limited quantity re-
quired, coupled with high specific value,
enable these and similar metals to move freely
in the channels of international trade, unless
tariffs and restrictive trade regulations are im-
posed to prevent it. In a sense, the whole
world draws upon the same sources of supply
of these metals.
Physical associations of metallic min-
erals An ore is a deposit of a metallic min-
eral (or one of its chemical compounds) suf-
ficiently concentrated so that it is profitable to
use it. Some metals, such as copper, oc-
casionally occur in the native state. But
more often the metallic elements are found in
chemical combination with other elements, in
such forms as sulphides, sulphates, oxides,
or carbonates, from which they must be set
free by processes of reduction called smelt-
ing. Usually the desirable minerals are also
intermingled with some quantity of unwanted
rock material, called gangue, from which
they must be separated by mechanical means.
The local concentration of metallic min-
erals is a result of the workings of a variety
of natural processes that can be grouped in
three general categories: (a) igneous activity,
(b) weathering, and (c) sedimentation. Com-
pounds of chromium, nickel, copper, lead,
zinc, and tin are examples of metals com-
monly found in association with crystalline
igneous rock masses. They may become con-
centrated within the mass itself during its
cooling and crystallization, or they may have
intruded or have been chemically precipi-
tated from circulating ground waters in the
rock zone adjacent to the cooling mass.
378
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
Weathering processes, the second category,
produce concentrations of metallic ores in sev-
eral ways: (a) The decomposition of rock
masses in the normal course of events may
involve a desirable chemical change, for ex-
ample, in the transformation of the unusable
silicate of aluminum to the usable ore of alumi-
num, a hydrous oxide called bauxite, (b) Un-
desirable components may be simply removed
by leaching, thereby concentrating the re-
mainder, for example, as when the removal
of a large portion of the silica from an iron
formation concentrates the iron oxide, (c) A
mineral may be leached from one zone and
precipitated in a more usable form at a lower
level, an example of which is the transforma-
tion of the insoluble sulphide of copper into
the soluble sulphate.
Finally, in the alluvial process useful min-
erals are transported by running water, and
because of their relative weight may become
concentrated as placer deposits in the present
or former beds of streams. For example, a
majority of the world's tin supply comes from
placer deposits.
In general, no matter how the ultimate con-
centration of a usable ore came about, it began
in the majority of instances by the segregation
of elements that occurs as a result of heat
and pressure in rock masses. It is not surpris-
ing, therefore, that ores containing metallic
minerals are commonly associated with re-
gions of igneous activity or where the proc-
esses of metamorphism have been accompanied
by great pressure and the development of
heat. Although there are some notable excep-
tions, it is broadly true that the great areas of
technically undisturbed sedimentary rocks
are poor in the ores of metals an exactly
opposite relationship from that regarding the
occurrence of the mineral fuels.
IRON
Iron is usually found in some chemical com-
bination, the more important ones being the
oxides named hematite, magnetite, and
limonite, and the carbonate named siderite.
The oxides are particularly abundant, and
they are scattered widely but thinly through
a large part of the regolith and give its com-
mon red, brown, and yellow colors to it, but
ordinary regolith has not enough iron per
unit volume to make it an ore, that is, to
give it the at least 30 to 35 per cent of the
metal most ores of iron must contain under
present economic conditions. Iron-bearing
minerals, such as hematite and magnetite,
contain as much as 70 per cent of iron, but
deposits seldom consist solely of these min-
erals; instead the iron content of a mass is re-
duced by the occurrence of associated gangue
minerals, especially silica.
Iron deposits of the world Iron is
more abundant than any other metallic min-
eral except aluminum, and iron deposits
occur widely, so that no large area of the
earth is far removed from iron-ore supplies.
The supply is sufficient for many years to
come. Among the outstanding deposits,
measured by their present contributions to
the world's iron industries, are those of the
United States, Canada, Venezuela, the west-
ern European countries, the U.S.S.R., and
India. There are many other places where
deposits of potential future significance occur.
But since at present iron must be separated
from the ore by the use of coke, it is impor-
tant to consider in what parts of the world
these two ingredients are found close together.
The distribution of the world's populated
plains is such that they contribute to the
commercial supremacy of the areas tributary
to the North Atlantic Basin which is the only
region in which abundant deposits of iron
ore and coking coal are known to be closely
associated. This area includes the eastern
United States, the countries of northwestern
Europe, and the U.S.S.R. In them are the
present world centers of heavy iron and steel
manufacture, as well as many other industries
that depend on cheap iron and steel. Some
of the world's greatest reserves of iron ore
are in Brazil and India, but Brazil has almost
no coking coal, and India has only a limited
supply. China apparently has large reserves
of excellent coal but no known supply of ore
of comparable importance. With respect to
the basic raw materials for iron and steel
manufacture the endowment of the United
States has indeed been fortunate.
Western Hemisphere In the Western
Hemisphere there are several regions of un-
usual present and potential future iron-ore
production. Outstanding are those associated
with the crystalline rocks of the Canadian
Shield, both in the United States and Canada.
United States: Lake Superior district The
United States has the most renowned iron
deposits, those of the Lake Superior district
(Fig. 19.13). These ores are mostly hematite
of high quality, and those mined until recent
years were very rich, the average iron content
being 50 per cent or over. They were con-
centrated by ground-water action, and some
of the deposits were so near the surface that
they could be easily mined by power shovel
(Fig. 19.14). The Lake Superior district has
been the most productive iron-ore deposit in
the world, and this region has supplied the
bulk of the ore used in the steel industry of
the United States. But the reserves of easily
mined, high-quality ore are limited, and al-
though they are not about to be depleted, an
Mineral resources 379
alternative ore, taconite, is now being used.
Taconite is the parent iron formation in
the Lake Superior district, an iron-bearing
silica rock, from which in some areas the
richer ores were derived through the removal
of silica by leaching. Because its iron content
is only about 25 per cent, such low-grade ore
is not suitable for direct shipment. Instead,
the taconite must be beneficiated, that is,
artificially concentrated. It is first quarried,
then crushed and processed into a concen-
trate containing some 60 per cent iron.
The relation of the Lake Superior ores to
regions of manufacture and market is fortu-
nate. The Great Lakes, with the canals con-
necting Lakes Superior and Huron, provide a
deep waterway almost from the mine to the
very margin of the Appalachian coal field
and the heart of the American industrial
region. Iron deposits are found elsewhere in
the United States than in the Lake Superior
district, but the reserves are limited. The
most used deposit is in Alabama, where iron
is mined in the same district with the coal
and limestone required in the smelting
(Fig. 19.15).
Iron ore moves so cheaply by water that
large supplies of foreign ores move to meet
abundant coal upon the eastern seaboard of
the United States for smelting there. Most of
these continually growing imports come from
other North or South American sources,
chiefly from Venezuela and Canada, which
together supply more than three-quarters of
the total.
Canada and Latin America Canada in-
cludes the larger part of the Canadian Shield,
but until recently it was not known to con-
tain such large and easily mined ore deposits
as those in the Lake Superior district of the
United States. Now, considerable deposits
380
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
WYOMING
UTAH
CALIFORNIA
PRINCIPAL
IRON
DEPOSITS
FIG. 19.13 The major Western Hemisphere iron deposits. The insets
show the locations of those in the Lake Superior and Labrador districts.
are being mined in the western Ontario region
in districts northwest and northeast of Lake
Superior. A most important and productive
recently discovered ore deposit lies in the
Knob Lake-Schefferville district in the eastern
part of the shield, in the boundary area be-
tween Quebec and Labrador some 360 miles
from the north shore of the Gulf of Saint
Lawrence (Fig. 19.13). This seems to be one
of the world's great reserves; much of the ore
is hematite with an iron content exceeding
60 per cent, and open-pit mining is practiced.
Venezuela supplies a large share of the iron-
ore imports of the United States. The deposits
Mineral resources 381
FIG. 19.14 Mining ore in an open pit in northern Minnesota. Open-pit
ore of high quality is no longer abundant in the Lake Superior district.
(Oliver Iron Mining Company.)
of high-grade ore are located near the lower
Orinoco River, so that the ore is easily
moved by water to the smelting centers of
the eastern United States.
Other Latin American deposits are located
in Brazil, Chile, Peru, Mexico, and Cuba.
The Brazilian deposits lie some 200 miles
north of Rio de Janeiro in the crystalline
rocks of the Brazilian plateau. They contain
iron minerals of the highest quality some
hematite, some magnetite and they rank
near the top of the world's great and rich re-
serves of iron ore. Their utilization is just
beginning.
Eastern Hemisphere Throughout the
Eastern Hemisphere are many known iron
deposits, many of which now produce
abundantly for local consumption (Fig.
FIG. 19.15 Distribution of essential minerals
in the Birmingham, Alabama, industrial region.
382
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
19.16). But only a few appear to rank among
the major world reserves.
Western Europe The iron industries of
western Europe depend primarily upon
European sources of ore. As in North
America, the greatest centers of iron manu-
facture are located in or close to the princi-
pal coal fields; but only in a few places are
the ore and coal found together, and one or
the other must usually be transported. Al-
though some of the countries contain both
iron ore and coking coal, the numerous
boundaries of western Europe have politically
fragmented some of the more important de-
posits, and much of the ore, especially the
high-grade ore, must move in international
trade to reach the principal smelting centers.
The large iron resources are in France and
Sweden. Sweden has less iron than France,
but the iron is superior in quality. Other im-
portant ore deposits are in England, Germany,
and Spain. Those of Germany and England
provide low-grade ore for local processing,
but those of the northern part of Spain, a
country of little coal, provide ore for export,
especially to England.
The iron ores of France include the largest
single iron reserve in western Europe and
one of the large ones of the world. They are
found in the northeastern part of the country
in the province of Lorraine, and extend across
the boundary into Luxembourg and slightly
into Belgium (Fig. 19.17). The Lorraine ores
are mainly limonite, a hydrous oxide of iron,
and are of relatively low quality, averaging
only about 25 to 35 per cent in iron content.
