Explanatory notes to accompany a new geological map of England and Wales

GEIKIE.
Explanatory Notes

8X

LIBRARY

THE UNIVERSITY
OF CALIFORNIA

SANTA BARBARA

PRESENTED BY

W.S.W. KEW

COMPANION MAP TO EKGLAKD AflD WALES

Price 7s. 6d. on cloth and in case. On rollers, varnished, I0i>.

Published with Government Authority.

A NEW

GEOLOGICAL MAP

SCOTLAND

Reduced from the
Ordnance and Geological Surveys

UNDER THE DIRECTION OF

SIR ARCHIBALD CEIKIE, D.SG., Ll.D., F.R.S.,

Director-Central of tht GtolagicAl Survey.

ACCOMPANIED BY DESCRIPTIVE MEMOIR.

Topography by JOHN BARTHOLOMEW, F.R.G.S.
Scab, 10 Miltt to an Inch.

JOHN BARTHOLOMEW & CO.,

Gbe EOinbur^b (Beograpbtcal institute,

EXPLANATORY NOTES

TO ACCOMPANY A NEW

GEOLOGICAL MAP

OF

ENGLAND & WALES

BY

SIR ARCHIBALD GEIKIE, D.C.L, D.Sc., F.R.S.

Director General of the Geological Survey

JOHN BARTHOLOMEW & CO.

<5eo0rapblcal Institute
1897

I •• )H,"AKY

UNIVERSITY OF CALIFORNIA
SANTA BARBARA

EXPLANATORY NOTES

TO ACCOMPANY THE

Geological Map of England
and Wales

SINCE the appearance in the year 1815 of the first general
geological map of England and Wales, by William
Smith, numerous maps of the whole and of parts of the
country on various scales have been published. The earliest
of these, by G. B. Greenough, was begun while that of William
Smith was in preparation, and appeared four years later — in
1819. It passed into the hands of the Geological Society of
London, has from time to time been improved, and is still on
sale. Its topography, however, compiled and engraved in the
early part of the century, was from the beginning imperfect,
and its geology has been so repeatedly and extensively altered
that its later forms bear only a general resemblance to the
original. The Geological Survey, begun in 1835, has now
completed the mapping of the whole country on the scale of
one inch to a mile, and is issuing a reduction on the scale of
four miles to one inch. All general maps now prepared are
based on the sheets of this official survey. In 1859, the late
Sir A. C. Ramsay, then Local Director of the Geological
Survey of Great Britain, published a general map of England
and Wales on the scale of twelve miles to an inch. It was a
reduction from the Survey maps as far as these had at the time
been published. In successive editions it has been improved,
and up till now it has been the best available map, but its

4 GEOLOGICAL MAP OF ENGLAND AND WALES

topographical basis was too inaccurate for the production of a
wholly satisfactory representation of the geology.

Five years ago, Mr. Bartholomew, who had with great labour
constructed from the sheets of the Ordnance Survey a general
topographical map of England and Wales on the scale of ten
miles to an inch, expressed to me his willingness to employ the
same plates for the production of a geological map of the country,
if I would superintend the reduction from the published maps
of the Geological Survey. His map is so good as a general
topographical representation of the country, that it appeared
to me likely to prove convenient and useful if the geology were
inserted on it. Accordingly, his proposal was eventually carried
into effect, and the map so prepared is now issued to the public.
While the reduction has been made by Mr. Bartholomew's able
assistant, Mr. Bosse, it has been repeatedly revised by some of
my colleagues in the Geological Survey, as well as by myself.
The late Mr. W. Topley, F.R.S., took a large share in the early
stages of this labour, and since his death the task has been
carried on by Mr. H. B. Woodward, F.R.S., Mr. W. Gibson,
and Mr. W. W. Watts. I have drawn the Sections which are
placed round the borders of the map to explain the geological
structure of the country.

The following brief Notes are intended merely for the
general reader or traveller who may have no special geological
knowledge, but may be induced to take the map with him as
a convenient guide in journeying across the country. The
meaning of the various colours and the nature of the rocks
they represent are here briefly explained.

A cursory glance at a geological map of England and Wales
reveals some of the fundamental features in the geology of the
country. In the first place, it shows that a line drawn in a
nearly north and south direction from the coast of Durham,
about the mouth of the Tees, to that of Devon, at the mouth
of the Exe, divides the region into two parts. To the west of
that line the colours are distributed in patches of widely dif-
ferent sizes, and apparently scattered at random. To the east,
on the other hand, the several tints are disposed in bands
which follow each other continuously across the island from
the shores of the North Sea to those of the English Channel.
In the western half of the country the rocks belong mostly to
the Palaeozoic formations. They have generally been greatly
disturbed, many of them having been plicated, crumpled, and

EXPLANATORY NOTES 5

squeezed. For the most part they consist of harder materials
than those to the east of them. Hence, partly owing to their
greater durability, and partly to the influence of their disturb-
ances and upheavals, they rise into the highest tracts of ground.
Beginning on the north, and tracing the series of older rocks
from the Scottish Border to Devonshire, we pass successively
over the hills of the Lake Country, the Pennine Chain, Wales,
and Dartmoor, and we note that these eminences usually form
detached groups, such as those of Cumberland, Westmoreland,
and North Wales.

In the eastern half of the island, on the other hand, the
rocks are arranged in successive bands which present their
edges towards the west, and sink below each other towards
the east. The harder members of the series, such as lime-
stone and sandstone, rise into long ridges, while the softer
clays, marls, and shales subside into valleys or spread out
into plains. The hills are not only less lofty than those of
the western side, but they are further distinguished by their
prolongation into continuous ridges. The most familiar
examples of this type of scenery are supplied by the chalk
hills. Thus the North Downs, which run for many miles
westward from the Dover cliffs, mark the trend of the chalk
through that part of the country, while the corresponding
range of the South Downs reveals the long unbroken outcrop
of the same rock from Beachy Head to Salisbury Plain.

It is thus evident that the landscapes of the country
depend for their character mainly upon the nature and dis-
tribution of the rocks underneath the surface. The traveller
or tourist who journeys with a geological map in his hand
can mark how each distinct change in topography arises from a
corresponding alteration in the character of the geology. The
variations of scenery consequently acquire for him a new mean-
ing and interest. There is no part of Europe where the relation
here described can be better seen than in England. Even
from a rapid railway journey much may be learnt here regard-
ing the dependence of topography upon geological structure.

