The Antiquity of Man Part 24

(* Charpentier, "Essai sur les Glaciers" page 63 1841.)

The evidence of the former sojourn of the sea upon the land after the commencement of the glacial period was formerly inferred from the height to which erratic blocks derived from distant regions could be traced, besides the want of conformity in the glacial furrows to the present contours of many of the valleys. Some of these phenomena may now, as we have seen, be accounted for by a.s.suming that there was once a crust of ice resembling that now covering Greenland.

The Grampians in Forfars.h.i.+re and in Perths.h.i.+re are from 3000 to 4000 feet high. To the southward lies the broad and deep valley of Strathmore, and to the south of this again rise the Sidlaw Hills to the height of 1500 feet and upwards. On the highest summits of this chain, formed of sandstone and shale, and at various elevations, I have observed huge angular fragments of mica-schist, some 3 and others 15 feet in diameter, which have been conveyed for a distance of at least 15 miles from the nearest Grampian rocks from which they could have been detached. Others have been left strewed over the bottom of the large intervening vale of Strathmore.*

(* "Proceedings of the Geological Society" volume 3 page 344.)

It may be argued that the transportation of such blocks may have been due not to floating ice, but to a period when Strathmore was filled up with land ice, a current of which extended from the Perths.h.i.+re Highlands to the summit of the Sidlaw Hills, and the total absence of marine or freshwater sh.e.l.ls from all deposits, stratified or unstratified, which have any connection with these erratics in Forfars.h.i.+re and Perths.h.i.+re may be thought to favour such a theory.

But the same mode of transport can scarcely be imagined for those fragments of mica-schist, one of them weighing from 8 to 10 tons, which were observed much farther south by Mr. Maclaren on the Pentland Hills, near Edinburgh, at the height of 1100 feet above the sea, the nearest mountain composed of this formation being 50 miles distant.*

(* Maclaren, "Geology of Fife" etc. page 220.)

On the same hills, also, at all elevations, stratified gravels occur which, although devoid of sh.e.l.ls, it seems hardly possible to refer to any but a marine origin.

Although I am willing, therefore, to concede that the glaciation of the Scotch mountains, at elevations exceeding 2000 feet, may be explained by land ice, it seems difficult not to embrace the conclusion that a subsidence took place not merely of 500 or 600 feet, as demonstrated by the marine sh.e.l.ls, but to a much greater amount, as shown by the present position of erratics and some patches of stratified drift. The absence of marine sh.e.l.ls at greater heights than 525 feet above the sea, will be treated of in a future chapter. It may in part, perhaps, be ascribed to the action of glaciers, which swept out marine strata from all the higher valleys, after the re-emergence of the land.

LATEST CHANGES PRODUCED BY GLACIERS IN SCOTLAND.

We may next consider the state of Scotland after its emergence from the glacial sea, when we cannot fail to be approaching the time when Man co-existed with the mammoth and other mammalia now extinct. In a paper which I published in 1840, on the ancient glaciers of Forfars.h.i.+re, I endeavoured to show that some of these existed after the mountains and glens had acquired precisely their present shape,* and had left moraines even in the minor valleys, just where they would now leave them were the snow and ice again to gain ground.

(* "Proceedings of the Geological Society" volume 3 page 337.)

I described also one remarkable transverse mound, evidently the terminal moraine of a retreating glacier, which crosses the valley of the South Esk, a few miles above the point where it issues from the Grampians, and about 6 miles below the Kirktown of Clova. Its central part, at a place called Glenarm, is 800 feet above the level of the sea. The valley is about half a mile broad, and is bounded by steep and lofty mountains, but immediately above the transverse barrier it expands into a wide alluvial plain, several miles broad, which has evidently once been a lake. The barrier itself, about 150 feet high, consists in its lower part of till with boulders, 50 feet thick, precisely resembling the moraine of a Swiss glacier, above which there is a ma.s.s of stratified sand, varying in thickness from 50 to 100 feet, which has the appearance of consisting of the materials of the moraine rearranged in a stratified form, possibly by the waters of a glacier lake. The structure of the barrier has been laid open by the Esk, which has cut through it a deep pa.s.sage about 400 yards wide.

