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OCCASIONAL PAPERS

OF THE

CALIFORNIA ACADEMY OF SCIENCES

No. 31, 24 pages, 6 tables. December 31, 1961

CYCLES AND GEOCHRONOLOGY*

Henry P. Hansen

Oregon State University

The phenomenon of the cyclic nature of the universe and its impact upon
the earth's inhabitants is practically inescapable. In fact much of our culture
has developed and evolved in response to the vast number of cycles that exist
in our environment. A cycle constitutes a sequence of events which progres-
ses until it attains the place or time where it began, but it need not exhibit
rhythmicity or periodicity. Many cycles do have rhythmicity and the annual
and diurnal cycles are perhaps the most important and significant in influ-
encing living systems. Variation in the seasonal photo-period is an excel-
lent example of a cycle that has a pronounced effect upon the reproductive
cycles of many plants and animals. There is evidence that sunspot cycles
have had strong influence in controlling not only biological periodicity in
various activities of organisms, but even social and economic trends. Then
there are the astronomic and cosmic cycles which involve the universe itself
and may be measured in terms of millions and even billions of years, caused
by the movements and relative positions of the components of the solar system
and other bodies of the universe. The direct cause of a rhythmic cycle may
be obscured because of the complexity of the ecological system of which it
is a part. There has been a well pronounced rhythm of 9.6 years in the abun-
dance of the lynx in Canada for 224 years, and in the abundance of rabbits,
tularemia, and ticks, all of which may be part of the ecological system of

•Presidential address presented at the 41st annual meeting of the Pacific Division;
AAAS, University of Oregon, Eugene; June 15, 1960.

2 CALIFORNIA ACADEMY OF SCIENCES (Occ. Papers

the lynx. A 9.6 year rhythm for tent caterpillars in New Jersey, Atlantic
Salmon in Canada, human heart disease in northeastern United States, and
the acreage planted to wheat in the United States, however, probably does
not relate to an ecological system in which these four components are invol-
ved. There are many economic cycles such as pig iron prices, cigarette pro-
duction, cotton prices, and business failures that are evident, but for which
there is no explanation at present. Dendrochronology has demonstrated a
close correlation between the annual ring growth in trees and sunspot cycles.
Dating of successive moraines by retreating Alaska glaciers in the past 200
years shows a close correlation with the 11-year sunspot cycle. In southwest
Africa, pre-Cambrian varves 500 million to 1 billion years old show a cyclic
rhythm of 11.5 years.

It is not my intention to discuss the causes of cycles but to review the
interpretations of some of the records left by plants and animals and their
chronological correlation with geological events and climatic trends. Only
a few of these events are recorded, and their chronology in most cases can
be only general and approximate. Man has always been interested in trying
to interpret prehistoric events and conditions and to correlate the paleoen-
vironment with the paleobiota. The sciences of paleontology, climatology,
paleoecology, paleogeography, archeology, geology, geobotany, geochemistry,
and palynology, are some of the tools which have helped him obtain a picture
of the past.

In the several billions of years of the earth's existence, there is evi-
dence of innumerable cycles. As one goes back in the earth's history, how-
ever, the magnitude and generalities of the cycles increase and become less
well defined because the record becomes more sparse and sporadic and more
difficult to interpret. In addition to the fossil record of plants and animals,
various earth processes such as diastrophism, volcanism, erosion, deposition,
weathering, and glaciation provide evidence for cycles and chronology. One
of the most interesting and intensely interpreted phases of past environments
is that of climate. Paleoclimate is recorded and reflected in a number of
ways by the fossil record and by geological processes, which in their inter-
relations may provide a very complex pattern which is not always easy to
decipher. There is evidence in the records that climate has followed a cyclic
pattern, and that these cycles have been of varying periods with the shorter
superimposed upon the longer ones. Climate in itself is an expression of the
conditions and characteristics of the atmosphere which are evanescent. It is
the sum total of the weather over a period of time, either long or short. The
atmospheric conditions of yesterday do not leave their record for long and in
many cases not at all. Many earth processes are directly or indirectly con-
trolled by climate which leaves its imprint physically in and on features of
the earth. A strong wind may leave its record in fossil wave ripples on a
sandy beach or playa lake, a heavy downpour may be recorded by a deposition

No. 3D HANSEN: CYCLES AND GEOCHRONOLOGY 3

of sediments, and a melting glacier may record its recession by moraines or
the lamination of sediments in a nearby glacial lake. Changes in plant and
animal populations representing biotic succession and migration of the past
are also indicators of climatic trends and fluctuations.

One of the most interesting and significant interpretations of life, geo-
logical processes, and events of the past is that of chronology. While the
evidence and records may be readily accessible, dating of their existence
and happenings is not always possible. The relative stratigraphic positions
of fossils indicate their time of existence in relation to one another, but not
the absolute dates. Estimates have been made with some degree of accuracy,
however, particularly of more recent events. They have been based upon
observed earth processes and applying the chronology to similar processes
of the past as evidenced by the strata and the stratigraphic position and re-
lationships. These include rate of delta building, retreat of earth features
by erosion, stream dissection, weathering, soil development, and deposition
of sediments including varved clays, peat, and other organic materials. Ab-
normal strata, whose occurrence indicates an interruption by some external
environmental change, also serve as chronological markers. These include
such strata as volcanic materials, soil horizons, forest beds, oxidized peat,
caliche, woody layers in peat, fire horizons, and others. These are especial-
ly valuable if their occurrence is fairly consistent and regional.