In the second place, a little closer study of the map will
show us that, on the whole, the oldest rocks lie along the
western shores, and the youngest along the eastern. If we were
to land in Anglesey and traverse the country to the coast of
Suffolk, we should pass successively from some of the most
ancient formations of Britain to some of the newest, as is illus-
trated by the section on the left side of the map. At Holy-

6 GEOLOGICAL MAP OF ENGLAND AND WALES

head we should find ourselves among quartzites and schists of
higher antiquity than even the most ancient of the Palaeozoic
formations, which further south, in Anglesey, are found resting
on the edges of these primeval rocks. Crossing into Caernar-
vonshire, we should come upon some of the oldest stratified
deposits in the country, belonging to the Cambrian system,
and from these, ranging into the great cone of Snowdon, and
thence into Montgomeryshire, we should ascend in the geo-
logical scale through the vast thickness of the Silurian system,
up to the base of the Old Red Sandstone. We should next
enter upon the still younger Carboniferous system, and pass in
succession into the Permian and Triassic formations, until, in
Leicestershire, we should reach the edge of the Jurassic series.
The various members of that series would stretch to northward
and southward of our line of traverse, the successive limestones
rising into long lines of escarpment and sinking gently eastward
under younger strata, until, beyond the alluvial fen-country
of Cambridgeshire and Huntingdon, the Chalk, with its long
escarpment, would present itself, undulating eastward through
Norfolk and Suffolk, and slipping gently under the much
younger Crag deposits, which are laid bare by the waves
along the East Coast.

In the third place, further examination of the map will
disclose the important fact that by far the largest part of the
framework of the country consists of sedimentary rocks, that
is, of deposits of fragmentary or detrital materials which have
been laid down under water, and mostly under the sea. The
largest part of these materials is composed of mechanical detritus,
like the gravels, sands, and muds of our present shores and
sea-floor, and it now appears solidified into such rocks as
sandstones, grits, conglomerates, or shales. A considerable
proportion, however, consists of the remains of once living
calcareous organisms, such as shells, crinoids, and corals, and
is now found in the form of limestone. The forms of surface
produced by the mechanical sediments are well exhibited by
the grits, greywackes, and slates of Wales, which have been
greatly plicated and cleaved, and by the horizontal Millstone
Grit of the picturesque dales of Yorkshire. The scenery of
the limestones is typically displayed by the Chalk downs, by
the Mountain-limestone uplands of Derbyshire, and by the
Jurassic ridges of the Cotteswold Hills and the moors of
Eastern Yorkshire.

In the fourth place, the map clearly shows that while most

EXPLANATORY NOTES 7

of the country is composed of sedimentary materials, it con-
tains also many detached masses of igneous origin. These
are mainly confined to the western side of the island. Some
of them are portions of large bodies of eruptive material which
probably never reached the surface, but was intruded frorn
below into the crust of the earth and solidified there. The
granite tracts of Devon and Cornwall may be taken as ex-
amples of this type. But a large proportion of the igneous
rocks are of truly volcanic origin — that is, they prove that
volcanic action was vigorous where they now occur. They
include many varieties of lavas and ashes. As they are inter-
calated in successive stratified formations, they mark widely
separated periods of volcanic activity. The oldest volcanoes
of which the geological epoch can be fixed were those which
have left their lavas and tuffs or ashes in the Cambrian system
of North and South Wales. The most prolonged and ex-
tensive eruptions took place in the Silurian period, and pro-
duced the masses which now form the more conspicuous hills
of the Lake district, as well as Snowdon, Glyder, the Arans,
Arenig, Cader Idris, and many more heights in Wales. There
are no known volcanic rocks in the Old Red Sandstone of
England or Wales ; but they occur on a diminished scale in
the Devonian system of Devonshire, and in the Carboni-
ferous series of the Isle of Man, Derbyshire, Somerset, and
Devonshire. The latest English volcanoes appear to have
been those which erupted certain dark basic lavas in the
Permian period near Exeter and Crediton ; but in Tertiary
time some of the fissures which were then opened in Britain
stretched across the North of England, and gave passage to the
uprise of long dykes of dark lava. The most conspicuous of
these dykes, that of Cleveland, may be seen on the map ex-
tending from near the Yorkshire coast to near Carlisle.

The accompanying Table represents the succession of the
various geological formations of England and Wales, arranged
in stratigraphical order, the youngest being placed at the top
and the oldest at the bottom. With regard to the Igneous
Rocks, the three great groups of Basic (Serpentine, Basalts,
Dolerites, and Gabbro), Intermediate (Syenite, Diorite, Ande-
sites), and Acid (Rhyolites, Felsites, Quartz-porphyry, Granite,
and Granophyre), are distinguished on the Map.

8 GEOLOGICAL MAP OF ENGLAND AND WALES

Table of the Geological Formations of England
and Wales in descending order.

Blown sand.

Recent fluviatile alluvium and river-terraces.

Peat, now forming in some places, the oldest portions

possibly belonging to the Glacial Period.
Cavern deposits.
Submerged forests.

Recent estuarine and marine deposits.
Raised beaches.

Plateau gravels. Eskers. Coombe-rock. Erratics of Selsea.
Moraines of the higher hills.

Upper boulder-clays. Middle sands and gravels. Lower
boulder-clay.

Forest Bed group.

Weybourne and Chillesford Crags.

Red and Norwich Crag.

St. Erth beds.

Coralline or Suffolk Crag.

Lenham Beds.

Hamstead group.
Bembridge group.
Osborne group.
Headon group.

Upper Bagshot and Barton group.

Middle Bagshot and Bracklesham group.

Lower Bagshot group.

London Clay.

Oldhaven beds, Woolwich and Reading group.

Thanet Sand.

Upper Chalk with flints (Senonian).
Middle Chalk without flints (Turonian).
Lower Chalk (Cenomanian).
Upper Greensand.
Gault.

Lower Greensand (Speeton clay, &c., Neocomian).
Wealden (Speeton clay, &c., Neocomian).

Purbeck group.

Portland group.

Kimeridge group.

Corallian group.

Oxford Clay and Kellaways rock.

Great or Bath Oolite group.

Fuller's Earth.

Inferior Oolite.

Lias — Upper, Middle, and Lower.

Rhsetic group.
Keuper series.
Bunter series.

EXPLANATORY NOTES

:z ( Magnesian Limestone.

1 1 1 Red Marls, Sandstones, and Breccias.

Coal-measures.
Millstone Grit.
Carboniferous Limestone series.

Muldfe.
Lower.

Ludlow group.
Wenlock group.
Llandovery group.

Bala and Caradoc group.
Llandeilo group.
Arenig group.

Upper or Olenus series (Tremadoc slates, Lingula

Middle or Paradoxides series (Menevian).

Lower or Olenellus series (Harlach and Llanberis)

Longmyndian rocks.

Uriconian volcanic series (? Charnwood Forest).

Schists, &c. (Dalradian ?), of Anglesey, Caernarvonshire, &c.

PRE-CAM BRIAN (i).1 — In various parts of the country cer-
tain ancient rocks, which rise to the surface in the midst of much
younger formations, may be classed together as pre-Cambrian,
though they probably represent widely separated periods of
geological time. Of these the largest display is to be found in
Anglesey and the western borders of Caernarvonshire. They
consist of various gneissic and schistose rocks, including green
silky schists, like those of the Scottish Highlands, and large
lenticular masses of white quartzite. The quartzite of Holy-
head has yielded some worm-burrows, like those found in the
west of Sutherland and Ross. It is difficult to fix the true strati-
graphical position of these rocks ; but as they are covered
unconformably by unaltered Lower Cambrian strata, they must
be older than the Cambrian period. They evidently consist
in great part of extremely metamorphosed sedimentary mate-
rials. They include also various igneous intrusions, while some
of the green schists suggest that they may perhaps have been

1 The letters and figures placed after the names of the systems and for-
mations refer to the symbols employed to distinguish them on the map.