FIG. 19.16 The major Eastern Hemisphere iron deposits.
PRIN C IPAL
IRON
DEPOSITS
Mineral resources 383
However, they lie near the German, Belgian,
and French coal fields and the great industrial
market of Europe.
The iron ores of Great Britain have been
greatly depleted. The remaining ores are
scattered, of different kinds, and mainly of
low grade. It has long been the practice of
British smelters to supplement the domestic
supply with imported ores. Nevertheless,
Britain has a supply of domestic low-grade
ores sufficient for many years. They are closely
associated with supplies of coal and limestone.
The iron ores of Sweden are noted for their
high quality. They are mainly magnetite and
average 55 to 65 per cent iron. High-quality
ore is obtained in central Sweden, but the
largest deposits are situated in the crystalline
rocks of the far northern part of the country
in the Kiruna district. Since there is almost
no coal and but relatively little iron manu-
facture in Sweden, much of the ore is
exported to other European countries.
U.S.S.R. and others The iron ores of the
U.S.S.R. include large reserves, the most im-
portant of which are found in three localities.
These are Krivoi Rog in the southern Ukraine,
the Kerch Peninsula in the Crimea, and
Magnitogorsk, near the southern end of the
Ural Mountains. The Krivoi Rog deposit is
the richest and normally the most productive.
It is located about 300 miles west of the Donets
coal basin, and thus contributes to the
Ukrainian region of heavy industry. The ores
at Magnitogorsk are used in association with
the coal from several small deposits farther
north in the Ural region, but especially with
the coal of the distant Kuznetsk and Karaganda
fields in Siberia. Smaller iron-ore deposits in
Siberia, such as that at Gornaya Shoriya
south of the Kuznetsk coal field, are becom-
ing more important in the growing industries
of the eastern U.S.S.R.
FIG. 19.17 Location of the great Lorraine
iron-ore field of France in relation to nearby coal
fields.
The iron ores of India constitute the only
major deposit elsewhere in the Eastern Hemi-
sphere. It lies adjacent to the principal coal
field of the country in the Deccan Plateau
about 150 miles west of Calcutta.
Throughout the other areas of Africa, Asia,
and Australia, iron-ore deposits are known to
exist in many places. Some of them now pro-
duce in sufficient quantity to provide for local
industry, as for example do those of southern
Australia, southern Africa, and Manchuria.
Also, it is probable, since iron is generally
widespread, that in localities as yet imper-
fectly explored other, and perhaps more sig-
nificant, resources will be found.
OTHER METALLIC MINERALS
THE FERROALLOYS
For the fabrication of finished products, the
modern industrial world does not use much
iron in the state in which it comes from the
blast furnace. Instead, iron is compounded
384
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
with various other metals and nonmetals to
make steels, which are, as already stated, the
most important alloys. Many of these ferroal-
loying elements including such metals as
nickel or tungsten and the nonmetals, carbon
and silicon are used to make the desirable
mixtures, but the two in greatest demand are
manganese and chromium.
The mixing of these materials with iron
imparts special qualities to the steel renders
it stainless, or makes it hard or tough, or
gives it the ability to hold a cutting edge at
high temperatures, and so on. These prop-
erties are so important that the major alloy-
ing metals are in great demand, although
they are not needed in large quantities. The
one most used, in terms of quantity, is
manganese, which is used in the manufacture
of all steel; about 13 Ib of manganese is
used in making each ton of steel in the United
States. Next to manganese in terms of
quantity used are chromium and nickel, in
that order.
More than 70 nations produce ores of the
alloying elements, and generalizations con-
cerning the rank of the producers and distri-
bution of the product are difficult. The major
known deposits of manganese are in the
U.S.S.R., which has in the southern Ukraine
and in Georgia the world's largest known
reserves. Other major producers are India,
the Union of South Africa, Ghana, Brazil,
and Morocco. The largest reserves of chro-
mium are in southern Africa, Turkey, the
U.S.S.R., and the Philippines. Canada is the
world's greatest nickel producer.
Although the United States produces a
greater number of alloying elements than any
other nation, and leads in the production of
several, virtually all its supplies of the most
used three manganese, chromium, and
nickel must be imported.
IMPORTANT NONFERROUS METALS
The list of metals other than iron used in
the modern arts and industries is very long,
and even summary descriptions of the uses
and regions of occurrence of all cannot be
included in this book. So comment will be
restricted to a few that illustrate some of the
many complexities of the geography of
minerals.
A great variety of earth conditions is favor-
able to the occurrence and discovery of the
ores of metals, but it is worth repeating that
the principal world regions of mineralization
are those where crystalline rocks are at or
near the surface or where there have been
recent crustal disturbances or igneous activ-
ity. Even this general rule has notable excep-
tions an apparent one being the deposits
of lead and zinc ores associated with sedi-
mentary rocks. Examples of these are the
lead and zinc deposits of southwestern Mis-
souri, southwestern Wisconsin, and adjacent
Illinois, and those of Belgium and Poland.
Another exception of great importance is the
occurrence of the ore of aluminum, bauxite.
Aluminum Aluminum, an even more
abundant element than iron, is a common
and widely distributed constituent of the
regolith. On the other hand, only in relatively
few places are there rich deposits of the ore.
Varieties of bauxite are of different origins,
but it seems clear that some are derived from
sedimentary clays that have been changed
through long-continued leaching by ground
water. Others are known to have been de-
rived by a process of natural beneficiation of
igneous rocks that originally were low in iron
and silica; of such origin are the limited de-
posits of the Ouachita Mountain region of
Arkansas.
Major deposits of bauxite are known to
exist in many parts of the world: northern
Australia, British Guiana, Brazil, Ghana,
Surinam, Jamaica, the East Indian region,
China, and the U.S.S.R. Large reserves also
exist in Hungary, Yugoslavia, and France,
and they provide abundantly for European
consumption. The United States, on the
other hand, must import much of the ore it
uses.
Copper Second to iron in amount pro-
duced, copper is quite the opposite of iron
and aluminum in its occurrence: whereas the
other two are widespread in the crust of the
earth, copper is not. But the occurrence of
copper ore conforms to the generalization re-
peated earlier; namely, it is found in regions
of crystalline rock or in areas of recent
tectonic activity. Because copper is the basis
of the modern electrical world it is much
sought after, and a deposit yielding as little
as 1 per cent of the metal is considered a
usable ore.
The largest known reserves are located in
three general regions, western North America,
western South America, and south central
Africa, in that order; among them they ac-
count for some three-quarters of the known
copper resource. None of the great industrial
nations, except perhaps the U.S.S.R., is self-
sufficient in copper production.
In North America, which produces ap-
proximately a third of the world's copper,
the copper reserves are located primarily in
the western cordilleran sections of the United
States and Canada. Other known deposits
are in the Canadian Shield area of northern
United States and southern Canada. South
American copper, in the cordilleran region of
that continent also, is concentrated in Chile
and southern Peru, the former having per-
haps the greatest single known deposit. In
Africa copper is located in the Katanga
Mineral resources 385
Province of the southern Congo and in adja-
cent Northern Rhodesia. Copper reserves else-
where in the world are small, although those
of the U.S.S.R. are estimated to be perhaps
one-fourth as great as those of North America.
SOURCE REGIONS OF METALLIC MINERALS
A survey of metallic minerals can at best
mention only a few of the many used today
in modern industry, so it is impossible to
treat in detail here the major known areas of
their present and potential supply. However,
some of these areas and the bases of their
world importance are noted below.
The Canadian Shield The Canadian Shield
is highly productive of metallic minerals and
has large possibilities for future discoveries.
From its ancient crystalline rocks are obtained
not only rich iron ores but a wealth of other
metals. These include most of the world's
supply of nickel and large amounts of gold,
silver, cobalt, copper, uranium, and others.
Important discoveries are made in this exten-
sive region each year, and the exploitation of
mineral resources is one of the principal
industries that has attracted people there.
The American cordilleran region The
American cordilleran region, from Alaska to
Cape Horn, is also an area noted for the
abundance and variety of its mineral products.
At least half the world's copper is found here
in deposits as far separated as Chile, Peru,
Arizona, Montana, and Alaska. Gold, silver,
lead, and zinc are sufficiently abundant that
Mexico, the United States, and Canada hold
high rank in the production of each of them.
The Andean countries of South America are
important producers of platinum, tin, and
tungsten, and have an appreciable output of
other metals. It was the gold of this region
that gave impetus to its conquest by Spain.
Central and southern Africa and other
386
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
regions In central and southern Africa the
crystalline rocks include several productive
mineral regions. Within that vast area are
The Rand, the world's leading gold-producing
district, and such important centers in the
production of copper as those of the Katanga
in Northern Rhodesia and the adjacent Congo.
There are also districts producing chromium,
manganese, and uranium, and important
localities from which most of the world's
diamonds are mined.
Other mineral regions of world renown
may only be mentioned. Among them are the
following: (a) Areas of igneous and crystalline
metamorphic rocks in southern and western
Australia which have yielded gold, silver,
lead, zinc, and minor quantities of other
metals, (b) The crystalline rocks of the high-
lands of eastern South America, in Brazil and
the Guianas. In addition to deposits of iron
ore and bauxite, these highlands yield impor-
tant quantities of manganese, gold, and
precious stones. They are known also to con-
tain deposits of several other metals which
are as yet little developed, (c) A large region
of crystalline rocks in eastern Asia. They ex-
tend from Korea on the south to the shores
of the Sea of Okhotsk on the north, and
thence westward in southern Siberia. From
this region is obtained a large part of the
gold that makes the U.S.S.R. one of the lead-
ing producers of that metal. The region con-
tains large areas that are as yet little explored
geologically, (d) The highlands of south-
eastern Asia. From them are now obtained
the larger part of the world's tin, tungsten,
and several other metals, (e) The cordilleran
region of southern Europe and the Med-
iterranean borderlands. In it are included im-
portant centers in the production of several
metals. (/) The Ural region of the U.S.S.R.