During the past ten years, the development of geochemical techniques
has provided the means of fairly accurately dating materials of great age. The
thousands of dates which have been obtained by geochemical means have
enabled the chronologist to attach absolute dates to prehistoric materials and
to construct a time table for many of the major events of the past million years.

The most significant and momentous geologic event of the Pleistocene
was glaciation. Curing the earth's history there have been at least four periods
when ice sheets formed and spread out from centers of accumulation, during
which time the climate was probably cooler than at present. These glacial
periods have been of comparatively short duration, however, and most of the
time the earth has had a genial climate favorable for the existence and evolu-
tion of life. Previous to the Pleistocene, there is evidence that glaciation
occurred during the late Proterozoic, the Carboniferous, and the Permian.

During the Pleistocene or "ice age" there were four or five major gla-
ciations covering a period estimated from 300,000 to 1,000,000 years. There
were at least four substages of the last glaciation known as the Wisconsin,
and it is probable that each major glaciation had a number of substages or
minor advances and retreats. Dating of deep-sea cores by geochemical tech-
niques suggest that the Riss-Illinoian glaciation ranges from 100,000 to
125,000 years ago, the Mindel-Kansan from 165,000 to 200,000 years ago,
and the Gunz-Nebraskan glaciation from 265,000 to 290,000 years ago. Radio-
carbon dating indicates that there were major glacial stages around 60,000

4 CALIFORNIA ACADEMY OF SCIENCES (Occ. Papers

years ago and 20,000 years ago, with substages about 13,500, 11,000 and
7,000 years ago (table 1). These are probably all stages of the late Wisconsin
glaciation, and the latter substages will be discussed later.

Because of the important part that radiocarbon dating has played in
developing a late glacial and postglacial chronology it seems pertinent to
present a brief discussion of the method. Radioactive carbon (C14) is formed
by cosmic rays bombarding nitrogen atoms in the earth's atmosphere. It emits
beta rays and disintegrates to nitrogen. Carbon14 has a half life of about
5,570 years, and by measuringthe amountofC14 in a substance, it is possible
to calculate the time elapsed since the active carbon was formed. Carbon
dioxide of the atmosphere, soil, and water contains a minute fraction of C14
and is absorbed by plants and synthesized in their tissues. Animals eat
plant material so they also contain C14. Living organisms maintain an equili-
brium between the rate of formation and the rate of decay, but upon death and
cessation of metabolism, radioactive disintegration takes place and the total
amount of C'4 is reduced with time. The amount of C14 remaining indicates
the amount of time elapsed since death of the organism. A maximum of 50,000
years can be dated with assurance of reasonable accuracy, but the possible
error increases with age. Artifacts of known age up to 5,000 years have been
radiocarbon dated, and the dates are reliable, while dates for prehistoric
materials show a consistence to warrant confidence in the method. In addition
to the source of laboratory errors, the interchange of C14 between organisms
and the environment obviously results in the re-use of older carbon as well
as dilution with ancient dead carbon. Percolation of ground water containing
young carbon may result in its absorption by old carbonaceous material, thus
presenting a younger date than is actually the case. A logical consistency
in an ever-increasing number of dates of many different materials in many
different situations vouches for the reliability and validity of the method.
Peat, wood charcoal, shells, and bone are most commonly dated, while inor-
ganic carbonates precipitated in saline lakes of the Great Basin have provided
a significant chronology of their pluvial and postpluvial history.

Before the development of geochemical dating techniques, including
radiocarbon assay, a fairly accurate chronology of the late glacial and post-
glacial time had been developed in northern Europe. Here the chronology
was worked out on practically an absolute time basis by the study of varves,
or layers of sediments deposited in standing bodies of water. In northern
Europe and North America varves are associated with the melting of glaciers
and are formed in glacial lakes as annual layers. The seasonal gradation of
size of particle provides a sharp demarcation between the finer particles de-
posited late in the season and the coarser particles laid down early in the
season of the following year. The thickness of the varves varies from year
to year and if they are exposed in cross section, they may be counted and
the number of years represented at a given site determined. The Swedish

No. 3D HANSEN: CYCLES AND GEOCHRONOLOGY 5

geologist, De Geer, recognized the potential value of varves in late glacial
and postglacial chronology and in 1879 began a thorough and systematic study
of varve beds. By measuring and counting the varves at one site he found
considerable variation in thickness, and by correlating sequences of varve
variation in thickness from one site to another, he was able to determine the
time required for the ice to retreat from that site to one farther north. This
correlation method is analogous to the cross-dating in tree-ring studies. A
Finno-Swedish varve chronology includes about 11,600 years, of which 10,150
are considered to represent the northern European postglacial. This is strik-
ingly similar to the radiocarbon date for the Two Creeks forest bed in Wis-
consin, which marks the Mankato-Valders stage of the late Wisconsin and is
generally accepted as the approximate beginning of the postglacial in North
America, as will be discussed later.