10 GEOLOGICAL MAP OF ENGLAND AND WALES

originally basic volcanic tuffs. Some of the more massive parts
of the igneous series to the east of Holyhead display in great
perfection the proofs of intense mechanical deformation. Cores
of the original rock may still be recognised amidst the crushed
material around them which has been converted into green schist.

On the eastern Welsh border, south and east from Shrews-
bury, a group of extremely ancient volcanic rocks has been
named Uriconian by Dr. Callaway. It forms a ridge on which
the Silurian and even the oldest Cambrian strata rest. Its
rocks are thus shown to be probably pre-Cambrian. They
consist of compact volcanic ashes and felsitic lavas, with rhyo-
litic structures, which point to some of the earliest eruptions
in the geological history of England and Wales.

Other small masses of pre-Cambrian rocks rise to the surface
in the Malvern Hills and at the Lizard Point, while a large
but much -obscured tract lies in Charnwood Forest, where
some striking volcanic agglomerates occur, probably of pre-
Cambrian age. The occurrence of these detached areas of
rocks that can claim so high an antiquity opens up some
interesting and difficult questions in the geological evolution
of the country. Where they appear from under the Cambrian
strata they were probably deeply buried beneath that system ;
but there would appear to have been some great terrestrial
movements during Palaeozoic time, whereby certain portions of
the pre-Cambrian platform were ridged up, and also enormous
denudation, by which these tracts had the whole of their over-
lying pile of sediment stripped off them. The Uriconian ridge
was exposed and buried again in Upper Silurian time. The
rocks of Charnwood Forest survived as a picturesque group of
peaks until they were entombed under the marls of the Triassic
waters. It is evident that the local disturbances during the
Palaeozoic periods must have been on a great scale, and that
while sedimentation went on with little interruption in certain
districts, it was interrupted by upheaval in others not far off,
and gave place to stupendous and prolonged denudation.

The thick mass of sedimentary strata forming the tract of
country known as the Longmynd was regarded by the Geolo-
gical Survey many years ago as of Cambrian age ; but more
recent investigation tends to place it below the Cambrian
system, and above the Uriconian volcanic series.

CAMBRIAN (a). — This great system of rocks occupies a
comparatively small space at the surface. It appears in

EXPLANATORY NOTES II

Anglesey, in Caernarvonshire and south-west Pembrokeshire,
emerging from underneath the overlying conformable Silurian
strata. It is seen also in a small tract at the Malvern Hills,
in Shropshire, and in Warwickshire. Probably a large part of
the Skiddaw Slates, which cover much of the northern part
of the Lake District, and reappear in the Isle of Man, is
to be referred to this ancient division of the geological
record.

The Cambrian system consists mainly of mechanical sedi-
ments, which in the lower part are somewhat coarse in texture,
forming grits and conglomerates, but which pass upward into
finer sandstones and shales or slates. It is usually grouped in
three divisions, each distinguished by a characteristic form of
trilobite. The lowest or Olenellus series has been determined
by Professor Lapworth to occur in the Shropshire area, where
it consists of thin quartzite passing up into flags, grits, shales,
and sandstones, and where it has yielded the characteristic
Olenellus. . In that region it is overlain with conglomerates
and limestones containing Paradoxides, forming the Middle
Cambrian group. The Upper group, composed of the Shineton
shales, has afforded specimens of Olenus, Dictyograptus, &c.
The total thickness of these three groups in the Shropshire
region is estimated at about 3000 feet. In Wales, however,
the system attains considerably greater dimensions, reaching
perhaps to 12,000 feet or more. The Lower group in the
Principality consists of massive sandstone, grits and conglome-
rates (Harlech, Llanberis), which lie upon and pass down into
a volcanic platform, consisting of tuffs, diabases, and quartz-
felsites (" Pebidian " of Dr. Hicks). The Middle or Menevian
group is well developed in South Wales, where along the coast
of Pembrokeshire its sandstones, shales, and slates, about 600
feet thick, have furnished a number of fossils, including the
typical Paradoxides. The Upper group consists of two divi-
sions. At the bottom, and passing down into the Menevian
strata, lie the Lingula Flags — bluish and black slates and flags,
which took their name from the abundance of a lingula (Lin-
guldla Davisii) in them. They are believed to sometimes
exceed 5000 feet in thickness. The Tremadoc slates, well
developed in the district from which they take their name, and
where they are about 1000 feet thick, contain a number of
trilobites, among which the characteristic Olenus occurs. The
top of the Cambrian system of Wales is now generally drawn
at the summit of the Tremadoc group, but there is no abrupt

12 GEOLOGICAL MAP OF ENGLAND AND WALES

break at this horizon, the Cambrian sediments passing gradu-
ally upward into the Silurian series.

SILURIAN (b). — This system occupies a large part of
Wales. Sinking underneath younger formations, it rises again
to the surface farther east, and appears in a number of de-
tached areas from the head of the estuary of the Severn north-
wards through Gloucestershire, Herefordshire, Worcestershire,
and Staffordshire ; but it no doubt extends under the Old Red
Sandstone and Carboniferous rocks across the whole of the
North of England, for it ascends once more to the surface
in Westmoreland and Cumberland, where it forms the hilly
ground of the Lake District. It reappears in the Isle of Man,
and extends under the Irish Sea into Ireland on the west, and
into Scotland on the north.

The Silurian rocks of England and Wales are divided into
two portions, Lower and Upper, each of these being further
grouped into separate formations, distinguished from each
other partly by lithological characters, but chiefly by their
typical organic remains. One distinguishing feature of these
formations is the presence in them of abundant Graptolites,
which vary both in genera and species from bottom to top of
the system. Double graptolites are specially found in the
Lower Silurian deposits, and single graptolites in the Upper.
The whole system has been arranged in zones, each marked
by the presence of some distinctive graptolite. Trilobites are
likewise abundant, and may be made use of to distinguish the
several formations.

I. The Lower Silurian formations are three in number —
Arenig, Llandeilo, and Bala or Caradoc.

(i.) The Arenig formation (b1) may reach a thickness of 4000
feet. It consists of dark shales, slates, flags, and sandstones,
which are typically developed in and around Arenig Mountain.
A prominent feature of this division of the system is the occur-
rence of abundant intercalated volcanic rocks, which prove
that while the Arenig sediments were deposited there were
active submarine volcanoes over the site of what is now the
heart of Wales. The lavas and tuffs of these eruptions rise
into conspicuous eminences, such as Arenig Mountain, the
Arans, Cader Idris, and others.

(2.) Th» Llandeilo formation (b2), sometimes perhaps 3000
feet thick, comprises dark argillaceous, sometimes calcareous
flagstones, sandstones, and shales, and was first named by

EXPLANATORY NOTES 13

Murchison, from its development about Llandeilo in Caermar-
thenshire. It extends into Pembrokeshire, and rises to the
surface in the Lower Silurian inlier of Builth.

