EARLIEST SIGNS OF MAN'S APPEARANCE IN EUROPE--CONTINUED.
Signs of extinct Glaciers in Wales.
Great Submergence of Wales during the Glacial Period proved by
Marine Shells.
Still greater Depression inferred from Stratified Drift.
Scarcity of Organic Remains in Glacial Formations.
Signs of extinct Glaciers in England.
Ice Action in Ireland.
Maps illustrating successive Revolutions in Physical Geography
during the Pleistocene Period.
Southernmost Extent of Erratics in England.
Successive Periods of Junction and Separation of England, Ireland,
and the Continent.
Time required for these Changes.
Probable Causes of the Upheaval and Subsidence of the Earth's Crust.
Antiquity of Man considered in relation to the Age of the existing
Fauna and Flora.
EXTINCT GLACIERS IN WALES.
The considerable amount of vertical movement in opposite directions,
which was suggested in the last chapter, as affording the most probable
explanation of the position of some of the stratified and fossiliferous
drifts of Scotland, formed since the commencement of the glacial
period, will appear less startling if it can be shown that independent
observations lead us to infer that a geographical revolution of still
greater magnitude accompanied the successive phases of glaciation
through which the Welsh mountains have passed.
That Wales was once an independent centre of the dispersion of erratic
blocks has long been acknowledged. Dr. Buckland published in 1842 his
reasons for believing that the Snowdonian mountains in Caernarvonshire
were formerly covered with glaciers, which radiated from the central
heights through the seven principal valleys of that chain, where striae
and flutings are seen on the polished rocks directed towards as many
different points of the compass. He also described the "moraines" of
the ancient glaciers, and the rounded masses of polished rock, called
in Switzerland "roches moutonnees." His views respecting the old extinct
glaciers of North Wales were subsequently confirmed by Mr. Darwin,
who attributed the transport of many of the larger erratic blocks to
floating ice. Much of the Welsh glacial drift had already been shown by
Mr. Trimmer to have had a submarine origin, and Mr. Darwin maintained
that when the land rose again to nearly its present height, glaciers
filled the valleys, and "swept them clean of all the rubbish left by the
sea."*
(* "Philosophical Magazine" series 3 volume 21 page 180.)
Professor Ramsay, in a paper read to the Geological Society in 1851,
and in a later work on the glaciation of North Wales, described three
successive glacial periods, during the first of which the land was much
higher than it now is, and the quantity of ice excessive; secondly, a
period of submergence when the land was 2300 feet lower than at present,
and when the higher mountain tops only stood out of the sea as a cluster
of low islands, which nevertheless were covered with snow; and lastly,
a third period when the marine boulder drift formed in the middle period
was ploughed out of the larger valleys by a second set of glaciers,
smaller than those of the first period. This last stage of glaciation
may have coincided with that of the parallel roads of Glen Roy,
spoken of in the last chapter. In Wales it was certainly preceded by
submergence, and the rocks had been exposed to glacial polishing and
friction before they sank.
Fortunately the evidence of the sojourn of the Welsh mountains beneath
the waters of the sea is not deficient, as in Scotland, in that complete
demonstration which the presence of marine shells affords. The late Mr.
Trimmer discovered such shells on Moel Tryfan, in North Wales, in drift
elevated more than 1300 feet above the level of the sea. It appears from
his observations, and those of the late Edward Forbes, corroborated by
others of Professor Ramsay and Mr. Prestwich, that about twelve species
of shells, including Fusus bamfius, F. antiquus, Venus striatula (Forbes
and Hanley), have been met with at heights of between 1000 and 1400
feet, in drift, reposing on a surface of rock which had been previously
exposed to glacial friction and striation.*
(* Ramsay, "Quarterly Journal of the Geological Society"
volume 8 1852 page 372.)
The shells, as a whole, are those of the glacial period, and not of the
Norwich Crag. Two localities of these shells in Wales, in addition
to that first pointed out by Mr. Trimmer, have since been observed
by Professor Ramsay, who, however, is of opinion that the amount of
submergence can by no means be limited to the extreme height to which
the shells happen to have been traced; for drift of the same character
as that of Moel Tryfan extends continuously to the height of 2300 feet.
[27]
RARITY OF ORGANIC REMAINS IN GLACIAL FORMATIONS.
The general dearth of shells in such formations, below as well as
above the level at which Mr. Trimmer first found them, deserves notice.