One of the most important research tools in the study of paleoclimatology,
history of vegetation, and chronology, especially for the Quaternary, is that
of pollen analysis. Since the time of its inception, the study of fossil pollen
in Quaternary deposits has been commonly spoken of as pollen analysis, but
with more extensive application of the method and the identification of fossil
spores of greater age, a broader, and more comprehensive and inclusive term
was needed. In 1944 the term "palynology" was suggested by Hyde and
Williams. Palynology from the Greek "paluno" means to strew or sprinkle;
cf. , pale, fine meal; cognate with the Latin pollen, flour, dust; the study of
pollen and other spores and their dispersal, and applications thereof. The
term "palynology" was readily adopted by workers in the field and has been
adopted as the official name for the science of pollen analysis and all of its
ramifications.

Modern pollen analysis per se made its debut in 1916 at Oslo, Norway,
when Lennart von Post presented the first modern percentage-pollen analysis
in a lecture to the Scandinavian scientists' meeting. Fossil pollen grains
were first observed in prequaternary sediments as early as 1836, and the
significance of the occurrence of pollen grains in postglacial sediments was
noted in 1893. The Swiss Geologist J. Fruh published a paper in 1885 on
characteristics of peat in which he listed many of the pollen grains present.
Other Germans and Scandinavians made early contributions to the literature
on pollen in sediments, but von Post deserves the credit for working out the
first pollen profiles in which changes in the pollen proportions were shown
from bottom to top.

The immediate and direct interpretation of pollen profiles is into terms
of vegetational succession during the time represented and within range of
pollen dispersal to the site of the sediments. The various stages of succes-
sion as recorded by the composition of the vegetation, indicate the environ-
mental influence upon the vegetation as well as the normal vegetation
succession controlled by the synecological and autecological characteristics

6 CALIFORNIA ACADEMY OF SCIENCES (Occ. Papers

of the species involved. Paleoclimate in its general trends is perhaps the
most significant direct interpretation of the vegetational record.

Considerable attention has been paid to the chronological aspects of
pollen analysis, and correlated with other sources of chronological data, a
rather definite and probably fairly accurate late glacial and postglacial se-
quence of events has been determined. Radiocarbon dates have been the
most significant factors in building this chronology. It is interesting to note,
however, that the general chronology and sequence of late glacial and post-
glacial events and climate as interpreted from pollen profiles and varves
before the advent of radiocarbon dating have been remarkably accurate. The
postglacial period of northern Europe, beginning about 10,000 years ago, as
based upon varved clay sequences and recurrence surfaces, is divided into
five phyto-climatic periods (table 2). The first stage is known as the Pre-
Boreal which persisted for about 600 years and was characterized by forests
of birch and pine and a cool wet climate. This was succeeded by the warmer
and dryer Boreal stage lasting about 1000 years during which time the forests
were composed largely of pine and hazel. In Sweden, this period may have
lasted for 1500 years. A longer period of continued warmth and greater moisture
was characterized by forests of oak, elm, and linden. Including a transition
stage, this period, known as the Atlantic, persisted for about 4000 years in
Denmark (table 2). A well defined interval, known as the Sub-Boreal succeeded
the Atlantic and the climate became warm and dry supporting forests of oak,
ash, and linden. The end of this stage is marked by a well oxidized stratum
of peat of wide spread occurrence in the peat beds of northern Europe, over-
lain by fresh and unoxidized peat. This layer has been dated at about 600
B.C. The final postglacial stage of perhaps 2500 years duration to the pre-
sent, saw a return to cooler and wetter climate supporting forests of oak and
beech in Denmark and alder, oak, and birch in the British Isles (table 2).

While these periods are marked by general climatic conditions, there
have been many lesser fluctuations during each stage, and during the last
several thousands of years there have been rather marked changes in glacial
movements that suggest corresponding changes in climate.

The warmest and driest stage during the postglacial in northern Europe,
the Sub-Boreal, has been called the xerothermic period. The end of this time
is marked by a recurrence horizon in the peat beds of northern Europe. In
fact there are numerous such horizons in the peat sediments, which are char-
acterized by a layer of oxidized woody peat, indicating a lowering of the
water table in the bogs, resulting in humidification of the organic material.
With a return to wetter conditions and subsequent raising of the water table,
the shrubby vegetation was replaced with bog mosses. In cross section, a
distinct horizon is evident. The Swedish postglacial chronology includes a
total of 5 recurrence surfaces, and probably more, dating back to about 3500
B.C. (table 3). Since recurrence surfaces constitute a change from drier to

No. 31) HANSEN: CYCLES AND GEOCHRONOLOGY

moister climate, they denote recurring dryness at general intervals of 500 to
600 years and 1000 to 1200 years in support of a fundamental climatic cycle
of about 550 years and another at about 1100 years interval. These periods
of alternating drought and moisture have been almost synchronous throughout
Europe since 2300 B.C., and may correspond to similar cycles of bog drying
and regeneration in North American bogs.