(3.) The Bala or Caradoc rocks (b3) have been computed
to reach a thickness of 6000 feet where fully developed. In
North Wales they consist of grey and dark slates, grits, and
sandstones, with two subordinate seams of limestone, and
attain a great development around Bala, in Merionethshire,
where they were first studied by Sedgwick. In that region
they include a vast mass of contemporaneously erupted vol-
canic rocks which tower into some of the finest scenery of
North Wales. Snowdon and the surrounding mountains con-
sist largely of these volcanic materials.

In the district of Caer Caradoc, Shropshire, the corre-
sponding strata consist of yellowish and grey sandstones. In
the Lake District an enormous mass of volcanic material,
estimated by some observers at 10,000 or 12,000 feet in thick-
ness, occupies the place of the Bala and Llandeilo rocks
below a limestone (Coniston), which from its fossils is regarded
as the equivalent of the Bala limestone of Wales.

II. The Upper Silurian formations are likewise three in
number — Llandovery, Wenlock, and Ludlow.

(i.) The Llandovery rocks (b5) consist of a lower group of
grey grits, from 600 to 1500 feet thick; a central group of
yellow and brown ferruginous sandstones (May Hill sand-
stone), about 800 feet thick, resting unconformably on the
older members of the system ; and an upper group of fine
smooth grey or blue shales (Tarannon shale), having an extreme
thickness of 1000 to 1500 feet.

(2.) The central division of the Upper Silurian series or Wen-
lock group (bc), in the typical Silurian district of Murchison,
consists of three distinct groups of strata— a lower limestone
and shale (Woolhope), well displayed in the valley of Woolhope,
Herefordshire, where it is about 30 or 40 feet in thickness ;
a central mass of grey and black shales (Wenlock Shale), trace-
able for ninety miles from the Severn near Coalbrookdale to
Caermarthen, and in the northern part of its course attaining
a depth of more than 2000 feet ; and an upper calcareous
deposit, known as the Wenlock Limestone — a thick-bedded,
flaggy* and concretionary rock, from 100 to 300 feet thick,
and abounding in fossils, particularly in corals. In North
Wales the soft shales and limestones of the Wenlock group
give place to hard grits, flags, sandstones, mudstones, and

14 GEOLOGICAL MAP OF ENGLAND AND WALES

shales (Denbighshire Grits), which sometimes reach a united
thickness of at least 3000 feet.

(3.) The Ludlow group (b7), in the typical district of Siluria,
is essentially composed of shales, with occasionally a central
band of limestone (Aymestry), the whole having an aggre-
gate thickness of about 1000 feet. The highest member of
the group consists of a band of fine yellow, red, and grey
micaceous sandstones (tilestones), which form a passage into
the overlying Old Red Sandstone. These rocks are best
seen in the Herefordshire region, where they were studied
by Murchison.

The remarkable change into hard, cleaved, and plicated
rocks which is observable when the Upper Silurian formations
are followed, even into North Wales, continues to be observ-
able northwards across the Lake District into the south of
Scotland. Nevertheless the stratigraphical position of the
northern Silurian formations is satisfactorily fixed by the occur-
rence of marked zones of graptolites. The Upper Silurian
formations are developed in the eastern and northern tracts
of Wales and in the southern part of the hills of the Lake
District.

OLD RED SANDSTONE AND DEVONIAN (c). — Along the
eastern border of Wales the highest member of the Upper
Silurian series passes upward conformably into the red strata
which form the base of the Old Red Sandstone — a vast
succession of red rocks which, stretching from Shropshire
across South Wales, attains a thickness of probably not less
than 10,000 feet. The lower parts of this system of red sedi-
ments consist of red and green shales and flagstones, with
sandstones and thin cornstones. The central and main por-
tion is made up of red and green sandy marls and clays, with
red sandstones and cornstones, the higher members consisting
of grey, red, brown, and yellow sandstones, and bands of
conglomerate, pass upward conformably into the base of the
Carboniferous system. In general the Old Red Sandstone of
England and Wales is barren of organic remains. Its lower
and central parts have yielded remains of fossil fishes (Cephal-
aspis, Pteraspis, &c.), while the higher portions have furnished
other genera of fishes (Pterichthys, Holoptychius) and remains
of land-plants.

In North Wales and the northern counties of England the
narrow belts and patches of Old Red Sandstone which appear

EXPLANATORY NOTES 15

between the Silurian and Carboniferous formations belong to
the Upper division of the system. They pass upward into the
Carboniferous strata above them, and lie with a violent uncon-
formability on the Silurian rocks below. In these regions,
therefore, a large part of the stratigraphical series is wanting,
which is supplied in South Wales and Shropshire.

The Devonian system in England is confined to Devon
and Cornwall. Though it presents a strong contrast to the
Old Red Sandstone, both in its lithological and its palseonto-
logical characters, it is regarded as the geological equivalent of
that system. The Old Red Sandstone appears to have ac-
cumulated in a series of lakes or inland seas. The Devonian
formations, on the other hand, are shown by their included
fossils to have been deposited in the sea. They are generally
grouped in three sections, named Lower, Middle, and Upper.

The Lower Devonian group consists of slates, grits, and
greywackes, seen at Cockington and elsewhere near Torquay,
and at Foreland and Lynton in North Devon. The Middle
group is best displayed in South Devon, where it consists
largely of massive fossiliferous limestones (Torquay, Plymouth),
which pass laterally into volcanic rocks (Ashprington). In
North Devon this group is represented by the grey silvery
slates of Ilfracombe, and the limestone grit and slate of Combe
Martin. The Upper Devonian rocks consist of red and grey
slates and grits sometimes with a fossiliferous limestone (Chud-
leigh). These strata pass upward into the so-called Culm-
measures, which belong to the Carboniferous system. The
chief horizons for Devonian fossils are to be found in the
limestones of Torquay and Plymouth, which have furnished a
large series of corals, together with trilobites and brachiopods.

CARBONIFEROUS (d1'5). — This important geological system
is extensively developed in England and Wales. It forms a
broad tract of country, extending from the borders of Scotland
southward through the Pennine Chain till it sinks below the
plain of the Midlands at Derby. In a broken fringe it encircles
Wales, from Anglesey to the mouth of the Dee, and thence
southward into Monmouthshire, where it spreads out into the
high tableland of the South Wales coal-field, and stretches to
the western shores of Pembrokeshire. It rises in a number of
detached tracts in the centre and south-west of England, and
covers a wide space in Devon and Cornwall. It is divided
into three main series of strata, each of which is distinguished

16 GEOLOGICAL MAP OF ENGLAND AND WALES

on the map — Carboniferous Limestone, Millstone Grit, and
Coal-measures.