Whether we can explain it or not, it is a negative character which seems
to belong very generally to deposits formed in glacial seas. The porous
nature of the strata, and the length of time during which they have been
permeated by rain-water, may partly account, as we hinted in a former
chapter, for the destruction of organic remains. But it is also possible
that they were originally scarce, for we read of the waters of the
sea being so freshened and chilled by the melting of ice-bergs in some
Norwegian and Icelandic fjords, that the fish are driven away, and all
the mollusca killed. The moraines of glaciers are always from the first
devoid of shells, and if transported by ice-bergs to a distance,
and deposited where the ice melts, may continue as barren of every
indication of life as they were when they originated.
Nevertheless, it may be said, on the other hand, that herds of seals and
walruses crowd the floating ice of Spitzbergen in latitude 80 degrees
north, of which Mr. Lamont has recently given us a lively picture,*nand
huge whales fatten on myriads of pteropods in polar regions.
(* "Seasons with the Sea-Horses" 1861.)
It had been suggested that the bottom of the sea, at the era of extreme
submergence in Scotland and Wales, was so deep as to reach the zero
of animal life, which, in part of the Mediterranean (the Aegean,
for example), the late Edward Forbes fixed, after a long series of
dredgings, at 300 fathoms. But the shells of the glacial drift of
Scotland and Wales, when they do occur, are not always those of deep
seas; and, moreover, our faith in the uninhabitable state of the ocean
at great depths has been rudely shaken, by the recent discovery of
Captain McClintock and Dr. Wallich, of starfish in water more than
a thousand fathoms deep (7560 feet!), midway between Greenland and
Iceland. That these radiata were really dredged up from the bottom, and
that they had been living and feeding there, appeared from the fact
that their stomachs were full of Globigerina, of which foraminiferous
creatures, both living and dead, the oozy bed of the ocean at that vast
depth was found to be exclusively composed. [28]
Whatever may be the cause, the fact is certain, that over large areas
in Scotland, Ireland, and Wales, I might add throughout the northern
hemisphere on both sides of the Atlantic, the stratified drift of the
glacial period is very commonly devoid of fossils, in spite of the
occurrence here and there, at the height of 500, 700, and even
1400 feet, of marine shells. These, when met with, belong, with few
exceptions, to known living species. I am therefore unable to agree with
Mr. Kjerulf that the amount of former submergence can be measured by the
extreme height at which shells happen to have been found.
GLACIAL FORMATIONS IN ENGLAND.
[Illustration: Figure 38. Dome-shaped Rocks]
(FIGURE 38. DOME-SHAPED ROCKS, OR "ROCHES MOUTONEES," IN THE VALLEY
OF THE ROTHAY, NEAR AMBLESIDE, FROM A DRAWING BY E. HULL, F.G.S.*
(* "Edinburgh New Philosophical Journal" volume 11 Plate 1 page 31
1860.))
The mountains of Cumberland and Westmorland, and the English lake
district, afford equally unequivocal vestiges of ice-action not only
in the form of polished and grooved surfaces, but also of those rounded
bosses before mentioned as being so abundant in the Alpine valleys of
Switzerland, where glaciers exist, or have existed. Mr. Hall has
lately published a faithful account of these phenomena, and has given
a representation of some of the English "roches moutonnees," which
precisely resemble hundreds of dome-shaped protuberances in North Wales,
Sweden, and North America.*
(* Hull, "Edinburgh New Philosophical Journal" July 1860.)
The marks of glaciation on the rocks, and the transportation of erratics
from Cumberland to the eastward, have been traced by Professor Phillips
over a large part of Yorkshire, extending to a height of 1500 feet above
the sea; and similar northern drift has been observed in Lancashire,
Cheshire, Derbyshire, Shropshire, Staffordshire, and Worcestershire. It
is rare to find marine shells, except at heights of 200 or 300 feet;
but a few instances of their occurrence have been noticed, especially
of Turritella communis (a gregarious shell), far in the interior, at
elevations of 500 feet, and even of 700 in Derbyshire, and some adjacent
counties, as I learn from Mr. Binney and Mr. Prestwich.
Such instances are of no small theoretical interest, as enabling us
to account for the scattering of large erratic blocks at equal or
much greater elevations, over a large part of the northern and midland
counties, such as could only have been conveyed to their present sites
by floating ice. Of this nature, among others, is a remarkable angular
block of syenitic greenstone, 4 1/2 feet by 4 feet square, and 2 feet
thick, which Mr. Darwin describes as lying on the summit of Ashley
Heath, in Staffordshire, 803 feet above the sea, resting on New Red
Sandstone.*
(* Ancient Glaciers of Caernarvonshire, "Philosophical
Magazine" series 3, 21 page 180.)