An excellent point of departure for considering the postglacial time in
North America seems to be about 10,000 to 12,000 years. One of the signifi-
cant radiocarbon dates is that of wood from the Two Creeks forest bed in
Wisconsin, located in wave-cut cliffs of Lake Michigan in northern Manitowoc
County, Wisconsin, about 25 miles within the maximum extent of the Mankato
ice. An average age of about 11,400 years for five samples of wood and peat
was determined. Inasmuch as the ice overrode the forest and moved another
25 miles south, the ice (Mankato maximum) is younger and a figure of about
11,000 years seems to be reasonable. Many additional radiocarbon dates
from materials that indicate a similar chronological relation to their encom-
passing drifts, suggest that 11,000 years for this maximum advance of the
last stage of the late Wisconsin glaciation was fairly consistent throughout
the northern United States. The Two Creeks forest interval probably represents
a warmer period more or less concurrent with the Allerbd of northern Germany
and Denmark, during which forests of birch and pine flourished between 11,000
and 12,000 years ago (table 2).

In eastern United States, Deevey has carefully worked out a chronology
of vegetation changes for at least 15,000 years showing a close chronological
correlation with the northern European sequence (table 2). The first was
tundra which persisted until 14,000 years ago, followed by forests of spruce,
pine, and birch for 1000 years or so. A brief return to tundra conditions is
suggested by pollen of tundra herbs, again to be invaded by forests consisting
of spruce, pine, fir, and oak during the Pre-Boreal. Continued warming favored
increase of pine during the Boreal, while persistent warmth accompanied by
increased moisture during the Atlantic, favored oak and hemlock for several
thousands of years. A warm but dryer climate permitted hickory to flourish
during the Sub-Boreal, while cooler and moister conditions during the sub-
Atlantic saw forests of oak and chestnut predominate the scene during the
past 2000 years. In the last few centuries the increase in spruce and fir may
indicate cooling.

There is little doubt that during the past 11,000 years, since the last
continental glaciers melted, there was an increase in temperatures to a degree
higher than at present, followed by cooler or wetter climate or both. In some
parts of the northern hemisphere there was also a decrease in moisture which
is well recorded by the increase in xerophytic vegetation, lowered lake levels,
and higher timberlines. This period of warmth and dryness, which varied in
length in various parts of the world, has been recognized by a number of terms,

CALIFORNIA ACADEMY OF SCIENCES

(Occ. Papers

GENERAL CHRONOLOGY OF THE PLEISTOCENE

Years B. P. *

North American
Glacial Stages

Northern European
Glacial Stages

6,500 -
7,500

Cochrane

Ragunda Pause

10,000 -
11,000

Mankato-Valders

Fenno-Scandian

13,500 -
14,500

Cary

Scanian

17,000 -
18,000

Tazewell

"Classical Wisconsin"

Pomeranian

30,000 -
40,000

Farmdale

Frankfurt
Brandenburg

45,000

Interglacial

55,000 -
70,000

Early Wisconsin (Iowan)

Warthe

100,000

Sangamon Interglacial

Interglacial

120,000

Illinoian Glacial

Saale

180,000

Yarmouth Interglacial

Interglacial

200,000

Kansan Glacial

Elster

260,000

Aftonian Interglacial

Interglacial

300,000 ?

Nebraskan

• The letters "B.P." as used here indicate "Before Present'

Table 1
Estimated dates of the major glaciations during the Pleistocene and the sub-
stages of the late Wisconsin beginning with the Tazewell. Dates are from many
sources including radiocarbon and other geo-chemical techniques, varves, peat strat-
igraphy, volcanic ash and pumice, lakes sediments, and pollen profiles.

No. 3D

HANSEN: CYCLES AND GEOCHRONOLOGY

Yrs.
B.P.

1000

Glacial
Sequence

Pollen

Sequence

Denmark

N. Germany

Pollen

Sequence

British

Isles

European

Cultural

Stages

Pollen

Sequence

N. E.

United States

Yrs.
B.P.
1000

1 -

2 -

— Glacial —

Sub-Atlantic

Beech-Oak
Cool-Wet

Sub-Atlantic

Alder-Oak

Birch

Cool-Wet

Iron

Sub-Atlantic
Oak-Chestnut

-1

Sub-Boreal
Oak-Hickory

3 -

X
H

Sub-Boreal

Oak-Ash

Linden

Warm-Dry

Sub-Boreal

Alder
Mixed Oak

Warm-Dry

Bronze

-3

4 -

Neolithic

-4

5 -

Atlantic

Oak-Elm

Linden

Warm-Wet

Atlantic
Oak-Elm

Oak

Pine

■5

6 -

Mesolithic

Atlantic
Oak-Hemlock

-6

7 -

Cochrane

■7

8 -

-8

9 -

Boreal
Pine-Hazel

Bo.real
Pine

10 -

Pre-Boreal
Birch-Pine

Pre-Boreal
Birch

Pre-Boreal

Spruce-Fir

Pine-Oak

"10

11 -

Mankato

H
<

Younger Dryas
Park-Tundra

Paleolithic

-1 1

12 -

Two Creeks
Interval

Allerod

Birch-Pine

Warmer

Allerod

Birch

Warmer

■ 12

13 -

Older Dryas

Tundra

•13

Boiling
Park-Tundra

Tundra

Spruce-Pine

Cary

14 -

Oldest Dryas

Tundra

■14

15 -

16 -

17-

•15

-16
-17

1 ft -

Tazewell

- 18

Table 2

Late-glacial and postglacial stratigraphy of northern Europe and northeastern
United States with substages of the late Wisconsin glaciation and cultural stages of
Europe. (From Deevey and Flint and Karlstrom).