It may once again be observed that the
world's principal centers of actual and poten-
tial production of the metallic minerals are
those associated with igneous or crystalline
rocks, in contrast to those regions which are
comprised mainly of sedimentary rocks.
Despite the fact that the regions of sedimen-
tary rocks contain the world's supplies of
mineral fuels and certain of the ores of iron,
aluminum, lead, and zinc, they are, in gen-
eral, poor in the ores of other metallic
minerals.
THE NONMETALLIC MINERALS
Modern use of nonmetals Man has
used the nonmetallic minerals of the earth's
crust longer than he has used either the
metallic minerals or the mineral fuels. He
very early learned to fashion implements and
utensils from stones and appreciated the value
of rock in the construction of many things.
Rock is still widely used today, but in
markedly different ways and in vastly greater
quantity.
Some nonmetallic minerals are used in
their natural states, while others pass through
processes of industrial manufacture and ap-
pear as components in goods having hundreds
of uses. Rocks, sands, clays, salts, abrasives,
fertilizers, gems-, and many others make up
the long list. Most of them are found in
a variety of grades. Nonmetallic minerals are
essential qualities of the natural equipment of
regions, and no limited portion of the earth
contains all of them; indeed, there are few
regions, if any, that contain all of even the
most essential.
Because of the great number of these sub-
stances and the variety of their occurrences,
many cannot be discussed in this brief treat-
ment. Only those considered relatively essen-
tial and those required in great quantity
such as the compounds and elements used in
large amounts by the chemical industries and
the rock, sands, limes, and clays used in the
construction and manufacturing industries
can be included.
Salt and its sources Salt, sodium chlor-
ide, is one of the common rock minerals, and
it is widely used in great volume as a food, a
food preservative, and a basic raw material
from which industry derives a number of the
useful compounds of sodium and chlorine.
Owing to its solubility in water, it is not
abundant in the zone of free ground-water
circulation in humid climates, but inexhaus-
tible supplies are available for human use
from the following sources: (a) the sea, which
contains 2% lb of salt for every 100 Ib of
water; (b) natural brines, which are the waters
of ancient seas that saturate deeply buried
sedimentary rocks cut off from ground-water
circulation; (c) deposits of rock salt, called
evaporites, which probably resulted from the
evaporation of water in the arms of ancient
seas or in former arid interior drainage basins
(these deposits now are sedimentary rocks
deep underground, where they are protected
by other sediments from the solvent action of
ground water); and (d) the present limited
surface incrustation of salt in interior drain-
age basins in dry climates. Salt is found in so
many places that few parts of the world are
without some local supply.
For industrial uses salt is obtained largely
by mining rock salt or by the pumping of
Mineral resources 387
brines, either natural brines or those pro-
duced by pumping water down to bodies of
rock salt. The industrial regions of North
America are supplied from abundant reserves.
Thick beds of rock salt underlie large areas
in central and western New York, northeastern
Ohio, southeastern Michigan, and peninsular
Ontario. Other large reserves are found in
the buried salt domes of the Louisiana and
Texas Gulf Coast, in deposits in central
Kansas, and at various places in the south-
western states.
The industrial centers elsewhere in the
world likewise are well provided with salt.
There are large deposits in western England,
central Germany, Austria, and the southern
U.S.S.R.
Sulphur Sulphur, especially in the form
of sulphuric acid, has many uses in modern
industry. It is variously used in connection
with the manufacture of steel, oil, paper,
rayon, rubber, and explosives, and in other
chemical industries. It has long been obtained
from pyrite (a mineral containing iron and
sulphur) and from deposits associated with
recent volcanic activity, and is mined from
these sources in Italy, Spain, Japan, and
Chile. In the United States the principal de-
posits occur in the Louisiana and Texas Gulf
Coast area. There native sulphur is recovered
by means of wells through which superheated
steam is pumped underground to the sulphur
beds, causing molten sulphur to be returned
to the surface. Alternatively, industrial sul-
phur is recovered from oil-refinery opera-
tions, and is a by-product of the smelting of
certain mineral ores in which the metals are
chemically combined with sulphur. Such are
certain ores of iron, copper, and zinc.
Mineral fertilizers Continuous crop-
ping and accelerated erosion remove the ele-
ments of soil fertility faster than they can be
388
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
resupplied by natural processes in the soil.
The most needed nutrients, moreover
calcium, nitrogen, potash, and phosphorus
are among those especially susceptible to
depletion. The volume of agricultural pro-
duction can be increased markedly in areas
of initial or induced low soil fertility, how-
ever, by adding these elements, especially
the last three. For each of the four, there are
known mineral deposits which are drawn
upon for the manufacture of fertilizers.
Calcium Calcium, in the form of calcium
carbonate, is readily available in the lime-
stones of many regions. The other three are
much less abundant, and notable deposits of
them are considered to be resources of great
importance.
Nitrogen Nitrogen is the most abundant
element in the atmosphere, but it is largely
unavailable to plants in the gaseous form: it
must be combined to form a soluble nitrogen
compound. Most natural inorganic nitrogen
compounds are soluble in water and so are
quickly lost when water seeps downward in
the soil. The great need for nitrogen com-
pounds for fertilizer (and for industrial use)
led to the discovery of ways of producing
such compounds from the nitrogen in the
atmosphere. This is now accomplished in a
number of ways, and synthetic nitrogen pro-
duction has practically superceded its pro-
duction from natural sources. Until recently,
however, the principal source was mineral
deposits in arid lands, chiefly those in the
Atacama Desert in northern Chile which are
accumulations from ages of seepage and sur-
face evaporation.
Potash Potash 4 is obtained in small
4 Potash is a term generally used to refer to simple potas-
sium-bearing compounds, such as potassium carbonate (lye),
potassium hydroxide, or potassium oxide.
quantities from the ashes of wood, seaweed,
and other substances, but the principal com-
mercial sources are deposits of complex min-
erals. Large deposits are located in western
Europe, mainly in central Germany and in
Alsace in northeastern France, where, until
recently, the greater part of the world's supply
has been obtained from mines 1,000 ft or
more beneath the surface of the earth. De-
posits of potash minerals are known to exist
in many areas; in the United States there is
a very large reserve in New Mexico.
Phosphorus Phosphorus is an indispensa-
ble constituent of all living cells, the prin-
cipal mineral sources of which occur as
calcium phosphates, mainly in rock form.
This rock is believed to have been formed
from the alteration of limestone by the chem-
ical action of ground water which had first
passed through ancient accumulations of bird
and fish remains.
Valuable beds of phosphate rock usually
occur as local pockets in limestone strata,
and useful deposits exist in several parts of
the world. The principal sources for Europe
are located near the Mediterranean Coast of
Africa in Tunisia, Algeria, and Morocco. The
United States is largely supplied from exten-
sive beds in western Florida and central
Tennessee. Other great reserves are known
to exist in the Northern Rocky Mountain
region of the United States, in the U.S.S.R.,
and in some of the islands of the Pacific
Ocean.
Sand, lime, gypsum, and clay Sand is
used in vast quantities in construction as an
ingredient of concrete, mortar, and plaster.
Also it is the chief raw material in the man-
ufacture of glass, and it shares with clay and
calcium compounds a place of great impor-
tance as a raw material of industry generally.
Lime (calcium carbonate that has been
calcined, i.e., heated to drive off the carbon
dioxide) and clay are required in the manu-
facture of cement; gypsum (calcium sulphate)
is widely used in plaster materials; and clay
is basic to the brick, tile, and pottery
industries.
These substances are of common occur-
rence. There are, for example, river sands,
beach sands, wind-blown sands, sands in
glacial deposits, and pure sandstones. There
are unconsolidated marls, soft chalks, and
other limestones as source materials from
which lime may be obtained. Deposits of
gypsum and anhydrite (anhydrous calcium
sulphate) are relatively common. There are
river clays, marine clays, residual clays, and
shale rocks. Indeed, not many regions are
without one or more of these minerals.
However, qualities differ, and needs for
particular grades of these minerals often can-
not be supplied locally or even regionally.
Glacial-lake clays are good enough for the
manufacture of ordinary brick and tile, but
other uses have more particular requirements.
Pottery clay especially must be pure and
burn white in the kiln. It is usually found in
residual deposits where it has weathered
from coarsely crystalline feldspars. Good
grades of glass sand, free from iron and clay,
may be sought hundreds of miles from the
centers of glass manufacture. Therefore, some
regions gain advantage from particular nat-
ural endowments suited to particular require-
ments. Some, indeed, have achieved inter-
national fame through their products. Such
are the regions of pottery clays in southern
England, northern France, or Bavaria.
Crude rock Many kinds of crude rock
are used in large quantities in architectural
and engineering structures; and some form'
Mineral resources 389
of cut stone, crushed rock, or gravels of
stream or glacial origin that will serve these
purposes is found in many parts of the earth.
Thus it may seem that rock, in this broad
sense, is one of the universal items of regional
equipment, like the air. That, however, is
not true. Some regions are endowed with
large quantities or with rock of unusual
quality; others have none at all.
Indeed, a few regions of considerable size
are practically devoid of any kind of rock.
Among these are the great deltas of the world,
where silt covers hundreds of square miles
and rock is buried to great depths. Much
larger still are certain plains of older alluvium
or regions of deep loess accumulation. Among
these are the loess-and-alluvium-covered
Pampa of Argentina and similar areas in the
American Corn Belt, where older glacial
drift and loess cover the rock strata deeply.
In these regions are localities that do not
even have any crude rock or gravel with
which to surface roads.
Where crude rock does exist, it seldom
moves far from its place of origin unless it
has some particular quality to recommend it
to a wider market, because crude rock is
heavy and of low value. Regions in which
rocks of special quality abound, however,
have a valuable resource, especially if they
also are near a large market for stone. Such
a region is New England. There, in a region
of igneous intrusion and metamorphosed
sediments, beautiful and massive granites,
slates of parallel cleavage, and excellent
marbles all are produced near a good market.
The even-textured and easily worked gray
limestones of southern Indiana have a na-
tional market, and some other stones of unique
quality, such as the statuary marble of Italy,
have practically world-wide markets.