SIGNS OF ICE-ACTION AND SUBMERGENCE IN IRELAND DURING THE GLACIAL
PERIOD.
In Ireland we encounter the same difficulty as in Scotland in
determining how much of the glaciation of the higher mountains should
be referred to land glaciers, and how much to floating ice, during
submergence. The signs of glacial action have been traced by Professor
Jukes to elevations of 2500 feet in the Killarney district, and to great
heights in other mountainous regions; but marine shells have rarely been
met with higher than 600 feet above the sea, and that chiefly in gravel,
clay, and sand in Wicklow and Wexford. They are so rare in the drift
east of the Wicklow mountains, that an exception to the rule, lately
observed at Ballymore Eustace, by Professor Jukes, is considered as a
fact of no small geological interest. The wide extent of drift of the
same character, spread over large areas in Ireland, shows that the
whole island was, in some part of the glacial period, an archipelago, as
represented in the maps, Figures 39 and 40.
Speaking of the Wexford drift, the late Professor E. Forbes states
that Sir H. James found in it, together with many of the usual glacial
shells, several species which are characteristic of the Crag; among
others the reversed variety of Fusus antiquus, called F. contrarius,
and the extinct species Nucula Cobboldiae, and Turritella incrassata.
Perhaps a portion of this drift of the south of Ireland may belong to
the close of the Pliocene period, and may be of a somewhat older date
than the shells of the Clyde, alluded to in Chapter 13. They may also
correspond still more nearly in age with the fauna of the uppermost
strata of the Norwich Crag, occurring at Chillesford. [29]
The scarcity of mammalian remains in the Irish drift favours the theory
of its marine origin. In the superficial deposits of the whole island,
I have only met with three recorded examples of the mammoth, one in
the south near Dungarvan, where the bones of Elephas primigenius, two
species of bear (Ursus arctos and Ursus spelaeus?), the reindeer, horse,
etc., were found in a cave;* another in the centre of the island near
Belturbet, in the county of Cavan.
(* E. Brenan and Dr. Carte, Dublin 1859.)
Perhaps the conversion into land of the bed of the glacial sea, and the
immigration into the newly upheaved region of the elephant, rhinoceros,
and hippopotamus, which co-existed with the fabricators of the St.
Acheul flint hatchets, were events which preceded in time the elevation
of the Irish drift, and the union of that island with England. Ireland
may have continued for a longer time in the state of an archipelago,
and was therefore for a much shorter time inhabited by the large extinct
Pleistocene pachyderms.
In one of the reports of the Geological Survey of Ireland, published in
1859, Professor Jukes, in explanation of sheet 184 of the maps, alludes
to beds of sand and gravel, and signs of the polishing and furrowing of
the rocks in the counties of Kerry and Killarney, as high as 2500 feet
above the sea, and supposes (perhaps with good reason) that the land
was depressed even to that extent. He observes that above that elevation
(2500 feet) the rocks are rough, and not smoothed, as if by ice. Some
of the drift was traced as high as 1500 feet, the highest hills there
exceeding 3400 feet. Mr. Jukes, however, is by no means inclined to
insist on submergence to the extent of 2500 feet, as he is aware that
ice, like that now prevailing in Greenland, might explain most, if not
all, the appearances of glaciation in the highest regions.
Although the course taken by the Irish erratics in general is such
that their transportation seems to have been due to floating ice or
coast-ice, yet some granite blocks have travelled from south to north,
as recorded by Sir R. Griffiths, namely, those of the Ox Mountains in
Sligo; a fact from which Mr. Jamieson infers that those mountains formed
at one time a centre of dispersion. In the same part of Ireland, the
general direction in which the boulders have travelled is everywhere
from north-west to south-east, a course directly at right angles to the
prevailing trend of the present mountain ridges.
MAPS ILLUSTRATING SUCCESSIVE REVOLUTIONS IN PHYSICAL GEOGRAPHY DURING
THE PLEISTOCENE PERIOD.
[Illustration: Figure 39. Map Of The British Isles]
(FIGURE 39. MAP OF THE BRITISH ISLES AND PART OF THE NORTH-WEST
OF EUROPE, SHOWING THE GREAT AMOUNT OF SUPPOSED SUBMERGENCE
OF LAND BENEATH THE SEA DURING PART OF THE GLACIAL PERIOD.
The submergence of Scotland is to the extent of 2000 feet,
and of other parts of the British Isles, 1300.