(i.) The Carboniferous Limestone (d1"3, formerly known as
the Mountain Limestone) forms conspicuous topographical
features in some parts of the country where it is well developed.
It surrounds the South Wales coal-field, where it passes down
into the Old Red Sandstone. In the Pennine Chain it attains
the great thickness of 4000 feet, yet its base is not there seen.
Spreading out over the high grounds of Derbyshire, Yorkshire,
and Lancashire, its gently inclined undulating strata form a
striking type of scenery, projecting to the surface in sheets and
knobs of bare pale stone, and winding in white escarpments
along the green slopes of the valleys. Even where it does not
spread so continuously over the ground, it sometimes reaches
a great development, as in the ridge of the Mendip Hills,
where it is 3000 feet thick. It consists of massive, well-bedded
limestone, composed of the remains of calcareous organisms.
Its fossils may often be seen projecting in enormous numbers
from exposed surfaces of the rock, where they have been
etched out by the weather. Traced northward, the limestone
is found to be more and more split up with intercalations of
shale and sandstone, which contain land-plants. We may infer
that, while a tolerably deep sea extended over the south of
England, a land clothed with vegetation lay somewhere to the
north, probably where the Scottish Highlands now rise, and
that from this terrestrial surface the sand, mud, and drifted
plants were derived. Some remnants of submarine volcanoes
occur in the Carboniferous Limestone of Derbyshire, where a
series of basic lavas and tuffs (toadstone) is associated with
necks of agglomerate. At the south end of the Isle of Man,
near Weston-super-Mare, and in the Culm-measures of Devon-
shire, other volcanic intercalations have been found.

The Millstone Grit (d4) comprises a persistent group of
grits and sandstones, with shales and clays, which overlie
and pass down into the limestone series below, and shade
upward into the coal -bearing series above. These strata
are from 400 to 1000 feet thick in South Wales, but in
North Staffordshire they have been estimated to be 4000
and in Lancashire 5500 feet in thickness. In Yorkshire and
Lancashire, where they are nearly flat, their successive beds
wind along the sides of the dales as long regular lines of
terrace.

The Coal-measures (d5) are made up of frequent alterna-

EXPLANATORY NOTES 17

tions of sandstones, shales, fireclays, coal-seams and ironstones,
and are distinguished as the chief repositories of workable
coal. In Lancashire they reach a thickness of 8000 feet ; in
South Wales they are 12,000 feet thick; but the true top of
the system is probably nowhere now to be seen in England,
having been removed by denudation, which appears to have
begun before the Permian period. The detached areas into
which, by movements of the terrestrial crust and by denuda-
tion, the Coal-measures have all separated, are known as
coal-fields. It will be seen from the map that on either side
of the Pennine Chain a series of coal-fields lies upon the Mill-
stone Grit, which dips on either side from the crest of that
anticlinal fold. These coal-fields are overspread by younger
formations, beneath which they are prolonged, so that the coal-
seams can be followed and worked far beyond the visible
limits of the coal-fields at the surface. In South Wales, on
the other hand, the margin of the coal-field is formed by the
underlying Millstone Grit, so that no coal can be found beyond
the actual visible margin of the field. In the centre of
England some of the coal-fields rest directly on ancient rocks,
without the intervention of any Millstone Grit, Carboniferous
Limestone, or Old Red Sandstone. In Warwickshire the
Coal-measures lie on Cambrian strata, and in Leicestershire
on still older rocks. There would thus seem to have been
an ancient ridge which ran through the centre of the country,
and was not submerged until the Coal-measures were deposited.
Recent boring operations have revealed the existence of Coal-
measures underneath the Secondary formations of the south-
east of Kent. It is possible that these Carboniferous rocks
extend across the south of England and join the Bristol
coal-field.

PERMIAN (e).— -The highest member of the series of Palaeo-
zoic systems is known as the Permian. It is found flanking
the Carboniferous rocks along both sides of the Pennine Chain,
continuously on the east side, and in disconnected strips on the
west side. It appears also overlying the Coal-measures along
the Welsh border and in the central counties, and it extends
in a belt across the peninsula of Devonshire.

Two distinct types of the system are observable in England.
On the west side of the island, from the Solway Firth to the
coast of Devonshire, it consists mainly of red and brown sand-
stones, with conglomerates and breccias (e), which in Cumber-

18 GEOLOGICAL MAP OF ENGLAND AND WALES

land reach a thickness of 3000 feet. In Devonshire these
strata include the traces of some contemporaneous volcanoes
in the form of thin sheets of lava. To the same period of
eruption may possibly belong the various basic sills which in
the Midlands and on the Welsh border have been intruded
into the Coal-measures.

The eastern type is quite different, and resembles that of
Germany. It displays a mass of Magnesian Limestone or dolo-
mite (e2"4), about 600 feet thick, underlain by some red and
variegated sandstones, hard brown shale (Marl Slate), and thin
limestones, and overlain by a thin group of sandstones, clays,
and gypsum, or anhydrite. There is a general dearth of or-
ganic remains in the Permian rocks. The Magnesian Lime-
stone is the chief fossiliferous repository. This formation is
best seen along the coast-line south of Tynemouth, where it
forms a range of picturesque cliffs.

TRIASSIC (f). — At the base of the great series of Second-
ary or Mesozoic formations lies the Trias, which, consisting,
like the Permian, mainly of red strata, was formerly classed
with that system as the New Red Sandstone. It occupies a
tolerably continuous area, which, beginning near Gloucester,
spreads over the great plain of the Midlands, and then divides
into two arms, one of which stretches up the west side of the
Pennine Chain to Morecambe Bay, while the other sweeps
along the eastern base of that ridge to the sea at the mouth
of the Tees. A detached area lies in the north-west of Cum-
berland, extending to the shores of the Solway Firth and
crossing into Scotland. Another series of patches extends
from the head of the Severn estuary across to the southern
coast of Devonshire.

The Trias consists of three divisions — (i) Lower or Bunter;
(2) Upper or Keuper; and (3) Rhaetic.

'(i.) The Bunter series (f1'3) is made up of three groups of
strata, having a united thickness of 1000 to 2000 feet. At
the bottom lie soft bright-red and variegated sandstones, from
80 to 650 feet thick. In the centre a group of harder reddish-
brown pebbly sandstones and conglomerates, from 60 to more
than 1000 feet in thickness, is known as the Pebble beds.
While at the top another series of bright -red and mottled
sandstones reaches a depth of from 200 to 700 feet.

(2.) The Keuper series (f5'6) is composed of a lower group
of red, white, and brown sandstones and marls (water-stones),

EXPLANATORY NOTES ig

from 150 to 250 feet thick (f5), and of an upper group of red
and grey shales and marls (f°), which form the most important
member of the Trias. They range in thickness from 800 to
sometimes as much as 3000 feet, and are particularly distin-
guished by including beds of rock-salt and gypsum. The salt-
works of Cheshire derive their supplies of brine from this group.
(3.) The Rhaetic formation (fg, Penarth Beds) is made up
of red, green, and grey marls, black shales, and pale limestone
(" White Lias "), which are often only a few yards and rarely
reach 150 feet in thickness. Yet in spite of its insignificant
proportions this group of strata runs with singular persistence
throughout England and Wales. One of its most interesting
seams is the " bone-bed " — a ferruginous and micaceous sand-
stone containing remains of fishes and saurians.