390 FUNDAMENTALS OF PHYSICAL GEOGRAPHY
SUGGESTED READING
Leet, L. Don, and Sheldon Judson: Physical Geology, Prentice-Hall, Inc., Englewood Cliffs, N. J.,
1958, chap. 20.
Miller, E. Willard: "Mineral Regionalism of the Canadian Shield," Canadian Geographer, no.
13, 1959, pp. 17-30.
: "World Patterns and Trends in Energy Consumption," Journal of Geography, vol. 58,
1959, pp. 269-279.
Pratt, Wallace E., and Dorothy Good: World Geography of Petroleum, American Geographical
Society and Princeton University Press, New York, 1950.
Riley, C. M.: Our Mineral Resources, John Wiley & Sons, Inc., New York, 1959.
Smith, J. Russell, M. Ogdon Phillips, and Thomas R. Smith: Industrial and Commercial
Geography, 4th ed., Henry Holt and Company, Inc., New York, 1955.
Van Royen, W., and Oliver Bowles: "The Mineral Resources of the World," Atlas of the
World's Resources, vol. 2, Prentice-Hall, Inc., Englewood Cliffs, N.J., 1952.
Zimmerman, E. W.: World Resources and Industries, rev. ed., Harper 8c Brothers, New York,
1951.
APPENDIX
A selected list
of United States
topographic
quadrangles
The topographic quadrangles indicated be-
low have been selected from those published by
the United States Geological Survey because
they illustrate in map form certain of the land-
surface types discussed in the text. Some of the
types discussed, ice-scoured plains, for ex-
ample, are not clearly illustrated in any of the
quadrangles now published and are therefore
omitted from the list.
Because of the great progress made during
the last two decades in accuracy of representa-
tion, recently published sheets have been se-
lected wherever possible. To provide uniformity
and to afford adequate-sized samples of the
terrain, the selection has been largely confined
to sheets on the scales of 1 : 62,500 or 1 : 63,360.
Scales other than those are noted where chosen.
In some instances two or three quadrangles are
391
392
required to show adequately the terrain type in
question. Such quadrangles are listed as a
series.
In recent years the Geological Survey has
begun issuing a number of sheets in shaded
relief as well as in contour editions. Because of
the excellence of these maps and their clarity
of terrain representation, they are especially val-
uable for teaching purposes. Quadrangles for
which shaded-relief editions are available are
marked with an asterisk in the list below.
All the quadrangles listed may be obtained
from the United States Geological Survey,
Washington 25, D.C.
PLAINS
STREAM-ERODED PLAINS
YOUTHFUL
Binger, Okla. (dendritic dissection)
Carlinville, 111. (dendritic dissection)
Florence West, S.C. (upland swamps)
Sandon, Kan. (flat; dissected edge)
MATURE
Marlow, Okla. (early mature)
Oxford, N.C. (mid mature)
Chatham, La. (late-mid mature)
Wiergate, Tex.-La. (late mature)
PLAINS WITH CUESTAS
Fredonia, Kan. (two well-marked escarpments)
Epes, Ala. (ragged escarpment)
Denmark and New Albany, Miss, (eroded, low)
Fond du Lac, Wis. (clean, glaciated escarpment)
WATER-LAID PLAINS
FLOODPLAINS
Clarksdale, Miss, (meander scars)
Davis Island, Miss.-La. (meanders, cutoffs,
scars)
Appendix
Fairbanks C-l and Fairbanks D-l, Alaska (me-
anders, cutoffs, braided channels)
Augusta, Mo. (narrow floodplain, bluffs)
ALLUVIAL TERRACES
Wabasha, Minn, (well-defined low terraces)
*Ennis, Mont, (high terraces)
DELTAS
Hahnville, La. (inner part large delta)
East Delta, La. (margin large delta)
Mount Vernon, Wash, (small delta)
Bouldin Island and Isleton, Calif, (diked delta
lands)
ALLUVIAL FANS; PIEDMONT ALLUVIAL
PLAINS
*Ennis, Mont, (well-defined fan)
Santaquin, Utah (several small fans)
Cucamonga and San Bernardino, Calif, (pied-
mont alluvial plain)
Unionville, Nev. (pied, alluvial plain; many fan
heads)
LAKE PLAINS
Grand Forks, N.D. (flat)
Wheaton, Minn, (beach ridges)
Merrill, Mich, (slightly dissected)
Perrinton, Mich, (margin and outlet)
COASTAL PLAINS
Limerick, Ga. (low, swampy)
Lake Drummond, Va.-N.C. (broad swamp)
Nixonville, S.C. (swampy; low terrace)
White Lake, N.C. (terrace, upland swamps)
GLACIALLY MODIFIED
PLAINS
TILL PLAINS
Gilman, Wis. (undulating; swampy; small mo-
raines)
Perry, la. (smooth; well-drained)
Lastrup, Minn, (undulating)
Beaver Dam, Wis. (drumlins)
MARGINAL MORAINES
Vergas and Pelican Rapids, Minn, (broad rough
moraine)
Noonan, N.Dak. (broad moraine)
Alma, Mich, (narrow and low; lake plain)
Arrowsmith, 111. (smooth, clayey moraine)
OUTWASH SURFACES
Three Rivers, Mich, (with moraine)
Schoolcraft, Mich, (pitted; with moraine)
Delavan and Manito, 111. (broad; terraces; sand
hills)
Saponac, Me. (esker)
PLAINS AFFECTED BY
UNDERGROUND SOLUTION
Interlachen, Fla. (large sinks, lakes)
*Mammoth Cave, Ky. (sinkholes on plains and
in hills)
Glendale, Fla. (swampy depressions, surface
streams)
Holt, Fla. (solution valleys, springs)
PLAINS AFFECTED
BY WIND
WIND-BLOWN SAND
*Ashby, Neb. (clumped sand hills)
Crescent Lake, Neb. (low sand hills)
Ogilby, Calif, (strip of live dunes)
Holland, Mich, (large coastal dunes)
LOESS SURFACES
Utica, Neb. (smooth depositional surface; dis-
sected edges)
St. Francis, Kan. (depositional surface; much
dissection)
Broken Bow SW, Neb. (1:24,000) (sharply
dissected)
Appendix 393
HILLS AND MOUNTAINS
STREAM-ERODED; LITTLE STRUCTURAL
CONTROL
* Dutchman Butte, Ore. (high relief; mature)
Round Spring, Mo. (moderate relief; mature)
Sparta, Wis. (moderate relief; late mature)
Cuny Table West, S.Dak. (1:24,000) (bad-
lands)
STREAM-ERODED; STRUCTURAL
CONTROL EVIDENT
*Orbisonia, Pa. (smooth monoclinal ridges)
*Waldron, Ark. (irregular monoclinal ridges)
*Maverick Spring, Wyo. (1:24,000) (eroded
structural dome)
Navajo Mountain, Utah- Ariz, (laccolith; joint
control of erosion)
FAULT SCARPS
Hurricane, Utah (relatively undissected scarp)
Mount Whitney, Calif. (1 : 125,000) (high dis-
sected scarp)
Mount Tom, Calif, (high dissected scarp)
Logan, Utah (1 : 125,000) (high straight scarp)
MODIFIED BY CONTINENTAL GLACIATION
*Old Speck Mountain, Me. (smoothed slopes)
*Monadnock, N.H. (smoothed knobs; ponds;
swamps)
*Ithaca West, N.Y. (1 : 24,000) (smooth slopes;
lake)
West Point, N.Y. (smoothed forms; lakes; water
MOUNTAIN VALLEY GLACIERS
Cordova C-3 and Cordova C-4, Alaska (large
glaciers; medial moraines)
Seldonia D-l and Seldonia D-2, Alaska (ice
field and many ice tongues)
Mount Rainier, Wash. (1:125,000) (radial
system on volcanic cone)
Fremont Peak, Wyo. (largest system in U.S.
Rockies)
394
MOUNTAINS MODIFIED BY VALLEY
GLACIATION
Mount Goddard and Mount Tom, Calif.