CALIFORNIA ACADEMY OF SCIENCES

(Occ. Papers

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CALIFORNIA ACADEMY OF SCIENCES

(Occ. Papers

Oregon

Utah

Nevada

California

Glacial

Summer Lake

Salt Lake

PyramidLake

Searles Lake

Tioga-Mankato

Tahoe
(Pre-Wisconsin)

Winter
Chewaucan

Provo
Bonneville

Dendritic
Lahontan

Parting Mud
(10-23,000 yrs.)

Bottom Mud
(32-46,000 yrs.)

Table 5

Pluvial (glacial) stages of Great Basin lakes and mountain glaciation in the Si-
erras and chronologically correlated with radiocarbon dates from Searles Lake, (from
Allison, Antevs, Blackwelder, and Flint and Gale).

including "xerothermic period," "climatic optimum," "thermal interval,"
"thermal maximum," "altithermal," "megathermal," and "hypsithermal."
It is not the purpose of this paper to discuss the semantics or evaluate these
terms in relation to which best describes this accepted concept of a post-
glacial temperature maximum. It suffices to say that such climatic develop-
ment did take place and that it was fairly consistently regional in its extent
and magnitude. It is also uncertain as to the exact period of time involved,
but it is immediately recognized that the time limits assigned are determined
by the individual's concept as to what the limits or boundaries of temperature
and moisture are and what latitudes and altitudes are involved. Upon the
basis of peat stratigraphy, varves, and recurrence horizons, before the advent
of pollen analysis, Scandinavian bogs were interpreted as showing a cool-dry
Boreal period, a warm-moist Atlantic, and a warm-dry Sub-Boreal period, the
latter designated as the "xerothermic" and the Atlantic as the "climatic
optimum." Chronological correlation with De Geer's varve sequences indi-
cates the climatic optimum between 6,000 and 4,000 years ago and the entire
time of the thermal interval or period as extending from about 9,000 to 2,500
years ago.

In the Pacific Northwest, pollen analyses of many peat sections located
in several different phytogeographic and climatic regions, correlated with
radiocarbon dates of bottom peats and volcanic ash and pumice horizons, re-
veal a remarkably consistent and regional sequence of climate and chronology.
Radiocarbon dates of bottom peat in the Puget Sound region indicate that
about 11,000 - 13,000 years comprise the time represented. Beyond the glacial
boundaries, physiographic events resulting from glacial retreat set a similar
date for peat sections in unglaciated areas. In general, the pollen profiles
indicate increasing warmth and desiccation to a maximum and then a return
to cooler and moister conditions in more recent time. It does not seem pos-
sible to interpret the profiles as the five or six climatic stages as has been

No. 3D

HANSEN: CYCLES AND GEOCHRONOLOGY

Name of Site

Location

Radiocarbon
Dates

Cultural Materials

Tule Springs

South Central
Nevada

23,000

Obsidian Flakes, Scrapers,
Camel, Bison, Horse,
Mammoth

Lindenmeier

Northeastern
Colorado

10,780

Folsom Points, Stone Im-
plements, Carved Bones,
Knives, Charcoal, Bison,
Camel Bones

Lehner Site

Southeastern
Arizona

12,000

Clovis Fluted Points, Char-
coal, Mammoth, Bison,
Horse, Tapir

Fort Rock
Cave

South Central
Oregon

9100

Points, Scrapers, Awls,
Atlatl Spur, Sagebrush
Sandals

Five Mile
Rapids

The Dalles
Oregon

9700

Points, Scrapers, Choppers,
Bird, Fish, Other Animal
Bones

Danger Cave

Wendover,
Utah

11,000

Stone Artifacts, Netting,
Mats, Basketry, Mountain
Sheep, Deer, Antelope

Leonard Rock
Shelter

Lovelock
Nevada

11,000

Atlatl Points, Shell Beads,
Scrapers, Etc.

Denbigh Flint
Complex

Cape Denbigh
Norton Sound
Alaska

5000 R.C.
8500 Est.

Microblades, Fluted Point

Frazer River
Canyon

British
Columbia

8000

Points, Bones, Artifacts
Related to Fishing

Table 6

Radiocarbon dates of materials from cultural sites in western North America,
(from Wormington).

14 CALIFORNIA ACADEMY OF SCIENCES (Occ. Papers

done for Europe and eastern North America. Apparently there was a single
major period of maximum warmth and desiccation, rather than the alternating
dry and wet warm of the Boreal, Sub-Boreal, and Atlantic.

In the Puget Sound region, the thermal interval is not well defined be-
cause of the influence of the Pacific Ocean. While increases and predomi-
nance of western hemlock during the past 4000 years and its partial replacement
of Douglas fir, suggest a preceding warmer and dryer climate, the postglacial
forest sequence in this region probably represents normal forest succession
from the pioneer invaders to the climax of the present. In eastern Washington
and Oregon, however, the thermal interval is well portrayed by a pronounced
expansion of drought-resisting vegetation consisting of grasses, chenopods,
and composites (table 4). Many of the plants represented in the pollen profile
are halophytic and emphasize the existence of xerophytic conditions, because
they probably invaded the dry beds of alkaline lakes. In the Willamette Val-
ley of Oregon, an influx and expansion of oak with decrease of conifers depicts
the warmer and dryer conditions. In south central British Columbia, a well-
defined yellow pine maximum further substantiates the development of warmth
and dryness, and attests to their widespread and regional occurrence.