In the map, the dark shade expresses the land which alone
remained above water. The area shaded by diagonal lines is
that which cannot be shown to have been under water at the
period of floating ice by the evidence of erratics, or by
marine shells of northern species. How far the several parts
of the submerged area were simultaneously or successively laid
under water, in the course of the glacial period, cannot, in
the present state of our knowledge, be determined.)
[Illustration: Figure 40. Map British Islands]
(FIGURE 40. MAP SHOWING WHAT PARTS OF THE BRITISH ISLANDS
WOULD REMAIN ABOVE WATER AFTER A SUBSIDENCE OF THE AREA
TO THE EXTENT OF 600 FEET.
The authorities to whom I am indebted for the information
contained in this map are--for:
SCOTLAND:
A. Geikie, Esquire, F.G.S., and T.F. Jamieson, Esquire,
of Ellon, Aberdeenshire.
ENGLAND:
For the counties of:
Yorkshire, Lancashire, and Durham: Colonel Sir Henry James, R.E.
Dorsetshire, Hampshire, and Isle of Wight: H.W. Bristow, Esquire.
Gloucestershire, Somersetshire, and part of Devon: R. Etheridge,
Esquire.
Kent and Sussex: Frederick Drew, Esquire.
Isle of Man: W. Whitaker, Esquire.
IRELAND:
Reduced from a contour map constructed by Lieutenant Larcom,
R.E., in 1837, for the Railway Commissioners.)
[Illustration: Figure 41. Map Of Part Of The North-West Of Europe]
(FIGURE 41. MAP OF PART OF THE NORTH-WEST OF EUROPE, INCLUDING
THE BRITISH ISLES, SHOWING THE EXTENT OF SEA WHICH WOULD BECOME
LAND IF THERE WERE A GENERAL RISE OF THE AREA TO THE EXTENT
OF 600 FEET.
The darker shade expresses what is now land, the lighter shade
the space intervening between the present coastline and the
100 fathom line, which would be converted by such a movement
into land.
The original of this map will be found in Sir H. de la Beche's
"Theoretical Researches" page 190, 1834, but several important
corrections have been introduced into it from recently
published Admiralty Surveys, especially:
1st. A deep channel passing from the North Sea into the
entrance of the Baltic.
2nd. The more limited westerly extension of the West Coast
of Ireland.)
The late Mr. Trimmer, before referred to, has endeavoured to assist our
speculations as to the successive revolutions in physical geography,
through which the British Islands have passed since the commencement
of the glacial period, by four "sketch maps" as he termed them, in the
first of which he gave an ideal restoration of the original Continental
period, called by him the first elephantine period, or that of the
forest of Cromer, before described. He was not aware that the prevailing
elephant of that era (E. meridionalis) was distinct from the mammoth. At
this era he conceived Ireland and England to have been united with each
other and with France, but much of the area represented as land in the
map, Figure 41, was supposed to be under water. His second map, of the
great submergence of the glacial period, was not essentially different
from our map, Figure 39. His third map expressed a period of partial
re-elevation, when Ireland was reunited to Scotland and the north of
England; but England still separated from France. This restoration
appears to me to rest on insufficient data, being constructed to suit
the supposed area over which the gigantic Irish deer, or Megaceros,
migrated from east to west, also to explain an assumed submergence of
the district called the Weald, in the south-east of England, which had
remained land during the grand glacial submergence.
The fourth map is a return to nearly the same continental conditions
as the first--Ireland, England, and the Continent being united. This he
called the second elephantine period; and it would coincide very closely
with that part of the Pleistocene era in which Man co-existed with the
mammoth, and when, according to Mr. Trimmer's hypothesis previously
indicated by Mr. Godwin-Austen, the Thames was a tributary of the
Rhine.*
(* Joshua Trimmer, "Quarterly Journal of the Geological
Society" volume 9 1853, Plate 13, and Godwin-Austen, ibid.
volume 7 1851 page 134 and Plate 7.)
These geographical speculations were indulged in ten years after Edward
Forbes had published his bold generalisations on the geological changes
which accompanied the successive establishment of the Scandinavian,
Germanic, and other living floras and faunas in the British Islands,
and, like the theories of his predecessor, were the results of much
reflection on a vast body of geological facts. It is by repeated efforts
of this kind, made by geologists who are prepared for the partial
failure of some of their first attempts, that we shall ultimately arrive
at a knowledge of the long series of geographical revolutions which have
followed each other since the beginning of the Pleistocene period.