JURASSIC (g). — Under this name are comprised the various
strata which led William Smith, in the neighbourhood of Bath,
to the discovery that the sedimentary rocks of England could
be identified from one district to another by means of their
enclosed organic remains. The sub-divisions made by him
are still on the whole retained, together with the local provincial
names which he attached to them. Four chief divisions are
recognisable, each of these being separated into minor groups.
They range across England from the coast-line of Yorkshire
to that of Dorset, and form some of the most characteristic
scenery of the country. In the north they rise to heights of
more than 1400 feet above the sea, and form the broad moor-
lands south of Middlesborough. They mount also in the south
into the range of the Cotteswold Hills. But throughout most
of their course they form a succession of low ridges and inter-
vening plains, the former marking the outcrop of the limestones
and other harder members of the series, while the latter indi-
cate the position of the softer shales and clays.

At the bottom of the whole series lies the (i.) Lias (g1'4),
which is composed of three tolerably well-marked groups of
strata. The lowest of these, known as the Lower Lias (g1),
consists of numerous thin blue and brown limestones with
partings of dark shale, having a total maximum thickness of
900 feet. In the centre comes the Marlstone (g2), or Middle
Lias (350 feet), composed of various limestones with under-
lying sands and clays, and sometimes, as in the Midlands and
in Yorkshire, including valuable ironstones. The Upper Lias
(g3"4. 4°° feet) 's made up of shales and clays, with nodular

20 GEOLOGICAL MAP OF ENGLAND AND WALES

limestones passing up into sands. Numerous palaeontological
zones have been recognised in the Lias, each of which has
received a name from the characteristic ammonite which it con-
tains. Ten such zones have been separated in the Lower Lias.

(2.) The Lower Oolites. This series contains three chief
sub-divisions. At the bottom the Inferior Oolite (g5, 260 feet),
in the south-western and central counties, consists mainly of
shelly (marine) limestones with clays and sandstones, but as it
is followed northward, increasing evidence of fresh-water and
terrestrial conditions are traceable, until in Yorkshire the strata,
swelling out to a thickness of 800 feet, are composed chiefly
of sandstones, together with shales and seams of coal and iron-
stone. Next in order comes the argillaceous deposit known
as the Fuller's Earth, which is only found in the southern and
south-western counties, where it attains a thickness of nearly
150 feet. The uppermost group of the Lower Oolites is well
developed around Bath, whence it has been called Bathonian
(g6'8). In Gloucestershire and Oxfordshire it displays three
sub-groups of strata, of which the lowest consists of thin-bedded
limestones and sands (Stonesfield slate), the middle of shelly
limestones (Great Oolite), and the uppermost of clays (Brad-
ford clay), shelly limestone (Forest marble), and earthy lime-
stone (Cornbrash). Of these sub-groups, the most persistent
is the Cornbrash (g9), at the top of the whole. Varying from
5 to 40 feet in thickness, this rock extends continuously into
Yorkshire.

(3.) The Middle or Oxford Oolites (g10'11) comprise two
well-marked formations. The lower of these consists of a local
calcareous sandstone (Kellaways Rock) and of a stiff blue
and brown clay, well developed in Oxfordshire, whence it has
received the name of Oxford Clay (g10). It ranges from 300
to 600 feet in thickness. The Upper or Corallian group
(g11, 250 feet) consists of limestone and calcareous grits, with
occasional clays, traceable with local modifications across the
country from the coast of Dorset to that of Yorkshire. It
received its name from the abundance of corals in its rubbly
limestone (Coral Rag).

(4.) The Upper or Portland Oolites (g1214) are divisible
into three groups. At the bottom comes the Kimeridge Clay
(g12, 600 feet), well developed on the Dorsetshire coast, whence
it can be followed into Yorkshire. In the centre lie the Port-
land Beds (g13), consisting of marls and sands towards the
base and limestones above, one of which is the well-known

EXPLANATORY NOTES 21

"Portland Stone," so largely used as a building material.
The highest group (Purbeck Beds, g", 350 feet) is best seen
in the Isle of Purbeck, where it consists chiefly of fresh-water
limestones and clays, including layers of soil with the stumps
of the trees which grew in them, but the central part of the
group contains marine shells. These strata have long been
celebrated for the mammalian remains which they have
yielded.

CRETACEOUS (h1'5). — This system of formations occupies
a well-defined area in the south and east of England. Cap-
ping the hills of eastern Devonshire it stretches eastward to
the coasts of Sussex and Kent From this southern band it
diverges at the broad tract of Salisbury Plain, and continues
in another band north-eastwards to the shores of Norfolk.
Beyond the Wash it reappears in Lincolnshire, crosses the
Humber, and stretching northward into the Wolds of York-
shire, is finally cut off in the sea-cliffs of Flamborough Head.

The Cretaceous system is divided into two main sections,
the Lower or Neocomian and the Upper. The Lower Cre-
taceous rocks present two distinct types in England. In the
southern counties they consist of a lower delta-formation
known as the Wealden, surmounted by some marine sandy
strata termed the Lower Greensand (h2). The Wealden group
has at its base the Hastings sands and clays (h), which pass
down into the Purbeck group. Above these comes the thick
deposit of Weald Clay (h1, 1000 feet) which forms a conspicuous
feature in the geological map of the south-east of England.
It represents the delta of an ancient river, and, like modern
delta-deposits, contains the remains of land plants and of
terrestrial animals (in this case deinosaurian reptiles), together
with fresh-water shells. In Yorkshire the " Speeton Clay "
consists of marine clays and shales, which in their lower part
are equivalents of the Kimeridge Clay of the south of England,
but which in their higher parts contain true Neocomian or
Lower Cretaceous fossils, while at the top they may even
belong to the higher division of the Cretaceous system.

The Upper Cretaceous series is divisible into three well-
marked groups of strata, distinguished from each other alike
by lithological characters and fossil contents. The lowest
group (h3) is known as the Gault (100 to 300 feet), a stiff
dark-blue clay, with thin seams of pyritous and phosphatic
nodules and occasional seams of green sand. This deposit

22 GEOLOGICAL MAP OF ENGLAND AND WALES

passes upward into certain sandy strata, often greenish in
colour, which have long been called Upper Greensand (h4).
Under this t£rm, however, have been included strata now
known to be equivalent to the Gault. The Upper group of
the system is composed mainly of the soft pulverulent lime-
stone called Chalk (h5), and forms topographically by far the
most conspicuous member of the whole system to which it
gave its name. It is this formation which rises into the undu-
lating area of Salisbury Plain, and stretches thence eastward
into the bare, smooth, treeless ranges of the North and South
Downs, and which reappears with the same kind of features in
the Wolds of Yorkshire. The Chalk can be separated into
three divisions — Lower (Cenomanian), consisting of the Glau-
conitic Marl, Chalk Marl, and Grey Chalk ; Middle (Turonian),
composed of white chalk without flints ; and Upper (Senonian),
also made of white chalk, but distinguished by the abundance
of its black flints.

