The Antiquity of Man Part 26

GLACIAL FORMATIONS IN ENGLAND.

[Ill.u.s.tration: 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 c.u.mberland 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 c.u.mberland 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 sh.e.l.ls, except at heights of 200 or 300 feet; but a few instances of their occurrence have been noticed, especially of Turritella communis (a gregarious sh.e.l.l), 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 sh.e.l.ls 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 sh.e.l.ls, 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 incra.s.sata.

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 sh.e.l.ls 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 ILl.u.s.tRATING SUCCESSIVE REVOLUTIONS IN PHYSICAL GEOGRAPHY DURING THE PLEISTOCENE PERIOD.

[Ill.u.s.tration: 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 sh.e.l.ls 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.)

[Ill.u.s.tration: 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 Suss.e.x: 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.)

[Ill.u.s.tration: 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 s.p.a.ce 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 pa.s.sing 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 a.s.sist our speculations as to the successive revolutions in physical geography, through which the British Islands have pa.s.sed 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 a.s.sumed 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. G.o.dwin-Austen, the Thames was a tributary of the Rhine.*

(* Joshua Trimmer, "Quarterly Journal of the Geological Society" volume 9 1853, Plate 13, and G.o.dwin-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 sh.e.l.ls, 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 sh.e.l.ls. In the lake district of c.u.mberland, 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 sh.e.l.ls 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 doc.u.ments, 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, a.s.sumed 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 sh.o.r.es, 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.*