The map, Figure 39, will give some idea of the great extent of land
which would be submerged, were we to infer, as many geologists have
done, from the joint evidence of marine shells, erratics, glacial striae
and stratified drift at great heights, that Scotland was, during part of
the glacial period, 2000 feet below its present level, and other parts
of the British Isles, 1300 feet. A subsidence to this amount can be
demonstrated in the case of North Wales by marine shells. In the lake
district of Cumberland, in Yorkshire, and in Ireland, we must depend on
proofs derived from glacial striae and the transportation of erratics
for so much of the supposed submergence as exceeds 600 feet. As to
central England, or the country north of the Thames and Bristol Channel,
marine shells of the glacial period sometimes reach as high as 600 and
700 feet, and erratics still higher, as we have seen above. But this
region is of such moderate elevation above the sea, that it would be
almost equally laid under water, were there a sinking of no more than
600 feet.
To make this last proposition clear, I have constructed, from numerous
documents, many of them unpublished, the map, Figure 40, which shows how
that small amount of subsidence would reduce the whole of the British
Isles to an archipelago of very small islands, with the exception
of parts of Scotland, and the north of England and Wales, where four
islands of considerable dimensions would still remain.
The map does not indicate a state of things supposed to have prevailed
at any one moment of the past, because the district south of the Thames
and the Bristol Channel seems to have remained land during the whole of
the glacial period, at a time when the northern area was under water.
The map simply represents the effects of a downward movement of a
hundred fathoms, or 600 English feet, assumed to be uniform over the
whole of the British Isles. It shows the very different state of the
physical geography of the area in question, when contrasted with the
results of an opposite movement, or one of upheaval, to an equal amount,
of which Sir Henry de la Beche had already given us a picture, in his
excellent treatise called "Theoretical Researches."*
(* Also repeated in De la Beche's "Geological Observer.")
His map I have borrowed (Figure 41), after making some important
corrections in it.
If we are surprised when looking at the first map, Figure 40, at the
vast expanse of sea which so moderate a subsidence as 600 feet would
cause, we shall probably be still more astonished to perceive, in Figure
41, that a rise of the same number of feet would unite all the British
Isles, including the Hebrides, Orkneys, and Shetlands, with one another
and the Continent, and lay dry the sea now separating Great Britain from
Sweden and Denmark.
It appears from soundings made during various Admiralty surveys, that
the gained land thus brought above the level of the sea, instead of
presenting a system of hills and valleys corresponding with those
usually characterising the interior of most of our island, would form
a nearly level terrace, or gently inclined plane, sloping outwards
like those terraces of denudation and deposition which I have elsewhere
described as occurring on the coasts of Sicily and the Morea.*
(* "Manual of Geology" page 74.)
It seems that, during former and perhaps repeated oscillations of level
undergone by the British Isles, the sea has had time to cut back the
cliffs for miles in many places, while in others the detritus derived
from wasting cliffs drifted along the shores, together with the sediment
brought down by rivers and swept by currents into submarine valleys,
has exerted a levelling power, filling up such depressions as may have
pre-existed. Owing to this twofold action few marked inequalities of
level have been left on the sea-bottom, the "silver-pits" off the mouth
of the Humber offering a rare exception to the general rule, and even
there the narrow depression is less than 300 feet in depth.
Beyond the 100 fathom line, the submarine slope surrounding the British
coast is so much steeper that a second elevation of equal amount (or of
600 feet) would add but slightly to the area of gained land; in other
words, the 100 and 200 fathom lines run very near each other.*
(* De la Beche, "Geological Researches" page 191.)
The naturalist would have been entitled to assume the former union,
within the Pleistocene period, of all the British Isles with each other
and with the Continent, as expressed in the map, Figure 41, even if
there had been no geological facts in favour of such a junction. For in
no other way would he be able to account for the identity of the fauna
and flora found throughout these lands. Had they been separated ever
since the Miocene period, like Madeira, Porto Santo, and the Desertas,
constituting the small Madeiran Archipelago, we might have expected
to discover a difference in the species of land-shells, not only when
Ireland was compared to England, but when different islands of the
Hebrides were contrasted one with another, and each of them with
England. It would not, however, be necessary, in order to effect the
complete fusion of the animals and plants which we witness, to assume
that all parts of the area formed continuous land at one and the same
moment of time, but merely that the several portions were so joined
within the Pleistocene era as to allow the animals and plants to migrate
freely in succession from one district to another.
SOUTHERNMOST EXTENT OF ERRATICS IN ENGLAND.
