MAPS ILLUSTRATING SUCCESSIVE REVOLUTIONS IN PHYSICAL GEOGRAPHY DURING THE PLEISTOCENE PERIOD.
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.)
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.)
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.