In addition to the radiocarbon date of 11,000 years for bottom peats in
the Puget Sound region, which marks the minimum date for deglaciation by
the ice of Mankato equivalent, an extremely important chronological marker
is present in most of the Washington peat sections in the form of a layer of
volcanic ash. The source of this ash has been traced to Glacier Peak in
north central Washington, and radiocarbon dates of peat immediately under-
lying the ash horizon range from about 6700 to 5300 years ago. The author,
before radiocarbon assay, estimated the date of this ash layer at about 6000
years, based upon its stratigraphic position in the peat sections and pollen
profiles (table 4). Another equally valuable radiocarbon date is that for the
eruption of Mount Mazama in south central Oregon, which left the caldera
holding Crater Lake. Charcoal from trees which were first charred by the
incandescent gases that rolled down the slopes and then buried by pumice,
have been dated at about 6500 years. Pumice from Mount Mazama, in peat sec-
tions in the northern Great Basin of southern Oregon, occur well above the
beginning of the thermal interval, and about in the same relative stratigraphic
position as the Washington ash from Glacier Peak (table 4). Thus in the
Pacific Northwest, both west and east of the Cascade Mountain range, there
is strong evidence that the thermal maximum can be bracketed between 8000
and 4000 years ago. This is consistent with Ernst Antev's dating of the
Altithermal of the Great Basin from 7500 to 4000 years ago (table 3). In
coastal Alaska, British Columbia, Washington, Oregon, and California, Heusser
interprets from many peat sections and pollen profiles a thermal interval
(hypsithermal) in the middle third of the recorded vegetational history. In
northern Alaska, Livingston relates an expansion of alder into the tundra to

No. 3D HANSEN: CYCLES AND GEOCHRONOLOGY 15

the thermal maximum within the time boundaries of that of the Pacific North-
west, while Tsukada believes that replacement of conifers by forests of oak,
beech, and elm in Japan reflect this same period of maximum warmth and
desiccation (table 3).

In the Great Basin of western United States, the stages of glaciation to
the north are considered to have been concurrent with pluvial periods during
which time the lakes were deepened and enlarged. Shorelines and terraces
of these ancient lakes are very evident and have received a great deal of
study and analysis by various workers, and have been chronologically cor-
related with glacial stages to the north. Antevs has set the postglacial in
the Great Basin as beginning at the time that postglacial temperatures had
reached about the same point as those of today. Chronologically this is based
upon the Finno-Swedish varve chronology of De Geer, Liden, and Sauramo
which sets about 10,150 years ago as the beginning of the postglacial in
Europe. This date also marks the beginning of the Pre-Boreal climatic stage.
Antevs has named the postglacial in the Great Basin the Neothermal which
he divides into a period of increasing warmth, the Anathermal; a period of
maximum warmth and desiccation, the Altithermal, from 7500 to 4000 years
ago; and a final period of greater moisture and lower temperatures, the Medi-
thermal (table 3). The end of the thermal interval is marked by the fact that
the present salinity of Great Basin lakes, Abert and Summer lakes in south
central Oregon and Owens Lake in east central California, is such that could
not have taken more than 4000 years to be attained. Antevs believes that
during the Altithermal these lakes dried up and their salt sediments were
either blown out or buried by sand. With the advent of a wetter climate these
lake basins were again filled and the lakes freshened. According to Matthes,
the fluctuations of glaciers in the western mountains are closely correlated
chronologically with the pluvial periods and the levels of the lakes in the
Great Basin. He suggests that the modern glaciers came into being within
the past few thousands of years after virtually disappearing during the warm
dry Altithermal.

One of the most conspicuous evidences of the high lake levels in the
Great Basin pluvial stages are well defined terraces considerably higher than
the present level of the lakes. These terraces represent long time stages of
lake levels that were maintained by the greater precipitation probably con-
temporaneous with the glacial stages to the north. These Pleistocene lakes
have been correlated, and some of the major ones are the lakes Provo and
Bonneville in the Salt Lake basin, Dendritic and Lahontan in the Pyramid
Lake basin of Nevada, and the Winter and Chewaucan lakes in south central
Oregon. All of the first named of each pair have been chronologically cor-
related with the Tioga mountain glaciation in the Sierra and the Mankato-
Valders of central United States, and the second ones may have been concur-
rent with the Sierra Tahoe and an earlier Wisconsin (Iowan) glaciation (table 5).

16 CALIFORNIA ACADEMY OF SCIENCES (Occ. Papers

Stratigraphic studies and radiocarbon dates of sediments in Searles Lake,
a dry lake in southeastern California, provide dates that have tentatively
been correlated with the pluvial stages of Great Basin lakes. Two deep lakes
are recorded implying a pluvial climate, each of which was followed by near-
desiccation representing a warmer and dryer climate or interpluvial stages.
The later pluvial climate persisted from about 23,000 to 10,000 years ago,
and was contemporaneous with the classical Wisconsin glaciation, while the
earlier pluvial from about 46,000 to 32,000 years ago was synchronous with
Lake Lahontan of the Tahoe-pre-Wisconsin glaciation (table 5).