In reference to that portion of the south of England which is marked by
diagonal lines in Figure 39, the theory of its having been an area of
dry land during the period of great submergence and floating ice does
not depend merely on negative evidence, such as the absence of the
northern drift or boulder clay on its surface; but we have also, in
favour of the same conclusion, the remarkable fact of the presence of
erratic blocks on the southern coast of Sussex, implying the existence
there of an ancient coast-line at a period when the cold must have been
at its height.
These blocks are to be seen in greatest number at Pagham and Selsea, 15
miles south of Chichester, in latitude 50 degrees 40 minutes north.
They consist of fragments of granite, syenite and greenstone, as well as
of Devonian and Silurian rocks, some of them of large size. I measured
one of granite at Pagham, 27 feet in circumference. They are not of
northern origin, but must have come from the coast of Normandy or
Brittany, or from land which may once have existed to the south-west, in
what is now the English Channel.
They were probably drifted into their present site by coast ice, and
the yellow clay and gravel in which they are embedded are a littoral
formation, as shown by the shells. Beneath the gravel containing these
large erratics, is a blue mud in which skeletons of Elephas antiquus,
and other mammalia, have been observed. Still lower occurs a sandy loam,
from which Mr. R.G. Austen* has collected thirty-eight species of marine
shells, all Recent, but forming an assemblage differing as a whole from
that now inhabiting the English Channel.
(* "Quarterly Journal of the Geological Society" volume 13
1857 page 50.)
The presence among them of Lutraria rugosa and Pecten polymorphus,
not known to range farther north in the actual seas than the coast of
Portugal, indicates a somewhat warmer temperature at the time when
they flourished. Subsequently, there must have been great cold when the
Selsea erratics were drifted into their present position, and this cold
doubtless coincided in time with a low temperature farther north. [30]
These transported rocks of Sussex are somewhat older than a sea-beach
with Recent marine shells which at Brighton is covered by Chalk rubble,
called the "elephant-bed" which I cannot describe in this place, but I
allude to it as one of many geological proofs of the former existence
of a seashore in this region, and of ancient cliffs bounding the channel
between France and England, all of older date than the close of the
glacial period. [31]
In order to form a connected view of the most simple series of changes
in physical geography which can possibly account for the phenomena of
the glacial period, and the period of the establishment of the present
provinces of animals and plants, the following geographical states of
the British and adjoining areas may be enumerated.
First, a continental period, towards the close of which the forest of
Cromer flourished: when the land was at least 500 feet above its present
level, perhaps much higher, and its extent probably greater than that
given in the map, Figure 41.
Secondly, a period of submergence, by which the land north of the Thames
and Bristol Channel, and that of Ireland, was gradually reduced to such
an archipelago as is pictured in map, Figure 40; and finally to such
a general prevalence of sea as is seen in map, Figure 39. This was the
period of great submergence and of floating ice, when the Scandinavian
flora, which occupied the lower grounds during the first continental
period, may have obtained exclusive possession of the only lands not
covered with perpetual snow.
Thirdly, a second continental period when the bed of the glacial sea,
with its marine shells and erratic blocks, was laid dry, and when
the quantity of land equalled that of the first period, and therefore
probably exceeded that represented in the map, Figure 41. During this
period there were glaciers in the higher mountains of Scotland and
Wales, and the Welsh glaciers, as we have seen, pushed before them and
cleared out the marine drift with which some valleys had been filled
during the period of submergence. The parallel roads of Glen Roy are
referable to some part of the same era.
As a reason for presuming that the land which in map, Figure 41, is only
represented as 600 feet above its present level, was during part of this
period much higher, Professor Ramsay has suggested that, as the previous
depression far exceeded 100 fathoms (amounting in Wales to 1400 feet,
as shown by marine shells, and to 2300, by stratified drift), it is not
improbable that the upward movement was on a corresponding scale.
In passing from the period of chief submergence to this second
continental condition of things, we may conceive a gradual change first
from that of Map 39 to Map 40, then from the latter phase to that of Map
41, and finally to still greater accessions of land. During this last
period the passage of the Germanic flora into the British area took
place, and the Scandinavian plants, together with northern insects,
birds, and quadrupeds, retreated into the higher grounds.
Judging from the evidence at present before us, the first appearance of
Man, when, together with the mammoth and woolly rhinoceros, or with the
Elephas antiquus, Rhinoceros hemitoechus, and Hippopotamus major, he
ranged freely from all parts of the Continent into the British area,
took place during this second continental period.