The Cretaceous system of England has undergone consider-
able disturbance in the southern counties, having been thrown
into a series of undulations which greatly affect its distribution
at the surface. Thus, as illustrated in the horizontal sections
on the map, the North and South Downs form the two sides of
a broad arch, from the crown of which the Chalk has been
entirely removed so as to reveal the Lower Cretaceous series
in the broad plain of the Weald. The denudation thus in-
dicated is likewise conspicuous along the western outcrop.
The fringe of detached patches of Chalk on that line are
remnants of a once continuous sheet of Chalk that formerly
stretched far to the west of the present limits of the formation.

The TERTIARY formations of England are almost entirely
confined to the south-eastern counties, where they occupy two
large districts. One of these, known as the London basin,
covers a wedge-shaped area, which, beginning in Wiltshire,
broadens out towards the east, keeping along the base of the
North Downs to within a few miles from the North Foreland,
and stretching north-eastward to the coast of Suffolk. The
other and much less extensive district, called the Hampshire
basin, is separated from that of London by a broad uprise
of the Chalk. Originally the Tertiary deposits were probably
continuous across the whole of the south-east of the country.
They have participated in the plications which have folded the
Cretaceous system and in the subsequent denudation. The

EXPLANATORY NOTES 23

extent to which they have been wasted is impressively indicated
by the numerous outliers scattered over the surface of the Chalk.
The English Tertiary formations have been divided into three
groups — Eocene, Oligocene, and Pliocene.

The EOCENE group (i1'7), as developed in the London basin,
differs in some degree from the type which it assumes in
Hampshire. At its base lies the Thanet Sand, not found in
Hampshire, and best seen in the eastern part of the London
basin. Next comes a variable series of plastic clay, loam, sand,
and pebble-beds, called the Woolwich and Reading, and the
Oldhaven Beds ( i1"2), succeeded by the chief member of the
Eocene group — the London Clay (i3), a stiff brown and bluish-
grey clay, with septaria of earthy limestone, which in Essex
attains a thickness of about 500 feet. This deposit is, in the
London basin, surmounted by the Bagshot Sands, but in Hamp-
shire and the Isle of Wight by a different and much thicker
succession of deposits (i4"7), the most interesting of which are
the Bracklesham Beds of the Sussex coast, long noted for the
abundance of their marine organisms. The highest Eocene
strata in the London basin are the uppermost Bagshot Sands ;
but in Hampshire they consist of clay (Barton), which on the
coast and in the Isle of Wight is 300 feet thick, and is remark-
able for the great number and excellent preservation of its
shells. The Barton Clay is overlain by from 140 to 200 feet
of pure siliceous sand, used for glass-making, which passes
upward into the base of the Oligocene series.

The OLIGOCENE strata (i8'11) of England occur only in
the Isle of Wight and part of the opposite county of Hamp-
shire. Resting conformably on the Eocene series, they consist
of thin seams of sand, clay, marl, and limestone, which may reach
a maximum thickness of 800 feet. They are divided into four
groups, each of which is named from the locality in the Isle of
Wight where it is typically exhibited. The lowest, called the
Headon group, consists of two series of fresh-water deposits,
separated by a middle stage, containing brackish water and
marine fossils. The Osborne Beds are likewise full of fresh-
water shells. The Bembridge group consists of two stages,
the lower composed of limestone, containing abundant land
and fresh-water shells, the upper presenting a thicker succes-
sion of marls, of fresh-water, estuarine, and marine origin.
The Hamstead Beds were chiefly deposited in fresh-water,

24 GEOLOGICAL MAP OF ENGLAND AND WALES

estuarine, or lagoon conditions, but their uppermost visible
strata were accumulated in the sea. Denudation has, however,
removed all the deposits which may originally have covered
these strata, and nothing has been left anywhere in England
to enable us to fill in the gap thus made in the geological
record.

Next in order of age come the PLIOCENE formations, there
being no representative in England of the Miocene series.
The English Pliocene is separable into two divisions, called Older
and Newer. The Older series (k1) comprises the White, Suf-
folk, or Coralline Crag, which covers a considerable space in
the east of Suffolk, where it consists of shelly sands, containing
about 60 per cent, of still living shells. On the southern edge
of the North Downs, near Lenham in Kent, patches of sand,
which lie on the Chalk and descend into pipes in that rock,
contain such an assemblage of shells as to prove them to be
of Pliocene age. Fragmentary though they are, they possess
much interest, inasmuch as they point to the submergence of
the south-east of England during older Pliocene time to the
extent of perhaps 860 feet below its present level.

The newer Pliocene groups (k2) include the Red and Nor-
wich Crag, specially developed in Norfolk and Suffolk, and
consisting of red and brown shelly sands and gravels, in which
the proportion of extinct shells is about sixteen, the Chilles-
ford Beds and the Forest Bed group. The fossils preserved
in these deposits indicate a gradual lowering of the climate of
Britain during Newer Pliocene time. Arctic species of shells
begin to make their appearance in the Red Crag, and Medi-
terranean forms rapidly diminish from the Coralline Crag
upwards. The Forest Bed group contains a flora that indicates
a mild and moist climate ; but immediately above it lie layers
of sand, loam, and gravel, with marine Arctic shells and a
fresh- water deposit containing mosses, dwarf -willows, and
birches, and pointing to a temperature as severe as that of
the North Cape.

Above the Pliocene, and covering a far more extensive pro-
portion of the surface of England and Wales, come the various
kinds of Drift — boulder-clays, gravels, sands, moraines, and
other detrital accumulations which record the passage of the
Ice Age. These superficial deposits are not shown upon the
map, because to insert them would in large measure obscure
the underlying geology of the country. In some districts, as,

EXPLANATORY NOTES 25

for instance, in large tracts of East Anglia, they entirely con-
ceal the rocks underneath. They fill up the valleys, and even
ascend to great heights among the hills.

Of the more recent deposits only two varieties are repre-
sented on the map — alluvium and blown sand. The alluvial
tracts along the river- valleys are shown by a special tint, which
serves to indicate some of the chief drainage lines of the
country.

NOTES ON THE HORIZONTAL SECTIONS

I. Section across England and Wales from Holy head to
Beachy Head. — This Section is drawn across the general strike
of the formations from north-west to south-east, and represents
the stratigraphical succession from the pre-Cambrian rocks
up to the top of the Mesozoic series. In Anglesey the most
ancient schists and quartzites are succeeded by Carboniferous
and Permian strata. Beyond the line of the Menai Strait the
Cambrian strata (which in Anglesey lie unconformably on the
schists) reappear with their underlying sheets of porphyry and
tuff, and pass under the Lower Silurian series in the valley of
Llanberis. There is on the whole an ascending section through
the Arenig and Bala rocks to the top of Snowdon, and in the
upper part of the series a great thickness of volcanic material
is included, together with massive intrusive sills. By another
vast fold of the terrestrial crust, the Arenig series is once more
brought up to the surface with its volcanic intercalations in the
ridge of Moel Wyn, while the Cambrian .series is likewise ex-
posed in a broad anticline. The Arenig rocks roll over to the
south-east, and with their volcanic sheets form the picturesque
ridge of Arenig Mountain, whence they plunge eastwards under
the Llandeilo and Bala groups. By a series of plications,
Lower and Upper Silurian rocks are made to succeed each
other, and to spread over the wide tract of country to the
valley of the Severn. East of Chirbury the Lower Silurian
lavas and tuffs, with the Arenig strata on which they rest,
rise to the surface and are accompanied by the great sill of
Corndon. Eventually the Cambrian quartzites make their
appearance in the ridge of the Stiper-stones, followed by the
underlying pre-Cambrian rocks of the Longmynd ridge. Some
of the enormous dislocations of this region are indicated on

26 GEOLOGICAL MAP OF ENGLAND AND WALES

the Section by the two faults which let down the wedge of
Wenlock and Ludlow strata between the Longmynd and the
Uriconian rocks of Hope Bowdler or Caer Caradoc.