(cirques, troughs; moraines; hornlike peaks)
*Holden, Wash, (troughs; small cirques; small
glaciers)
*Holy Cross, Colo, (cirques; troughs; moraine
loop)
Glacier National Park, Mont. (1:125,000)
(cirques; troughs; sharp peaks and ridges;
lakes)
VOLCANIC CONES
Lassen Volcanic National Park, Calif, (cones;
flows)
Amboy Crater, Calif, (cinder cone; flow)
*Umnak, Alaska (1:250,000) (huge caldera;
glaciated cones)
TABLELANDS
UPLANDS AND VALLEYS
Hatch Point, Utah (broad upland; cliffs;
canyons)
*Portage, Mont, (low; narrow valleys)
Grand Canyon National Park, Ariz. (2 sheets)
(great canyon)
Mouth of Dark Canyon, Utah (several canyons)
ESCARPMENTS AND OUTLIERS
Boot Mesa and Agathla Peak, Ariz, (escarpment
and many outliers)
The Spur, Utah (escarpment and outliers)
*Anvil Points, Colo. (1:24,000) (high, dis-
sected escarpment)
Promontory Butte, Ariz, (high, dissected escarp-
ment)
PLAINS WITH HILLS
OR MOUNTAINS
EROSIONAL VARIETIES
*Warm Springs, Ga. (residual ridges; rolling
plain)
Appendix
Greenville, S.C. (residual mountain; rolling
plain)
Saponac, Me. (residual mountains; glaciated)
Cooperton, Okla. (exhumed granite knobs)
TECTONICALLY PRODUCED VARIETIES
* Antelope Peak, Ariz, (isolated peaks; pedi-
ments)
Sonoma Range, Nev. (1 : 125,000) (basin and
range)
*Bray, Calif, (volcanic cones on plains)
Ship Rock, N.M. (volcanic neck; dikes)
COASTAL FEATURES
ESTUARIES AND BAYS
Kilmarnock, Va. (branching estuaries; bottom
contours)
Empire and Coos Bay, Ore. (large estuary)
Foley and Ft. Barrancas, Ala.-Fla. (large
branching estuary)
Boothbay, Me. (drowned glaciated coast)
FIORDS (all sheets have bottom contours)
Kodiak B-6 and Kodiak C-6, Alaska (basins;
sills; moraines)
Seldovia B-2 and Seldovia C-2, Alaska (branch-
ing)
Blying Sound D-8, Alaska (large; sill; glaciers)
SEA CLIFFS AND TERRACES
Orick, Calif, (cliffs; bay bars; beach)
Pt. Reyes, Calif, (high cliffs; rocky islets)
Suffolk and Smithfield, Va. (3 terrace levels)
Limerick, Hinesville, and Glennville, Ga. (3
terrace levels)
BEACHES AND BARS
Edgartown, Mass. (1 : 31,080) (bay bars; hook)
Eureka, Calif, (large bay bar; inlet)
Toms River, N J. (offshore bar; inlets)
Potrero Cortado, Tex. (broad, duned offshore
bar)
Index
Abrasion by glaciers, 66
Adiabatic cooling, 177
Advection, 145-146
temperature effects of, 145-146
Advection fog, 176-177
Africa, land-form pattern, 40-41
rift valleys, 45, 47
sand-dune areas, 97
Agassiz, Lake, 86-87
Agents of gradation, 5 1
Agonic line, 10
Air drainage, 149
relation to frost, 149
Air masses, 188-192
classification, 1 90- 1 92
distribution, 191
in middle latitude cyclones, 195-196
source regions, 189
Air photographs, 22-26
world coverage, 24
Alluvial fans, 58-59, 83-84, 118
Alluvial soils, 345, 355-356
Alluvium, 57
Aluminum, 384-385
American cordillera, ores in, 385
Animals in soil formation, 336
Annual surface runoff, 298
Annuals, 319
Antarctic Circle, 7
Anthracite, 362, 364, 365, 369
Anticyclone, cold, 159
middle-latitude, 192-202
origin, 194
temperature effects, 199-200
types, 194
wind system, 193, 197
subtropical, 158
thermal, 159
Anticyclonic circulation of subtropics, 165
Appalachian coal province, 365-366
Appalachian Highlands, 45, 49, 99, 101-105
Aquifer, 310, 314, 315
Arc of great circle, 3-4
Arctic Circle, 7
Arrangement as characteristic of land form, 30-33
Artesian wells, 315-316
395
396
Index
Atmosphere, capacity for water vapor, 172-173
circulation, 155-170
composition, 134-135
disturbances, 192-208
heating and cooling processes, 143-146
warming by heat of condensation, 145
zones, of convergence, 163
of divergence, 163
Atmospheric disturbances, middle-latitude, 192-202
tropical, 202-204
Atmospheric stability and instability, 178-179
Atolls, 128-130, 132
Australia, land-form pattern, 41
Azimuth, 10
Azimuthal map projection, 19
Azonal soils, 345, 346, 352
Badlands, 101
Barchans, 96
Bars, coastal, 125-127
offshore, 126-127
Base lines in survey system, 11-14
Baselevel, 57
Basin and Range province, 87, 117-118
Bauxite, 384
Bays, 122-124
Beaches, 125-127
Bed load of streams, 55-56
Bituminous coal, 362, 365, 369
Blowouts, 95
Bog soil, 351
Bonneville, Lake, 87
Braided channels, 58, 60, 79-80, 83
Brazil, iron deposits, 381
Brazilian Highlands, 39
Broadleaf forest of middle latitudes, 324-325
Buttes, 113-114
Calcite, 344
Calcium for fertilizer, 388
Calderas, 109, 111
Calendar time, 14-16
California, Central Valley, 83
Canada, 374
petroleum, 372-373
Canadian iron deposits, 379-380
Canadian Shield, ores, 385
Cancer, Tropic of, 7
Canyons, 99-100, 113-115
Capillary fringe, 307-308
Capricorn, Tropic of, 7
Caribbean petroleum field, 373-374
Cascade Range, 109-111
Caverns, 93-94
Channels (see Stream channels)
Chernozem soil, 353
Chernozemic-desertic soils, 345, 354-355
Chernozemic soils, 345, 353-354
Chromium, 384
Circle of illumination, 4
Cirques, 106-108
Classification, of climates, 209-212
of soils, 344-346
Clay, in soil, 339-340
for industry, 388-389
Claypan in soil, 344
Cliffs, coastal, 124-125
Climate, classification, 209-212
controls, 135
definition, 135
dry, 223-234
dry-summer subtropical, 236-242
effects on vegetation, 318-319
elements of, 135
highland, 272-276
humid continental, 255-265
humid subtropical, 242-247
icecap, 271-272
polar, 269-272
relation to soil occurrence, 347
in soil formation, 336
subarctic, 265-269
tropical humid, 212-222
tropical wet, 213-217
tundra, 270-271
Climatic groups, 210-212
Climatic regions, 210-212
Climatic types, 209-212
Climax vegetation, 318
Cloud cover, effects on air temperature, 145
Cloud forms, 180
Clouds, formation, 177-179
Coal, classes, 361-362
coking, 362
constituents, 361-362
mining, 362
origin, 361
world reserves, 362-363
Index 397
Coal regions, central and western Europe, 363, 367-
368
eastern and southern Asia, 370
North America, 364-367
South America, Africa, and Australia, 370
U.S.S.R., 368-370
Coastal features, 120-133
bays, 122-124
beaches and bars, 125-127
cliffs and terraces, 124-125
coral reefs, 127-128
harbors, 128-129
islands, 129-133
Coastal plains, 84-86
Coking coal, 362, 365, 368, 369
Cold front, 190
Colloids in soil, 339, 340
Color of soil, 343
Compass, declination of, 10
Compass bearing, 10
Condensation, 175-179
heat of, 172
origin, 175-176
Cones, volcanic, 47-51, 109-112, 117, 130, 132-133
Conformal map projection, 18
Coniferous forest, 325-329
in subarctic, 326
Conservation of soil, 357-358
Continental land-form patterns, 38-41, Plate 3
Continental shelf, 120-122
Continental slope, 120-121
Contour, 20-22
Contour interval, 21
Controls of climate, 135
Convergence, intertropical, 163
Coordinate system, 5-8
Copper, 385
Coral reefs, 127-130, 132
Cordilleran belts, 38, 51
of individual continents, 38-41
Craters, volcanic, 109
Creep, 62-64
Crust of earth, 2-3,43-51
deformation, 43-51
density, 3
disturbances of, effects, 48-49
world pattern of, 49-51
faulting, 44-49
folding, 44-49, 103
molten material in, 47-48
Crust of earth, movements, 44-47
nature, 2-3, 43-44
Cuestas, 77-78
Currents, ocean, 285-287
Cyclone, middle-latitude, 192-202
air masses, 195-196
appearance, 193
isotherms, 200
movement, 193-194
occlusion, 195-196
origin, 194
precipitation, 197-199
relation to jet stream, 168
structure, 194-196
temperature effects, 199-200
tracks, 201-202
weather associated with, 200-201
wind shift, 196-198
wind system, 196-197
tropical, 203-204
tracks, 202
Date line, international, 16
Deciduous plants, 319
Declination of compass, 10
Decomposition of rocks, 51-53
Deflection of winds, 160-161
Deltas, 58, 81-83
Dendritic stream pattern, 76-77, 100-101
Deposition, essential nature, 54-55
by glaciers, 67-69
by water, 56-58
by waves and currents, 71
by wind, 69-70
Deposits, of glaciers, 67-68, 88-92, 105-109
of streams, 57-58, 79-84, 91-92
of waves and currents, 71, 125-127
of wind, 69-70, 95-97
Desert climate, 223, 228-230, 233
Desert pavement, 95
Desert shrub, 332
Desert vegetation, 332
Desertic soils, 345, 355
Deserts, erosional plains, 78
Differential erosion, 54
Dike, intrusive, 49
Dimensions as characteristics of land form, 33
Direction on earth, 5-6, 9-10
Disintegration of rocks, 51-52
398
Index
Disturbances, atmospheric, 192-208
tropical, 202-204
Doldrums, 161-162
Domes, structural, 44, 49, 112
Drainage (see Surface water)
Drift, glacial, definition, 68
nature and deposition, 67-68
surface features, 88-92, 105
Drifts and currents, oceanic, 285-287
Drowned valleys, 123-124
Drumlins, 90-91
Dry climates, 223-234
cool marine variety, 229-230
definition, 223
location, 224
in low latitudes, 225-230
in middle latitudes, 230-234
precipitation, 224
soils, 234, 346-348
temperature, 224
vegetation, 234
winds, 224-225
Dry lands, erosional plains, 78
Dry realm, resource potentialities, 233-234
Dry-summer subtropical climate, 236-242
fronts, 238
location, 236-237
precipitation, 239-240
snow, 241
soils, 242
temperature, 237-239
vegetation, 242
weather, 240-241
Dry-summer subtropical realm, resource potentialities,