I have also given an account of another striking feature in the physical geography of Perths.h.i.+re and Forfars.h.i.+re, which I consider to belong to the same period; namely, a continuous zone of boulder clay, forming ridges and mounds from 50 to 70 feet high (the upper part of the mounds usually stratified), enclosing numerous lakes, some of them several miles long, and many ponds and swamps filled with sh.e.l.l-marl and peat.

This band of till, with Grampian boulders and a.s.sociated river-gravel, may be traced continuously for a distance of 34 miles, with a width of 3 1/2 miles, from near Dunkeld, by Coupar, to the south of Blairgowrie, then through the lowest part of Strathmore, and afterwards in a straight line through the greatest depression in the Sidlaw Hills, from Forfar to Lunan Bay.

Although no great river now takes its course through this line of ancient lakes, moraines, and river gravel, yet it evidently marks an ancient line by which, first, a great glacier descended from the mountains to the sea, and by which, secondly, at a later period, the princ.i.p.al water drainage of this country was effected. The subsequent modification in geography is comparable in amount to that which has taken place since the higher level gravels of the valley of the Somme were formed, or since the Belgian caves were filled with mud and bone-breccia.

[Ill.u.s.tration: Figure 35. Oval And Flattish Pebbles In Deserted Channels]

(FIGURE 35. OVAL AND FLATTISH PEBBLES IN DESERTED CHANNELS.)

Mr. Jamieson has remarked, in reference to this and some other extinct river-channels of corresponding date, that we have the means of ascertaining the direction in which the waters flowed by observing the arrangement of the oval and flattish pebbles in their deserted channels; for in the bed of a fast-flowing river such pebbles are seen to dip towards the current, as represented in Figure 35, such being the position of greatest resistance to the stream.*

(* Jamieson, "Quarterly Journal of the Geological Society"

volume 16 1860 page 349.)

If this be admitted, it follows that the higher or mountainous country bore the same relation to the lower lands, at the time when a great river pa.s.sed through this chain of lakes, as it does at present.

We also seem to have a test of the comparatively modern origin of the mounds of till which surround the above-mentioned chain of lakes (of which that of Forfar is one), in the species of organic remains contained in the sh.e.l.l-marl deposited at their bottom. All the mammalia as well as sh.e.l.ls are of recent species. Unfortunately, we have no information as to the fauna which inhabited the country at the time when the till itself was formed. There seem to be only three or four instances as yet known in all Scotland of mammalia having been discovered in boulder clay.

Mr. R. Bald has recorded the circ.u.mstances under which a single elephant's tusk was found in the unstratified drift of the valley of the Forth, with the minuteness which such a discovery from its rarity well deserved. He distinguishes the boulder clay, under the name of "the old alluvial cover," from that more modern alluvium, in which the whales of Airthrie, described in Chapter 3, were found. This cover he says is sometimes 160 feet thick. Having never observed any organic remains in it, he watched with curiosity and care the digging of the Union Ca.n.a.l between Edinburgh and Falkirk, which pa.s.sed for no less than 28 miles almost continuously through it. Mr. Baird, the engineer who superintended the works, a.s.sisted in the inquiry, and at one place only in this long section did they meet with a fossil, namely, at Cliftonhall, in the valley of the Almond. It lay at a depth of between 15 and 20 feet from the surface, in very stiff clay, and consisted of an elephant's tusk, 39 inches long and 13 in circ.u.mference, in so fresh a state that an ivory turner purchased it and turned part of it into chessmen before it was rescued from destruction. The remainder is still preserved in the museum at Edinburgh, but by exposure to the air it has shrunk considerably.*

(* "Memoirs of the Wernerian Society" Edinburgh volume 4 page 58.)