The late glacial and postglacial saw the extinction of most of the large
mammals that existed abundantly during the Pleistocene in North America.
Radiocarbon dates of animal remains, dung, and associated organic materials
indicate that most of the extinction took place between 10,000 and 3,000 years
ago, varying in different parts of the continent. Seven species have persisted
as late as 2000 years ago in Florida, but no remains from Alaska have been
dated at less than 16,000 years ago. In western United States, most of the
large mammals became extinct before 6000 years ago. These include the
mastodon, woolly mammoth, giant bison, horse, sloth, shrub-ox, dire wolf,
big beaver, bear, cats, tapir, camel, cave deer, and others. The cause of
their extinction is a mystery and a number of theories have been expressed,
such as early hunters, climatic changes, and epidemics. Mastodon bones
buried in a shallow swamp near Silverton, Oregon, have been dated on the
basis of their stratigraphic position in the pollen profile well above the begin-
ning of the thermal maximum as indicated by its recorded influx and expansion
of oak (table 4). They have been dated at not older than 6000 years, which
would be one of the most recent occurrences in the western United States.

Perhaps one of the most interesting and fascinating aspects of Pleisto-
cene chronology is that concerning the time of arrival of man on the scene.
It is now generally accepted that man (Homo) made his first appearance in the
Pleistocene, possibly 300,000 years ago during the Giinz (Nebraskan) glacial
stage. This is based upon the occurrence of hominid fossils in stratigraphic
sequence that have been correlated with the Pleistocene glacial stages as
based upon temperature variations of equatorial, tropical, and temperate marine
surface waters determined by oxygen isotopic analyses of pelagic foraminifera
from deep sea cores, radiocarbon and ionium dates of deep sea cores, and
comparison with insolation curves. When Homo sapiens appeared is a problem
of taxonomy as well as dating, however, and it is possible that he did not
evolve more than 100,000 years ago and probably less. The last hominid
forms to evolve seem to have been Neanderthal man and Cro-Magnon man,
which were contemporaneous. The former became extinct by the end of the
last glaciation, however, while Cro-Magnon persisted and has flourished. In
terms of European cultural stages, Paleolithic man was able to persist through
the late Wisconsin glaciation, which culture may have survived until the end

No. 3D HANSEN: CYCLES AND GEOCHRONOLOGY 17

of the late glacial Fenno-Scandian of northern Europe (Mankato) during tun-
dra conditions (table 2). The beautiful paintings in the Lascaux Cave in
central France are at least 15,000 years old as determined by radiocarbon
dates of charcoal found in the cave. The beginning of the postglacial about
10,000 years ago, has also been designated as the end of the Paleolithic
and the dawn of the Mesolithic cultural stage, which saw ameliorating cli-
matic conditions in the North Temperate zone. The Neolithic stage in Den-
mark, northern Germany, and the British Isles began near the end of the warm
wet Atlantic between 5000 and 6000 years ago. This was followed by the
Bronze Age from 3500 to 2500 years ago which ended with the warm dry Sub-
Boreal delineated by the recurrence horizon known as the Grenz-Horizont of
the Sernander climatic sequence (table 3). The cooler and wetter climate of
the sub-Atlantic is thought to have been concurrent with the Iron Age, and
includes the Roman occupation of parts of the British Isles. In the Mediter-
ranean Basin, radiocarbon dates indicate that man has existed there for at
least 50,000 years, the present limits of determination by radiocarbon tech-
niques.

The chronology of man in North America did not receive the intensive
study that it did in Europe, and until the advent of radiocarbon dating, one
or two millennia were considered to be sufficient antiquity to cover the period
of occupation. The first discovery that provided evidence for a probable
greater antiquity was that of pollen analysis upon which are based the clima-
tic sequences correlated with those of Europe. In Lower Klamath Lake, Oregon,
pollen analysis of peat sections shows a fossil lake bed under 13 feet of
peat which had been occupied by early man, evidently resulting from the
warmth and drought of the Thermal interval. This interval is substantiated
on the basis of the aforementioned fluctuation of Great Basin lakes. It is
obvious, however, that the hominid record in North America does not begin
with pie-Homo sapiens forms. This fact immediately presents the problem
as to the time and source of the invasion of the New World by early man.
While a number of mythical hypotheses have been advanced, they are hardly
worth mentioning in view of the vast amount of evidence that has been un-
covered which refutes them. This evidence, as studied and interpreted by
archeologists, includes tools and implements that depict technological stages,
methods of subsistence indicating the use of the environment in getting food,
clothes and shelter which point to their adaption to climate, and their cul-
tural attributes which outline their socio-economic patterns. The vast amounts
of time and space involved and the comparatively small number of widely-
spaced occupational sites discovered further complicate the problem. There
seems to be little doubt, however, that the paleontologic, geologic, and geo-
graphic eyidence of Tertiary and Quaternary times points with assurance and
confidence to the conclusion that the Old and New Worlds have at intervals,
and for fairly long periods, been connected by a land bridge across Bering

18 CALIFORNIA ACADEMY OF SCIENCES (Occ. Papers

Strait between Alaska and Siberia. The land connection between the two
continents was controlled by glaciation and deglaciation, resulting from fluc-
tuation of sea level as ice either melted or accumulated. Paleontological
evidence suggests that a seaway existed during the middle Eocene but that
during the glacial stages they became connected by a land bridge. This
means that during the past 100,000 years there have been at least two long
periods during which such a land bridge existed. The first was in response
to an early Wisconsin glaciation, perhaps 75,000 to 50,000 years ago, and a
second during the late Wisconsin between 25,000 and 10,000 years ago, since
which time no connection has existed. Although the land bridge must have
existed during a period of an Arctic climate and must have supported only
tundra vegetation, it provided a pathway for early man into the New World and
for the migration of plants and animals back and forth.