From this part of the Section an ascending succession of
strata can be followed through the Upper Silurian formations
into the Old Red Sandstone, which is seen to undulate for
many miles to the east. On Brown Clee and Titterstone Hills
outliers of Coal-measures have been preserved under intrusive
sheets of basalt, and a larger tract of the same formation,
spreading out into the Forest of Wyre Coal-field, lies on the
Old Red Sandstone and extends to the vale of the Severn
near Worcester. Here the Section enters the Mesozoic series
of strata. First come the Keuper and Bunter groups, fol-
lowed by the thin band of the Rhaetic deposits, which crops
out to the east of Worcester and dips under the Lias. Their
gentle inclinations enable the strata to spread out over wide
areas. Outliers of the Lower Oolites are seen capping the
Lias many miles in advance of the main outcrop of these
groups. The members of the Jurassic series are successively
traversed, until to the south-east of Oxford, the Cretaceous
system sets in with its prominent escarpment of Chalk rising
above the Gault and Upper Greensand. The London Ter-
tiary basin is shown by the descent of the Chalk below the
Tertiary deposits on the one side and its uprise from beneath
them on the other. The angle of inclination, however, is so
gentle that, if drawn on a true scale, the strata would seem to
be nearly horizontal. On the south side of the basin the Chalk
mounts into the ridge of the North Downs, and presents another
bold escarpment to the great plain of the Weald, where the
Wealden groups are exposed in a broad anticline. The ridge
of the South Downs is formed by the outcrop of the Chalk on
the south limb of the fold. There cannot be any doubt that
the Chalk was once continuous across the Weald, and that its
disappearance is entirely owing to prolonged denudation.

II. Section across the centre of England from the borders
of Wales at Denbigh over the Cheshire Plains, the Derbyshire
Hills, and the plains of the Trent, to the coast of Lincolnshire
at Saltfleet. — At the west end of this Section the Denbighshire
grits of the Upper Silurian series are seen to be uncon-
formably covered by the Upper Old Red Sandstone and
Carboniferous Limestone, against which the Trias of the
vale of Clwyd is let down by faults on either side of the
depression. On the east side the three members of the

EXPLANATORY NOTES 27

Carboniferous system are seen to dip away from the Silurian
ridge under the Triassic plains of Cheshire, which stretch
eastwards to the base of the broad Pennine Chain. The
great denudation of the Carboniferous rocks is shown by the
outliers of Millstone Grit and Coal-measures left detached
on the hill-tops. East of Buxton the Carboniferous Lime-
stone appears, with its bands of volcanic loadstone, and after
undulating for some miles finally slips below the Yorkshire
coal-field. Still farther east the Permian series dips under
the Triassic groups, which occupy the low plain of the Trent,
until they pass under the Jurassic series. From the fine
escarpment at Lincoln that series sinks gently eastward under-
neath the Cretaceous system, which extends to the coast.

III. Section from the Solway across the Lake District and
the Pennine Chain to the coast of Yorkshire at Flamborough
Head. — This Section explains the structure of the ground in
the north of England. At the west end the Trias rises from
under the Solway Firth, and is faulted against the Whitehaven
coal-field, which in turn has been let down by a dislocation
on the east side. From under the unconformable Car-
boniferous strata, the Lower Silurian volcanic rocks are seen
to emerge, and to be followed by the great mass of Skiddaw
slate, with its intrusive granite. After various folds and
fractures the rocks roll over to the south-east, and the
volcanic series again makes its appearance, and swelling out
to its maximum thickness, occupies the high grounds of the
Lake District, until it dips below the Coniston or Bala Lime-
stone. Then come the Upper Silurian formations, which
extend eastwards for many miles as the platform on which
the Carboniferous system there reposes. The Section affords
an illustration of the great denudation of that system, Ingle-
borough and Penygent remaining as colossal monuments of
the amount of material removed. It will be seen that some
of the valleys have been excavated completely through the
thick Carboniferous series, and have laid bare the Silurian
platform below. Towards Ripon, the Permian band sets
in, and is soon overspread by the Trias of the plain of the
Ouse, which in turn dips under the Jurassic system of the
East Yorkshire hills. . When the Section reaches the uplands
of the Wolds it enters the district of the Chalk, which covers
all the rest of the ground until it ends abruptly at the line
of precipice that culminates at Flamborough Head.

IV. Section across the Isle of Wight. — This Section is drawn

28 GEOLOGICAL MAP OF ENGLAND AND WALES

in a north and south direction across the Isle of Wight, on a
much larger scale than the others, with the view of showing
the succession of the Tertiary and Cretaceous formations of the
south of England. At the north end, the four groups of the
Oligocene series are seen to occupy the low ground stretching
from the Solent to the chalk downs in the middle of the island.
At the top lie the Hamstead Beds, with their denuded upper
surface; underneath them comes the Bembridge group of
marls and limestone, which, as the dip is gently northwards,
are succeeded by the Osborne Beds, emerging towards the
northern shore. It will be observed, however, that these low
angles of inclination rapidly change towards the south, until
the strata become vertical, or even slightly overhang. Thus
the lowest Oligocene group, that of the Headon Beds, is
allowed to rise from under the sea. It is followed by the
Eocene series — Barton Clay, Bracklesham Beds, Bagshot Sands,
London Clay, and Woolwich and Reading Beds — all vertical or
nearly so, along the coast cliffs of the western and eastern
ends of the island. Next comes the Chalk, also in vertical
beds, which strike from the Needles to Culver Cliff. The
various members of the Cretaceous system appear successively
in their proper order, but as the inclination rapidly lessens,
they occupy progressively broader tracts of the surface, until the
Lower Cretaceous formations cover a wide space of ground, re-
vealing on the western coast a part of the Wealden series. It
will be seen from the dotted line that this tract lies over a
broad flat anticline, and that on the south side the higher
members once more appear in a detached outlier at St. Cathe-
rine's Down. The blue Gault gives rise to extensive landslips
of the Upper Greensand and Chalk above, and thus produces
the picturesque scenery of the Undercliff. One of the most
impressive lessons conveyed by this Section is the evidence it
supplies that extensive terrestrial disturbances have affected
the south of England since older Tertiary time. Another lesson
is the proof furnished of the vast amount of denudation which
has taken place during the same interval.

GEOLOGICAL SURVEY OFFICE,
28 JERMYN STREET, LONDON.

June 1897.

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