In 1817, two other tusks and some bones of the elephant, as we learn from the same authority (Mr. Bald), were met with, 3 1/2 feet long and 13 inches in circ.u.mference, lying in an horizontal position, 17 feet deep in clay, with marine sh.e.l.ls, at Kilmaurs, in Ayrs.h.i.+re. The species of sh.e.l.ls are not given.*

(* Ibid. volume 4 page 63.)

In another excavation through the Scotch boulder clay, made in digging the Clyde and Forth Junction Railway, the antlers of a reindeer were found at Croftamie, in Dumbartons.h.i.+re, in the basin of the river Endrick, which flows into Loch Lomond. They had cut through 12 feet of till with angular and rounded stones, some of large size, and then through 6 feet of underlying clay, when they came upon the deer's horns, 18 feet from the surface, and within a foot of the sandstone on which the till rested. At the distance of a few yards, and in the same position, but a foot or two deeper, were observed marine sh.e.l.ls, Cyprina islandica, Astarte elliptica, A. compressa, Fusus antiquus, Littorina littorea, and a Bala.n.u.s. The height above the level of the sea was between 100 and 103 feet. The reindeer's horn was seen by Professor Owen, who considered it to be that of a young female of the large variety, called by the Hudson's Bay trappers the caribou.

The remains of elephants, now in the museums of Glasgow and Edinburgh, purporting to come from the superficial deposits of Scotland have been referred to Elephas primigenius. In cases where tusks alone have been found unaccompanied by molar teeth, such specific determinations may be uncertain; but if any one specimen be correctly named, the occurrence of the mammoth and reindeer in the Scotch boulder-clay, as both these quadrupeds are known to have been contemporary with Man, favours the idea which I have already expressed, that the close of the glacial period in the Grampians may have coincided in time with the existence of Man in those parts of Europe where the climate was less severe, as, for example, in the basins of the Thames, Somme, and Seine, in which the bones of many extinct mammalia are a.s.sociated with flint implements of the antique type.

PARALLEL ROADS OF GLEN ROY IN SCOTLAND.

[Ill.u.s.tration: Plate 2. Glen Roy and Glen Spean]

(PLATE 2. VIEW OF THE MOUTHS OF GLEN ROY AND GLEN SPEAN, BY SIR T. d.i.c.k LAUDER.

VV. Hill of Bohuntine.

VVV. Glen Roy.

V(inverted)V. Mealderry.

V. Entrance of Glen Spean VV(superscript)V. Point of division between Glens Roy and Spean.)

Perhaps no portion of the superficial drift of Scotland can lay claim to so modern an origin on the score of the freshness of its aspect, as that which forms what are called the Parallel Roads of Glen Roy. If they do not belong to the Recent epoch, they are at least posterior in date to the present outline of mountain and glen, and to the time when every one of the smaller burns ran in their present channels, though some of them have since been slightly deepened. The almost perfect horizontality, moreover, of the roads, one of which is continuous for about 20 miles from east to west, and 12 miles from north to south, shows that since the era of their formation no change has taken place in the relative levels of different parts of the district.

[Ill.u.s.tration: Figure 36. Map of Glen Roy]

(FIGURE 36. MAP OF THE PARALLEL ROADS OF GLEN ROY OR LOCHABER.

A. five miles distant south-west from this point is Fort William, where the Lochy joins an arm of the sea, called Loch Eil.

Vertical lines. Cols or watersheds at the heads of the glens--once the westward outlet of the lakes.

Dots. Conspicuous delta deposits as laid down by Mr. T.F. Jamieson.)

Glen Roy is situated in the Western Highlands, about 10 miles east-north-east of Fort William, near the western end of the great glen of Scotland, or Caledonian Ca.n.a.l, and near the foot of the highest of the Grampians, Ben Nevis. (See map, Figure 36.) Throughout nearly its whole length, a distance of more than 10 miles, three parallel roads or shelves are traced along the steep sides of the mountains, as represented in the annexed view, Plate 2, by the late Sir T. d.i.c.k Lauder, each maintaining a perfect horizontality, and continuing at exactly the same level on the opposite sides of the glen. Seen at a distance, they appear like ledges, or roads, cut artificially out of the sides of the hills; but when we are upon them, we can scarcely recognise their existence, so uneven is their surface, and so covered with boulders. They are from 10 to 60 feet broad, and merely differ from the side of the mountain by being somewhat less steep.