The inundation of the land bridge at the termination of the last major
stage of Wisconsin glaciation (Mankato) eliminated access to the New World
by terrestrial flora and fauna, including man. Entry of man into the New World
from Siberia must have taken place during one of the glacial stages, the latest
of which could have occurred less than 10,000 to 15,000 years ago. Man also
could have come during one of the earlier glacial intervals, about 50,000 years
ago. Radiocarbon dates of 10,000 years for materials associated with man's
culture substantiate the forgoing evidence for his migration over the land
bridge during the late Wisconsin or at an earlier glacial stage. The central
part of Alaska and western Yukon Territory were not glaciated during the
late Wisconsin stage and in spite of the cold climate that must have existed
the summers were sufficiently warm so as to maintain a food chain composed
of both plants and animals that permitted man to subsist. Slow migration east-
ward into interior Alaska and the Yukon during both glacial and interglacial
intervals was possible, and then southward movement along the east flank
of the Rocky Mountains in British Columbia took place, although the archeo-
logical evidence for the latter is quite sparse. The existence of an ice-free
corridor during most of the late Wisconsin is evidenced by pollen analysis
of peat sections along the present Alaska Highway and in western Alberta,
in which forest-tree pollen is present near and at the bottom of muskegs as
much as six meters deep. This enabled continued migration southward during
the late Wisconsin by any migrants whose ancestors had taken advantage of
the Bering land bridge during or before the early Wisconsin glaciation. Al-
though there is no direct evidence for this, it is more than conjecture. It is
substantiated by many radiocarbon dates that man reached Central and South
America more than 10,000 years ago. This does not suggest any phenomenal
speed of migration, however, because at a rate of one mile per year, the
southern tip of South America could be reached in about 10,000 years.

While most of the radiocarbon dates for occupation materials range in
the vicinity of 10,000 years, several are greater, and the Tule Springs site

No. 3D HANSEN: CYCLES AND GEOCHRONOLOGY 19

in south central Nevada has been dated at 23,000 years (table 6). If this is
true.it adds support to the idea thatman's arrival in theNewWorld was before
the late Wisconsin maximum of about 15,000 years ago. Early man and the
megafauna of the late Pleistocene were apparently contemporaneous until
about 8000 years ago, and since then, man had to get along without whatever
these large mammals furnished him in the way of food, clothing, and shelter.

Many thousands of years after the first invasion by Homo sapiens of the
New World from the west, another invasion from the east took place. The
earliest record of this migration is found in Greenland and in the records set
down by the Norsemen who founded a colony numbering as many as 3000 per-
sons in southwestern Greenland about 1000 years ago. The colony thrived
for about three hundred years, and then a cooling period began about 1600
A.D. and lasted until at least the middle of the 18th century. During this
time of cooling, the so-called "Little Ice Age," glaciers in Alaska and northern
Europe probably advanced to their greatest maximum since before the Thermal
maximum. From 1750 to 1800 A.D. there was a pronounced retreat, with a
readvance attaining a maximum about 1850, and then general retreat again
until the present. In a detailed study of the retreat of Herbert Glacier 25
miles north of Juneau, Alaska, Lawrence discovered a series of about twenty
parallel crescentic moraines marking a recession of the glacier beginning
about 1740 A.D. He ascribed the systematic morainal deposition as correla-
tive with a 11-year sunspot cycle with moraine formation occurring with low
sunspot number intervals which reflected increased glacier nutrition, while
retreat is correlated with a high sunspot number resulting from reduced nu-
trition.

In summary, I have briefly, and with many gaps, pointed out some of the
cyclic and chronological events of the Pleistocene, a time of marked climatic
changes. This was a period of perhaps a million years that saw four or five
major periods of glaciation in the northern hemisphere. It was a time during
which man apparently evolved his present state of mental and physical de-
velopment. It was a time of great displacement of biota by the ice sheets
as they radiated from their centers of accumulation northward as well as south-
ward. The latter part of this ice age, saw the extinction of a large number of
species of megafauna which grew up with early man, and undoubtedly pro-
vided him with a source of food, clothing, and shelter before he developed
primitive agricultural practices. This same early man in western United States
witnessed several volcanoes eject vast quantities of lava, ash, and pumice,
the last two of which serve as excellent chronological markers and provide
us with the basis for some fairly good dates for postglacial forest history
and climatic fluctuations. There is no reason not to believe that we will
hava more glacial stages within the next 10,000 years, more or less.

20 CALIFORNIA ACADEMY OF SCIENCES (Occ. Papers

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