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 Forfarshire, 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 mass 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 passage about 400 yards wide.
I have also given an account of another striking feature in the physical geography of Perthshire and Forfarshire, 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 shell-marl and peat. This band of till, with Grampian boulders and associated 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 principal 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.
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 passed 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 shell-marl deposited at their bottom. All the mammalia as well as shells 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 circumstances 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 Canal between Edinburgh and Falkirk, which passed for no less than 28 miles almost continuously through it. Mr. Baird, the engineer who superintended the works, assisted 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 circumference, 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 circumference, lying in an horizontal position, 17 feet deep in clay, with marine shells, at Kilmaurs, in Ayrshire. The species of shells 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 Dumbartonshire, 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 shells, Cyprina islandica, Astarte elliptica, A. compressa, Fusus antiquus, Littorina littorea, and a Balanus. 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 associated with flint implements of the antique type.