Fourthly, the next and last change comprised the breaking up of the
land of the British area once more into numerous islands, ending in
the present geographical condition of things. There were probably many
oscillations of level during this last conversion of continuous land
into islands, and such movements in opposite directions would account
for the occurrence of marine shells at moderate heights above the level
of the sea, notwithstanding a general lowering of the land. To the close
of this era belong the marine deposits of the Clyde and the Carses of
the Tay and Forth, before alluded to.
In a memoir by Professor E. Forbes, before cited, he observes, that the
land of passage by which the plants and animals migrated into Ireland
consisted of the upraised marine drift which had previously formed the
bottom of the glacial sea. Portions of this drift extend to the eastern
shores of Wicklow and Wexford, others are found in the Isle of Man full
of arctic shells, others on the British coast opposite Ireland. The
freshwater marl, containing numerous skeletons of the great deer,
or Megaceros, overlie in the Isle of Man that marine glacial drift.
Professor Forbes also remarks that the subsequent disjunction of Ireland
from England, or the formation of the Irish Channel, which is less than
400 feet in its greatest depth, preceded the opening of the Straits of
Dover, or the final separation of England from the Continent. This he
inferred from the present distribution of species both in the animal and
vegetable kingdoms. Thus, for example, there are twice as many reptiles
in Belgium as in England, and the number inhabiting England is twice
that found in Ireland. Yet the Irish species are all common to England,
and all the English to Belgium. It is therefore assumed that the
migration of species westward having been the work of time, there was
not sufficient lapse of ages to complete the fusion of the continental
and British reptilian fauna, before France was separated from England
and England from Ireland.
For the same reason there are also a great number of birds of short
flight, and small quadrupeds, inhabiting England which do not cross
to Ireland, the Irish Channel seeming to have arrested them in their
westward course.*
(* E. Forbes, Fauna and Flora of British Isles, "Memoir of
the Geological Survey" volume 1 1846 page 344.)
The depth of the Irish Channel in the narrower parts is only 360 feet,
and the English Channel between Dover and Calais less than 200, and
rarely anywhere more than 300 feet; so that vertical movements of slight
amount compared to some of those previously considered, with the aid of
denuding operations or the waste of sea cliffs, and the scouring out
of the channel, might in time effect the insulation of the lands above
alluded to.
TIME REQUIRED FOR SUCCESSIVE CHANGES IN PHYSICAL GEOGRAPHY IN THE
PLEISTOCENE PERIOD.
The time which it would require to bring about such changes of level,
according to the average rate assumed in Chapter 3, however vast, will
not be found to exceed that which would best explain the successive
fluctuations in terrestrial temperature, the glaciation of solid rocks,
the transportation of erratics above and below the sea-level, the height
of arctic shells above the sea, and last, not least, the migration of
the existing species of animals and plants into their actual stations,
and the extinction of some conspicuous forms which flourished during
the Pleistocene ages. When we duly consider all these changes which
have taken place since the beginning of the glacial epoch, or since the
forest of Cromer and the Elephas meridionalis flourished, we shall find
that the phenomena become more and more intelligible in proportion to
the slowness of the rate of elevation and depression which we assume.
The submergence of Wales to the extent of 1400 feet, as proved by
glacial shells, would require 56,000 years, at the rate of 2 1/2 feet
per century; but taking Professor Ramsay's estimate of 800 feet more,
that depression being implied by the position of some of the stratified
drift, we must demand an additional period of 32,000 years, amounting in
all to 88,000; and the same time would be required for the re-elevation
of the tract to its present height. But if the land rose in the second
continental period as much as 600 feet above its present level, as in
Figure 41, this 600 feet, first of rising and then of sinking,
would require 48,000 years more; the whole of the grand oscillation,
comprising the submergence and re-emergence, having taken about 224,000
years for its completion; and this, even if there were no pause or
stationary period, when the downward movement ceased, and before it was
converted into an upward one.
I am aware that it may be objected that the average rate here proposed
is a purely arbitrary and conjectural one, because, at the North Cape,
it is supposed that there has been a rise of about 5 feet in a century,
and at Spitsbergen, according to Mr. Lamont, a still faster upheaval
during the last 400 years.*
(* "Seasons with the Sea-Horses" page 202.)
But, granting that in these and some exceptional cases (none of them as
yet very well established) the rising or sinking has, for a time, been
accelerated, I do not believe the average rate of motion to exceed that
above proposed. Mr. Darwin, I find, considers that such a mean rate of
upheaval would be as high as we could assume for the west coast of South
America, where we have more evidence of sudden changes of level than
anywhere else. He has not, however, attempted to estimate the probable
rate of secular elevation in that or any other region.