241-242
Dunes, 70, 95-97
Earth, area, 1,2
average elevation, 2
coordinate system, 5-8
density of crustal segments, 2-3
departure from sphericity, 1
directions on, 5-6, 9-10
distance from sun, 4
form, 1
internal structure, 2-3, 43
location on, 5-14
movements, 4-5
orbit, 5
Earth, polar flattening, 1
radius, 1
relation to celestial bodies, 4-5
revolution, 5
size, 1
surface materials, 2, 29-30
Earth flows, 61-63
Earth-sun relations, 137-139
Earthquakes, 46-47, 50-51
Eastern Hemisphere, iron deposits, 381-383
petroleum regions, 374-376
Ecliptic, plane of, 5
Effluent streams, 311, 312
Elements, of climate, 135
elements in soils, 337, 338
Eluviation in soil, 340
Equal-area projection, 18
Equatorial westerlies, 165, 167
Equidistant projection, 19
Equivalent map projection, 18
Erosion, differential or selective, 54
essential nature, 51, 54
by glaciers, 66-67
by running water, 55-57
of soil, 356-358
by water, 293
by waves and currents, 70-71
by wind, 69
Escarpments, of cuestas, 78
in tablelands, 113-114
Estuaries, 123-124
Eurasia, former continental glaciers, 65
land-form pattern, 39-40
sand-dune areas, 97
European petroleum fields, 376
Evaporation, effect, on ground water, 310, 311
on surface water, 296-301
Evaporation-condensation cycle, 172
Evapotranspiration, 300
definition, 279
general distribution, 280-281
Evergreen vegetation, 319
Extrusive vulcanism, 47-48
Fans, alluvial, 58-59, 83-84, 118
Fault-block mountains and valleys, 45-47, 101, 116-
118
Fault scarps, 45-47, 49, 101, 113
Faulting of crust, 44-49
Index 399
Ferroalloys, 383-384
Fertility in soils, 337-339
Fiords, 124
Fish, 289-291
Fisheries, 290-291
Flat polar quartic equal-area projection, 19
Floodplains, 57-58, 79-81, 91
Fog, 176-177
Folding of crust, 44-46, 48-49, 103
Forests, climate, 321
coniferous, 325-329
lighter tropical, 322
middle-latitude, 323-329
temperate broadleaf, 324-325
tropical, 321-323
types, 321-329
France, iron deposits, 382
Freezing of water as rock-breaking agent, 52
Front, 182, 188-190
cold, 190
occluded, 196
warm, 190
Frost, 149-151
in dry-summer subtropical climate, 238
in humid subtropical climate, 244-245
in low-latitude dry climates, 228
in marine climate, 249-250
Fuels, fossil, 360-376
mineral, 360-376
Gas (see Petroleum and natural gas)
General circulation of atmosphere, 166-168
Glacial drift (see Drift)
Glacial landforms, cirques, 106-108
drumlins, 90-91
fiords, 124
glacially modified plains, 88-92
glaciated valleys, 66-67, 105-109
lakes, 88-92
moraines, 67-68, 88-91, 106-109
in mountains, 104-109
outwash surfaces, 91-92
Glaciated areas, lakes in, 303
Glaciation, in humid continental realm, 265
Pleistocene, 65-66, 68, 88, 104, 122, 125
effects on sea level, 122, 125
Glaciers, 64-68, 88, 104-109
continental (Pleistocene), 65-66, 88-93, 105, 122
deposition by, 67-68, 88-92, 105-109
Glaciers, erosion by, 66-67, 92, 105-109
mountain valley, 104-106
effects, 105-109
movement, 64
nature and development, 64
transportation by, 67
Globes, 17
Gradational agents, 51
Gradational processes, 43, 51-71
Gradient of valleys, 57
Granite, weathering, 53-54
Graphite, 362
Grasslands, 329-332
climate, 321
middle-latitude, 331-332
tropical, 329-331
Gravity as gradational agent, 51, 58-64
Great Britain, iron deposits, 383
Great circle, 3, 9
Great Salt Lake, 87
Greenhouse effect of atmosphere, 144
Greenwich time, 14
Ground water, 307-316
discharge and recharge, 310-312
hardness, 313
hydraulic gradient, 309
in hydrologic cycle, 278-280
minerals in, 313
movement, 308, 309
occurrence, 309-310
pollution, 314
as resource, 312-316
solution by, 92-95
springs, 312, 313
zones, 307-308
Ground-water table (see Water table)
Growing season (see Frost)
Guiana Highlands, 39
Gulf coast petroleum field, 372-373
Hail, 181, 206
Harbors, 128-129
Hardness of water, 313
Hardpan in soil, 344
Hawaiian Islands, 48, 109-110, 130
Heat of condensation, 145, 172
Heating and cooling processes, conduction, 143-144
earth radiation, 144-145
Hematite, 378
400
Index
High Plains, 73-74, 79, 83-84
Highland climates, 272-276
Hills, 34-35, 37-38, 98-112
characteristic features, 99-104
definition, 34, 38
distribution, 35, 38-41, Plate 3
effects of structure in, 101-104
glaciated, 105
origin and development, 98-99
stream-eroded, 99-104
Hogback ridges, 106
Horizon in soils, 335, 336, 343, 345
Horse latitudes, 162, 165
Humid climates, soils, 346-348
Humid continental climate, 255-265
glaciation, 265
location, 255-256
precipitation, 256-258
snow cover, 257
soils, 265, 347, 349-351
subdivisions, 260-263
temperature, 256
vegetation, 264-265
weather types, 258-260
Humid continental realm, resource potentialities,
264-265
Humid mesothermal climates (see Mesothermal cli-
mates)
Humid microthermal climates (see Microthermal cli-
mates)
Humid subtropical climate, 242-247
frost, 244-245
location, 243
precipitation, 245-246
soils, 247
temperature, 243-245
vegetation, 247
weather, 246-247
Humid subtropical realm, resource potentialities, 247
Humidity (see Water vapor)
Humus, 341
Hurricane, 203-204
Hydraulic gradient, 308-309
Hydrologic cycle, 278-281, 295-296, 308
Hydrosphere, 277-278
Hygrophytes, 319
Icecap climate, 271-272
Icecaps, distribution, 35, Plate 3
Ice sheets, continental, 65-66, 88-93, 105, 122
Illuviadon, 340
Inclination of earth, 5
India, iron deposits, 383
Industrial Revolution, 293, 377
Influent streams, 311, 312
Insolation (see Solar energy)
Instability, 178-179
Intermittent streams, 298
International date line, 16
Intertropical convergence, 161-165
Intrazonal soils, 345, 346, 352
Intrusive vulcanism, 47-48
Inversions of temperature, 148-149
Iron deposits, 378-383
Brazil, 381
Canada, 379-380
Eastern Hemisphere, 381-383
France, 382
Great Britain, 383
India, 383
Lake Superior, 379, 380
Latin America, 379-381
Lorraine, 382
Sweden, 383
U.S.S.R., 383
Venezuela, 380-381
Western Europe, 382-383
Western Hemisphere, 379-381
Isarithm, 20-22
Isarithmic interval, 21
Island arcs, 132-133
Islands, 129-133
Isobaric charts, 158-159
Isobars, 157
Isogonic lines, 10
Isoline, 20
Isopleth, 20
Isostasy, 3
Isothermal charts, 151-153
Isotherms, 151
Jet stream, 168
Joints in rocks, 52, 66, 77, 93, 103
Katanga, 385, 386
Kettle Moraine, 90
Lagoons. 127-128
Index 401
Lake plains, 84-87
Lake Superior iron deposits, 379, 380
Lakes, 301-304
effect on runoff, 303
formerly existing, 84-87
on glaciated plains, 88-92
man-made, 304
occurrence, 303
on solution-marked plains, 94
Land form, in soil formation, 336
world pattern, 34-35, 38-41, Plate 3
(See also Land-surface form)
Land-surface form, 28-133
characteristics, 28-33
development, 42-71
varieties, 28-41
Land surfaces, characteristics, 28-33
types, 33-38
distribution, 34-35, 38-41, Plate 3
Landslides, 61-63
Lapse rate, 177
Latent energy, 172
Laterite, 348, 351
Latin American iron deposits, 379-381
Latitude, 5-7
determination, 8
lengths of degrees, 6-7
Latosolic soils, 345, 351-352
Lava, 47-49, 109-112, 114
Leaching, in ore-deposit formation, 378
in soils, 338, 342
Leeward, 160
Levees, natural, 79-82
Lightning, 206-207
Lignite, 361-362
Lime, accumulation in soil, 343, 348, 354
industrial, 388
Limestone, solution features, 93-95
weathering, 53-54
Limonite, 378
Lithosphere, 277-278
Loam, 339-340
Local relief as characteristic of land form,
33
Loess, definition, 70
surface features, 96-97
Longitude, 5-8
determination, 8
lengths of degrees, 7-8
Lorraine iron deposits, 382
Los Angeles water supply, 305
Low-latitude dry climates, 225-230
frost, 228
location, 235-236
precipitation, 226-227
temperature, 227-228
weather, 228-229
Magnetic poles, 10
Magnetite, 378
Manganese, 384
Map Information Section of U.S. Geological Survey,
25
Map projections, 17-19
Map scale, 16-17
Maps, topographic, 22-26
varieties, 19-20
March of temperature, daily and seasonal, 146-147
Margins of lands, 120-133
Marine climate, 248-252
frost, 249-250
location, 248
precipitation, 250-251
soils, 252
temperature, 248-250
vegetation, 252
Marine climatic realm, resource potentialities, 251-
252
Marshes (see Swamps)
Mass movement, 59-64
causes, 59-60
importance and results, 62-64
kinds, 60-62
Mass wasting (see Mass movement)
Mature erosion surfaces, 73-75
Mean solar time, 14
Meandering channels, 58, 60, 79-80
Mechanical breaking of rocks, 51-52
Mediterranean climate (see Dry-summer subtropical
climate)
Mediterranean woodland, 323-324
Mercator projection, 1 9
Meridians, 6
Mesas, 113-114
Mesothermal climates, 235-252
location, 235-236
soils, 347
Metallic minerals, 376-386
Meter length, 7
402
Index
Metes and bounds, 7, 8
Microorganisms in soil, 341
Microthermal climates, 253-269
location, 253-254
precipitation, 254-255
soils, 347
temperature, 254
effects of snow cover on, 254
Midcontinent petroleum field, 372-373
Middle East petroleum fields, 374-376
Middle-latitude dry climates, 230-234
location, 230-231
precipitation, 231-233
temperature, 231
weather, 233
Middle-latitude forests, 323-329