On closer inspection, we find that these terraces are stratified in the ordinary manner of alluvial or littoral deposits, as may be seen at those points where ravines have been excavated by torrents. The parallel shelves, therefore, have not been caused by denudation, but by the deposition of detritus, precisely similar to that which is dispersed in smaller quant.i.ties over the declivities of the hills above. These hills consist of clay-slate, mica schist, and granite, which rocks have been worn away and laid bare at a few points immediately above the parallel roads. The lowest of these roads is about 850 feet above the level of the sea, the next about 212 feet higher, and the third 82 feet above the second. There is a fourth shelf, which occurs only in a contiguous valley called Glen Gluoy, which is 12 feet above the highest of all the Glen Roy roads, and consequently about 1156 feet above the level of the sea.*

(* Another detached shelf also occurs at Kilfinnan. (See Map, Figure 36.))

One only, the lowest of the three roads of Glen Roy, is continued throughout Glen Spean, a large valley with which Glen Roy unites. (See Plate 2 and map, Figure 36.) As the shelves, having no slope towards the sea like ordinary river terraces, are always at the same absolute height, they become continually more elevated above the river in proportion as we descend each valley; and they at length terminate very abruptly, without any obvious cause, or any change either in the shape of the ground or in the composition or hardness of the rocks.

I should exceed the limits of this work, were I to attempt to give a full description of all the geographical circ.u.mstances attending these singular terraces, or to discuss the ingenious theories which have been severally proposed to account for them by Dr. Macculloch, Sir T. Lauder, and Messrs. Darwin, Aga.s.siz, Milne, and Chambers. There is one point, however, on which all are agreed, namely, that these shelves are ancient beaches, or littoral formations, acc.u.mulated round the edges of one or more sheets of water which once stood for a long time successively at the level of the several shelves.

[Ill.u.s.tration: Figure 37. Section Through Side of Loch]

(FIGURE 37. SECTION THROUGH SIDE OF LOCH.

AB. Supposed original surface of rock.

CD. Roads or shelves in the outer alluvial covering of the hill.)

It is well known, that wherever a lake or marine fjord exists surrounded by steep mountains subject to disintegration by frost or the action of torrents, some loose matter is washed down annually, especially during the melting of snow, and a check is given to the descent of this detritus at the point where it reaches the waters of the lake. The waves then spread out the materials along the sh.o.r.e, and throw some of them upon the beach; their dispersing power being aided by the ice, which often adheres to pebbles during the winter months, and gives buoyancy to them. The annexed diagram (Figure 37) ill.u.s.trates the manner in which Dr. MacCulloch and Mr. Darwin suppose "the roads" to const.i.tute mere excrescences of the superficial alluvial coating which rests upon the hillside, and consists chiefly of clay and sharp unrounded stones.

Among other proofs that the parallel roads have really been formed along the margin of a sheet of water, it may be mentioned, that wherever an isolated hill rises in the middle of the glen above the level of any particular shelf, as in Mealderry, Plate 2, a corresponding shelf is seen at the same level pa.s.sing round the hill, as would have happened if it had once formed an island in a lake or fjord. Another very remarkable peculiarity in these terraces is this; each of them comes in some portion of its course to a col, or parting ridge, between the heads of glens, the explanation of which will be considered in the sequel.

Those writers who first advocated the doctrine that the roads were the ancient beaches of freshwater lakes, were unable to offer any probable hypothesis respecting the formation and subsequent removal of barriers of sufficient height and solidity to dam up the water. To introduce any violent convulsion for their removal was inconsistent with the uninterrupted horizontality of the roads, and with the undisturbed aspect of those parts of the glens where the shelves come suddenly to an end.