Little progress has yet been made in divining the most probable causes
of these great movements of the earth's crust; yet what little we
know of the state of the interior leads us to expect that the gradual
expansion or contraction of large portions of the solid crust may be
the result of fluctuations in temperature, with which the existence
of hundreds of active and thousands of extinct volcanoes is probably
connected.
It is ascertained that solid rocks, such as granite and sandstone,
expand and contract annually, even under such a moderate range of
temperature as that of a Canadian winter and summer. If the heat should
go on increasing through a thickness, say only of 10 miles of the
earth's crust, the gradual upheaval of the incumbent mass may amount to
many hundreds of feet; and the elevation may be carried still farther,
by the complete fusion of part of the inferior rocks.
According to the experiments of Deville, the contraction of granite, in
passing from a melted, or as some would say its plastic condition, to a
solid state, must be more than 10 per cent.*
(* "Bull. Societe Geologique France" 2nd series volume 4
page 1312.)
So that we have at our command a source of depression on a grand scale,
at every period when granitic rocks have originated in the interior
of the earth's crust. All mineralogists are agreed that the passage of
voluminous masses, from a liquid or pasty to a solid and crystalline
state, must be an extremely slow process. It may often happen that, in
the same series of superimposed rocks, some are expanding while still
solid or while partially melting, while others are at the same time
crystallising and contracting; so that the alterations of level at
the surface may be the result of complicated and often of conflicting
agencies. The more gradually we conceive such changes to take place, the
more comprehensible they become in the eyes of the chemist and natural
philosopher who speculates on the changes of the earth's interior; and
the more fertile are they in the hands of the geologist in accounting
for revolutions on the habitable surface.
We may presume, that after the movement has gone on for a long time
in one determinate direction, whether of elevation or depression, the
change to an opposite movement, implying the substitution of a heating
for a refrigerating operation, or the reverse, would not take place
suddenly; but would be marked by a period of inaction, or of slight
movement, or such a state of quiescence, as prevails throughout large
areas of dry land in the normal condition of the globe.
I see no reason for supposing that any part of the revolutions in
physical geography, to which the maps above described have reference,
indicate any catastrophes greater than those which the present
generation has witnessed. If Man was in existence when the Cromer forest
was becoming submerged, he would have felt no more alarm than the Danish
settlers on the east coast of Baffin's Bay, when they found the poles,
which they had driven into the beach to secure their boats, had subsided
below their original level.
Already, perhaps, the melting ice has thrown down till and boulders
upon those poles, a counterpart of the boulder clay which overlies the
forest-bed on the Norfolk cliffs.
We have seen that all the plants and shells, marine and freshwater,
of the forest bed, and associated fluvio-marine strata of Norfolk,
are specifically identical with those of the living European flora and
fauna; so that if upon such a stratum a deposit of the present period,
whether freshwater or marine, should be thrown down, it might lie
conformably over it, and contain the same invertebrate fauna and flora.
The strata so superimposed would, in ordinary geological language, be
called contemporaneous, not only as belonging to the same epoch, but as
appertaining strictly to the same subdivision of one and the same
epoch; although they would in fact have been separated by an interval of
several hundred thousand years.
If, in the lower of the two formations, some of the mammalia of the
genera elephant and rhinoceros were found to be distinct in species
from those of the same genera in the upper or "recent" stratum, it might
appear as though there had been a sudden coming in of new forms, and a
sudden dying out of old ones; for there would not have been time in the
interval for any perceptible change in the invertebrate fauna, by which
alone we usually measure the lapse of time in the older formations.
When we are contrasting the vertebrate contents of two sets of
superimposed strata of the Cretaceous, Oolitic, or any other ancient
formation in which the shells are identical in species, we ought never
to lose sight of the possibility of their having been separated by such
intervals or by two or three thousand centuries. That number of years
may sometimes be of small moment in reference to the rate of fluctuation
of species in the lower animals, but very important when the succession
of forms in the highest classes of vertebrata is concerned.
If we reflect on the long series of events of the Pleistocene and Recent
periods contemplated in this chapter, it will be remarked that the
time assigned to the first appearance of Man, so far as our geological
inquiries have yet gone, is extremely modern in relation to the age
of the existing fauna and flora, or even to the time when most of the
living species of animals and plants attained their actual geographical
distribution. At the same time it will also be seen, that if the advent
of Man in Europe occurred before the close of the second continental
period, and antecedently to the separation of Ireland from England and
of England from the Continent, the event would be sufficiently remote to
cause the historical period to appear quite insignificant in duration,
when compared to the antiquity of the human race.