Middle West, American, structure, 44-45
surface features, 74-75, 88-92
Minerals, coal, 361-370
as components of rocks, 53-54
ferroalloys, 383-384
for fertilizers, 387-388
as fuels, 360-376
iron, 378-383
natural gas, 370-376
nonferrous, 384-386
nonmetallic, 386-389
petroleum, 370-376
resistance to weathering, 53-54
as resources, 359-360
salt, 387
sulphur, 387
Mississippi River, delta, 46, 81, 84
floodplain, 80-81
Moisture regions, 186
Molten material in crust, 47-48
Monadnocks, 75-76, 116
Monsoons, 162-163, 169-170
Moraines, 67-68, 88-91, 106-109
marginal, 89-90
of mountain- valley glaciers, 105-109
Mountains, 34-35, 37-38, 98-112
alpine, 108-109
characteristic features, 99-112
definition, 34, 38
distribution, 35, 38-41, Plate 3
effects of structure in, 101-104
fault-block, 45-47, 101, 117-118
glacial features, 104-109
origin and development, 98
Mountains, slopes, peaks, and ridges, 100-101
soils, 355
stream-eroded, 99-104
volcanic, 109-112, 117
Natural gas (see Petroleum and natural gas)
Natural levees, 79-82
Natural vegetation (see Vegetation)
Necks, volcanic, 112
Needleleaf forest, 325-329
Netherlands, 83-84
New York City water supply, 305-306
New Zealand land forms, 41
Nickel, 384
Nitrogen in soil, 341
Nitrogen sources, 388
Nonferrous metals, 384-386
North America, former continental glaciers, 65
land-form pattern, 38-39
Occlusion, 195-196
Ocean drifts and currents, 285-287
Ocean temperatures, 287-289
Offshore bars, 126-127
Oil (see Petroleum)
Oil shale, 371, 376
Old-age erosion surfaces, 75-76, 115-116
Ores of metals, 377-378
aluminum, 384-385
concentration, 377
copper, 385
ferroalloys, 383-384
iron, 378-383
nonferrous, 384-386
regions of occurrence, 385-386
Organic matter in soils, 340
Outliers, 113-114
Outwash, glacial, nature and deposition, 68-69
surface features, 91-92
Pamir knot, 35, 38, 40
Pan layers in soil, 343-344
Parallelism of earth's axis, 5
Parallels, 5
Parent material, 335
Patagonian plateau, 39
Pattern as characteristic of land form, 30-31
Index 403
Peat, 361-362
Peneplain, 75
Perennials, 319
Permanent streams, 298
Permeability, of regolith, 310
of soil, 310
Petroleum and natural gas, structural associations,
371-372
world reserves, 371
Petroleum regions, California, 372-373
Canada, 374
Caribbean, 373-374
Eastern Hemisphere, 374-376
European, 376
Gulf Coast, 372-373
midcontinent, 372-373
Middle East, 374-376
U.S.S.R., 374-376
Western hemisphere, 372-374
Phosphate rock, 388
Phosphorus sources, 388
Photographs, air, 22-26
Piedmont alluvial plains, 83-85
Placer ore deposits, 378
Plains, 34-36, 72-97
alluvial, 57-58, 79-84
piedmont, 83-85
coastal, 84-86
definition, 34, 38, 72
distribution, 35, 38-41, Plate 3
with features of considerable relief, 34, 112-118
glacially modified, 88-92
glaciated, with little drift, 92
with hills or mountains, 34-36, 38, 112, 115-118
definition, 34, 38
distribution, 35, 38-41, Plate 3
erosional varieties, 116
origin, 112, 115-118
tectonic varieties, 116-118
lake bottom, 84-87
origin, 72-73
outwash, 91-92
with solution features, 92-95
stream-eroded, 73-78
till, 88-91
water-laid, 79-87
wind-shaped, 95-97
Plane of ecliptic, 5
Plankton, 289-291
Planosol soil, 351
Plant formations, 318-319
(See also Vegetation)
Plants in soil formation, 336
Plateaus (see Tablelands) .
Pleistocene glacial period, 65-66, 68, 88, 104, 122,
125
Plucking by glaciers, 66
Podzol soils, 345, 349-350
Podzolic-latosolic soils, 345, 352-353
Podzolic soils, 345, 350-351, 353
Polar climates, 269-272
Polar front, 166
Polar highs, 158
Poles, geographical, 1, 10
magnetic, 10
Pollution, of ground water, 314
of surface water, 305
Pore space in soils, 340
Porosity of regolith, 310
Potash sources, 388
Prairie, 331-332
Prairie soil, 353
Precipitation, 171-187
amounts, 183
characteristics, 183-184
of convective origin, 181-182
in cyclones, 183
distribution, 184-187
in dry climates, 224
in dry-summer subtropical climate, 239-240
effect, on ground water, 310
on surface water, 296-300
forms, 179-181
frontal, 182-183
in highland climates, 275-276
in humid continental climate, 256-258
in humid subtropical climate, 245-246
in low-latitude dry climates, 226-227
in marine climate, 250-251
in microthermal climates, 254-255
in middle-latitude cyclone, 197-199
in middle-latitude dry climates, 231-233
origin, 181-182
orographic, 181
seasonal, 186-187
in subarctic climate, 267-268
in thunderstorm, 206
in tropical cyclone, 204
in tropical wet climate, 214-216
variability, 183
404
Index
Precipitation, in weak tropical disturbances, 203
Pressure, 155-161
distribution, horizontal, 157-159
seasonal, 158-159
vertical, 157
in highland climates, 275-276
importance, 155-156
measurement, 156
origin of differences, 156-157
relation to winds, 159-160
Pressure gradient, 157, 159-160
Prime meridian, 7
Principal meridians, 11-14
Profile, as characteristic of land form, 31-32
in soil, 335, 343
Projection, azimuthal, 19
conformal, 18
equal-area, 18
flat polar quartic, 1 9
equidistant, 19
- Mercator, 19
Properties of map projections, 17-19
Quarrying by glaciers, 66
Quartzite, resistance, 54
Radius of earth, 1
Rain, 179
Rainfall (see Precipitation)
Rainforest, tropical, 321-322
Ranges in survey system, 11-14
Reactivity of soils, 339
Rectangular survey system, 11-14
Red Sea, nature of basin, 45, 47, 122
salinity, 282
Reefs, coral, 127-130, 132
Regolith, 334, 335
permeability, 310
porosity, 310
Relative humidity, 173-175
Relief, factors affecting, 72-73, 98-99
local, 33
Representative fraction (RF), 16
Resistance of rocks, 53-54, 77-78
Revolution of earth, 5
Rift valleys of Africa, 45, 47
Rock salt, 387
Rock structure, definition and origin, 48-49
effects on gradation, 77-78, 92, 100-104, 112, 113
Rocks, breakdown, 51-54
characteristics and types, 53-54
petroleum in, 371, 372
resistance, 53-54, 77-78
Rocky Mountains, 45, 65, 103-106, 109, 111-112
glaciation, 65, 104-105
Roots, effectiveness in rock breaking, 52
Rotation of earth, 4-5
Rurioff (see Surface water)
Running water, deposition by, 56-58
erosion by, 55-57
as gradational agent, 54-58
hills and mountains shaped by, 99-104
plains shaped by, 72-84
transportation by, 55-56
(See also Streams)
Salinity of sea water, 281-282
Salt, 387
Sand, for industry, 388-389
in soil, 339-340
Sand dunes, 70, 95-97
Sandstone, weathering, 54
Savanna, 329-331
Savanna climate (see Tropical wet-and-dry climate)
Scale on maps, 16-17
Scarps, fault, 45-47, 49, 101, 113
Scottish Highlands, 49
Sculpturing processes, 51-71
Sea breeze, 170
Sea cliffs, 124-125
Sea-level changes, 121-125
Sea water, composition and salinity, 281-282
density, 282
surface temperatures, 287-289
Seas, 281-291
coastal features, 120-133
drifts and currents, 285-287
islands, 129-133
life forms, 289-291
surface salinity, 281-282
surface temperature, 287-289
tides, 282-285
waves, 70-71, 282
Sections in survey system, 12
Sedimentation, forma don of ore deposits by, 377-378
Seifs, 96
Index 405
Sensible temperature, 153-154
in tropical wet climate, 214
Sheet wash, 298
of soil, 357
Shelf, continental, 120-122
Shore lines, 121-124
(See also Coastal features)
Siderite, 378
Sierra Nevada Range, California, 45-46, 101
Silt in soil, 339-340
Sinkholes, 94-95
Sleet, 181
Slope, as characteristic of land form, 29
continental, 120-121
Slopes, gentle, origin, 73
steep, origin, 99
Small circle, 4
Snow, 179
in dry-summer subtropical climate, 241
Snow cover, 181
effects on atmospheric temperatures, 145
in humid continental climate, 257
in microthermal climates, 254
Soil creep, 62-64
Soils, alluvial, 345, 355-356
bog, 351
chemical elements, 337
chernozemic, 345, 353-354
chernozemic-desertic, 345, 354-355
classification, 344-346
colloids, 339, 340
colors, 343
components, 337-344
conservation, 357-358
desertic, 345, 355 /
in dry climates, 234, 346-348
in dry-summer subtropical climate, 242
effects on vegetation, 319-320 /
eluviation, 340 S
erosion, 356-358 */
fertility, 337-339 y
formation, 335-337 /
horizons, 335, 336, 343, 345
in humid climates, 346-348
in humid continental climate, 265
in humid subtropical climate, 247
humus, 341
illuviation, 340
latosolic, 345, 351-352
leaching, 338, 342
Soils, in marine climate, 252
maturity, 337
microorganisms, 341
organic components, 340
pan layers, 343-344
phases, 345, 346
planosol, 351
podzol, 345, 349-350
podzolic, 345, 350-351, 353
podzolic-latosolic, 345, 352-353
pore space, 340
prairie, 353
profile, 335, 343, 345
reaction, 338, 348
reactivity, 339
series, 345-346
sheet wash, 357
significance, 335
specific surface, 339
structure, 340
in subarctic climate, 269
texture, 339-340
in tropical climate, 217
in tropical wet-and-dry climate, 221-222
tundra, 345, 349-350
water, 341-343
wind erosion, 357
Solar energy, 136-146
absorption by atmosphere, 143
distribution, 136-139, 140-142
factors determining, 136-137
effects of atmosphere on, 139-140
gradients, 142
heating earth's surface by, 142
in highland climates, 272-273
land and water reactions to, 142-143
nature, 136
Solar radiation (see Solar energy)
Solar time, 5, 14
Solifluction, 61-62
Solution, in land-form development, 55, 92-95
by water, 293
Source region for air masses, 189
South America, land-form pattern, 39
Specific humidity, 173
Specific surface of soils, 339
Spits, 126-127
Springs, 312
Stability, atmospheric, 178
Standard time, 14-15
406
Index
Steppe climate, 223
Stream channels, braided, 58, 60, 79-80, 83
gradient, 57
meandering, 58, 60, 79-80
pattern, 30, 58, 60, 76-77, 79-80
Stream-eroded plains, 73-78
Stream flow, 298-299
Streams, channels, 57-58, 60, 79-80, 83
deltas, 58, 81-83
deposition by, 56-58
deposits, 57-58, 79-84, 91-92
erosion by, 55-57
gradient, 57
intermittent, 298
pattern, 30-31, 58, 60, 79-80, 83
permanent, 298
(See also Running water)
Structure, of earth, 2-3, 43
geological (see. Rock structure)
in soils, 340
Subarctic climate, 265-269
soils, 269
vegetation, 269
Subarctic forest, 326
Subarctic realm, resource potentialities, 208-209
Submergence, coastal, 122-125
Subpolar lows, 158
Subsidence, atmospheric, 148-149, 163
relation to temperature inversions, 148-149
Sulphur, 387
Surface of discontinuity, 189
Surface material, as characteristic of land form, 29-30
moving, 54-55
Surface water, 295-306
in hydrologic cycle, 295-296
impurities, 305
as resource, 304-306
runoff, 297-304
climatic factors, 299
computation, 297
by continents, 301
effects of land surface on, 300
by latitude, 300
regime, 299
stream flow, 298-299
of United States, 298
water bodies in, 301-304
of world, 299-302
storage, 295-297
Survey, rectangular system, 11-14
Suspended load of streams, 55-56
Swamps, 301-304
on floodplains and deltas, 79-84
on glaciated plains, 88-89, 91
on lake and coastal plains, 85-86
Sweden, iron deposits, 383
Tablelands, 34-36, 38, 112-115
characteristic features, 113-114
definition, 34, 38
distribution, 35, 38-41, Plate 3
escarpments, 113-114
occurrence, 114-115
origin, 112-113
uplands, 113
Taconite, 379
Taiga, 326
Talus slopes, 63, 100, 102
Tar sands, 371, 376
Tectonic processes, 43-5 1
Temperature, 134-154
advection, 145-146
annual range, 153
as climatic control, 136
as climatic element, 136
daily and seasonal march, 146-147
distribution, 151-154
geographical, 147-148
temporal, 146-147
in dry climates, 224
in dry-summer subtropical climate, 237-239
effects on, of cloud cover, 145
of humid atmosphere, 145
in highland climates, 273-275
in humid continental climate, 256
in humid subtropical climate, 243-245
January and July, 152-153
in low-latitude dry climates, 227-228
in marine climate, 248-250
in microthermal climates, 254
in middle-latitude anticyclone, 199-200
in middle-latitude cyclone, 199-200
in middle-latitude dry climates, 234
sea surface, 287-289
effects on climate, 288-289
sensible, 153-154
in subarctic climate, 265-267
in tropical wet-and-dry climate, 218-219
in tropical wet climate, 213-214
vertical transfer, 145
Index 407
Temperature changes, effectiveness in rock break-
ing, 52
Temperature gradient, 142, 153
Temperature inversions, 148-149
Temperature range in tropical wet climate, 213-214
Temperature zones in highland climates, 274-275
Tennessee River drainage basin, 304
Terraces, alluvial, 81
coastal or marine, 124-125
Terrain types (see Land surfaces, types)
Texture, as characteristic of land form, 32-33
of soils, 339-340
Thermal anticyclones, 159
Thorium, 376
Thunder, 206-207
Thundersquall, 207
Thunderstorms, 204-208
distribution, 207-208
origin, 204-206
precipitation, 206
Tidal currents, 285
Tides, 282-285
causes, 282-284
occurrence, 282-285
Till, glacial, nature and deposition, 67-68
surface features, 88-91
Till plains, 88-91
Time, calendar, 14-16
geologic, 42-43, 49
Greenwich, 14
in soil formation, 336
standard, 14-15
Topographic maps, 22-26
world coverage, 23
Tornado, 207
Townships in survey system, 11-14
Trade winds, 161-164
atmospheric disturbances in, 164
weather in, 164
Transportation of earth materials, 54-56, 67, 69, 71
by glaciers, 67
by running water, 55-56
by waves and currents, 71
by wind, 69
Tributary valleys, development, 73-74, 113
pattern, 76-77
Tropical cyclone, 203-204
Tropical disturbances, 192-203
Tropical easterlies, 161-164
Tropical forests, 321-323
utilization, 322-323
Tropical grasslands, 329-331
Tropical humid climates, 212-222
upland variety, 222
Tropical rainforest, 321-322
Tropical-rainforest climate (see Tropical wet climate)
Tropical wet climate, 213-217
location, 213
precipitation, 214-216
soils, 217, 347, 348, 351-352
temperature, 213-214
vegetation, 217
weather, 214, 216
Tropical wet-and-dry climate, 217-222
location, 218
precipitation, 219-221
soils, 221-222, 347, 348
temperature, 218-219
vegetation, 221
weather, 220
Tropical wet-and-dry realm, resource potentialities,
221-222
Tropical wet realm, resource potentialities, 216-217
Tropics, weather, 202-203
Tundra climate, 270-271
Tundra soils, 345, 349-350
Tundra vegetation, 332-333
Typhoon (hurricane), 203-204
U.S.S.R., coal deposits, 368-370
iron deposits, 383
petroleum fields, 374-376
United States, soil erosion, 357
soils, 346
Uranium, 376
Valleys, drowned, 123-124
fault-block, 45-47, 116-118
glaciated, 66-67, 105-109
stream-eroded, development, 56-57, 74-76, 99-100
form and size, 74-76, 99-100, 113-114
gradient, 57
pattern, 76-78
Variability of precipitation, 183-184
Vegetation, 317-333
annual types, 319
causes of regional variations, 318-320
classification, 320-321
408
Index
Vegetation, deciduous, 319
desert, 332
in dry climates, 234
in dry-summer subtropical climate, 242
effects, of climate, 318-319
of organisms, 320
of soils, 319-320
evergreen, 319
formations, 318
in humid continental climate, 264-265
in humid subtropical climate, 247
landscape qualities, 317-318
perennial types, 319
in soil formation, 336, 347, 349, 350, 352-355
in subarctic climate, 269
in tropical wet climate, 217
in tropical wet-and-dry climate, 221
tundra, 332-333
Vegetation climax, 318
Venezuela, iron deposits, 380-381
Vesuvius, 47, 109
Volcanic activity, 47-48
Volcanic ash, 48-49, 109-110
Volcanic cones, 47-51, 109-112, 117, 130, 132-133
Volcanic necks, 112
Volcanoes, 47-51, 109-112, 117, 130, 132-133
Vulcanism, 47-51, 109-112
extrusive, 47-48
intrusive, 47-49
Warm front, 190
Water, capillary, 342
cyclical behavior, 278-281
dissolving power, 293
erosion by, 293
general occurrence, 277-281
gravitational, 342-343
ground (see Ground water)
hygroscopic, 342
importance to rock weathering, 52-53
need for, 293-295
nonwithdrawal, 294, 303
properties, 292
as resource, 293
runoff (see Surface water)
of seas, 281.-282
in soils, 341-343
surface (see Surface water)
uses, 293-295
Water, withdrawal, 294, 305
Water balance of earth, 296-297
Water-laid plains, 79-87
Water supply of cities, 304-305
treatment, 305
Water table, 307, 308
cone of depression, 315
configuration, 308
effect of wells on, 314
fluctuations, 309, 315
in soil, 343
Water vapor, 171-175
absorption by, of earth radiation, 144-145
of solar energy, 139-140, 142
capacity of air for, 172-173
effects on climate, 171
functions, 134-135
latent energy, 1 72
sources, 172
Waves, 70-71, 282
and currents as gradational agents, 70-71
Weather, definition, 135
in dry-summer subtropical climate, 240-241
in highland climates, 276
in humid continental climate, 258-260
in humid subtropical climate, 246-247
in low-latitude dry climates, 228-229
in tropical latitudes, 202-203
in tropical wet-and-dry climate, 220
Weathering, chemical, 51-53
formation of ore deposits by, 377-378
resistance to, 53-54
selective, 53-54
Wells, petroleum, 371-372
water, 314-316
Westerlies, equatorial, 165
middle-latitude, 161-162, 165-166
Western European iron deposits, 382-383
Western Hemisphere, iron deposits, 379-381
petroleum regions, 373
Whales, 290-291
Winds, anticyclonic, 161
as climatic control, 155-156
as climatic element, 155-156
convergence, 163
converging system, 161
cyclonic, 161
deflection by earth rotation, 160-161
deposition by, 69-70
divergence, 163
Index 409
Winds, diverging system, 161
.doldrums, 161-162
in dry climates, 224-225
of earth's surface, 161
erosion by, 69, 357
general circulation, 166-168
as gradational agents, 69-70, 95-97
in ITC, 164-165
in January and July, 162-163
latitudinal shifting, 169
in middle-latitude anticyclone, 193, 197
in middle-latitude cyclone, 193, 195-197
middle-latitude westerly, 161-162
of middle latitudes, 165-166
monsoons, 162-163, 169-170
polar, 166
prevailing, 161
relation to pressure, 159-160
seasonal, 162-163
of subtropics, 165
Winds, surface features produced by, 95-97
terrestrial modifications, 168-170
trade, 161-164
in tropical cyclone, 204
tropical easterlies, 161-164
in tropics, 163-165
zonal pattern, 161-162
Windward, 160
Xerophytes, 319
Youthful erosion surfaces, 73-75, 113
Zonal soils, 345, 346
Zone, of aeration, 307-308
of soil water, 307-308
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