The upper boulder-clay of our islands is the product of another mer de glace, which in Scotland would seem to have been hardly less thick and extensive than its predecessor. Like the latter, it covered the whole country, overflowed the Outer Hebrides, and became confluent with the Scandinavian inland-ice on the bed of the North Sea. But it did not flow so far to the south as the earlier ice-sheet.
It is well known that this later mer de glace was succeeded in our mountain-regions by a series of large local glaciers, which geologists generally believe were its direct descendants. It is supposed, in short, that the inland-ice, after retreating from the low-grounds, persisted for a time in the form of local glaciers in mountain-valleys. This view I also formerly held, although there were certain appearances which seemed to indicate that, after the ice-sheet had melted away from the Lowlands and shrunk far into the mountains, a general advance of great valley-glaciers had taken place. I had observed, for example, that the upper boulder-clay is often well developed in the lower reaches of our mountain-valleys—that, in fact, it may be met with more or less abundantly up to the point at which large terminal moraines are encountered. More than this, I had noticed that upland valleys, in which no local or terminal moraines occur, are usually clothed and paved with boulder-clay throughout. Again, the aspect of valleys which have been occupied by large local glaciers is very suggestive. Above the point at which terminal moraines occur only meagre patches of till are met with on the bottoms of the valleys. The adjacent hill-slopes up to a certain line may show bare rock, sprinkled perchance with erratics and superficial morainic detritus; but above this line, if the acclivity be not too great, boulder-clay often comes on again. These appearances are most conspicuously displayed in the southern Uplands of Scotland, particularly in south Ayrshire and Galloway, and long ago they led me to suspect that the local glaciers into which our latest mer de glace was resolved, after retreating continuously towards the heads of their valleys, so as to leave the boulder-clay in a comparatively unmodified condition, had again advanced and ploughed this out, down to the point at which they dropped their terminal moraines. Subsequent observations in the Highlands and the Inner and Outer Hebrides confirmed me in my suspicion, for in all those regions we meet with phenomena of precisely the same kind. My friends and colleagues, Messrs. Peach and Horne, had independently come to a similar conclusion; and the more recent work of the Geological Survey in the north-west Highlands, as they inform me, has demonstrated that after the dissolution of the general ice-sheet underneath which the upper boulder-clay was accumulated, a strong recrudescence of glacial conditions supervened, and a general advance of great valley-glaciers took place—the glaciers in many places coalescing upon the low-grounds to form united mers de glace of considerable extent.
The development of these large glaciers, therefore, forms a distinct stage in the history of the Glacial period. They were of sufficient extent to occupy all the fiords of the northern and western Highlands, at the mouths of which they calved their icebergs, and they descended the valleys on the eastern slopes of the land into the region of the great lakes, at the lower ends of which we encounter their outermost terminal moraines. The Shetland and Orkney Islands and the Inner and Outer Hebrides at the same time nourished local glaciers, not a few of which flowed into the sea. Such, for example, was the case in Skye, Harris, South Uist, and Arran. The broad Uplands of the south were likewise clothed with snow-fields that fed numerous glaciers. These were especially conspicuous in the wilds of Galloway, but they appeared likewise in the Peeblesshire hills; and even in less elevated tracts they have left more or less well-marked traces of their former presence.
It is to this third epoch of glaciation that I would assign the final scooping out of our lake-basins and the completion of the deep depressions in the beds of our Highland fiords. All the evidence, indeed, leads to the conviction that the epoch was one of long duration.
It goes without saying that what holds good for Scotland must, within certain limits, hold good also for Ireland and England. In Wales and the Cumberland lake district, and in the mountain-regions of the sister island, we meet with evidence of similar conditions. Each of those areas has obviously experienced intense local glaciation subsequent to the disappearance of the last big ice-sheet.
Attention must now be directed to another series of facts which help us to realise the general conditions that obtained during the epoch of local glaciation. In the basin of the estuary of the Clyde, and at various other places both on the west and east coasts of Scotland, occur certain clays and sands, which overlie the upper boulder-clay, and in some places are found wrapping round the kames and osar of the last great ice-sheet. These beds are charged with the relics of a boreal and arctic fauna, and indicate a submergence of rather more than 100 feet. In the lower reaches of the rivers Clyde, Forth, and Tay the clays and sands form a well-marked terrace, and a raised sea-beach, containing similar organisms, occurs here and there on the sea-coast, as between Dundee and Arbroath, on the southern shores of the Moray Firth, and elsewhere. When the terraces are traced inland they are found to pass into high-level fluviatile gravels, which may be followed into the mountain-valleys, until eventually they shade off into fluvio-glacial detritus associated with the terminal moraines of the great local glaciers. It is obvious, in short, that the epoch of local ice-sheets and large valley-glaciers was one also of partial submergence. This is further shown by the fact that in some places the glaciers that reached the sea threw down their moraines on the 100-feet beach. It must have been an epoch of much floating ice, as the marine deposits contain now and again many erratics, large and small, and are, moreover, frequently disturbed and contorted as if from the grounding of pack-ice.
The phenomena which I have thus briefly sketched suffice to show that the epoch of local glaciation is to be clearly distinguished from that of the latest general mer de glace. I have long suspected, indeed, that the two may have been separated by as wide an interval of time as that which divided the earlier from the later epoch of general glaciation. Again and again I have searched underneath the terminal moraines, in the faint hope of detecting interglacial accumulations. My failure to discover these, however, did not weaken my conviction, for it was only by the merest chance that interglacial beds could ever have been preserved in such places. I feel sure, however, that they must occur among the older alluvia of our Lowlands. Indeed, as I shall point out in the sequel, it is highly probable that they are already known, and that we have hitherto failed to recognise their true position in the glacial series.
Although we have no direct evidence to prove that a long interglacial epoch of mild conditions immediately preceded the advent of our local ice-sheets and large valley-glaciers, yet the indirect evidence is so strong that we seem driven to admit that such must have been the case. To show this I must briefly recapitulate what is now known as to the glacial succession on the Continent. It has been ascertained, then, that the Scandinavian ice has invaded the low-grounds of Germany on two separate occasions, which are spoken of by Continental geologists as the “first” and “second” glacial epochs. The earlier of these was the epoch of maximum glaciation, when the inland ice flowed south into Saxony, and overspread a vast area between the borders of the North Sea and the base of the Ural Mountains. This ice-sheet unquestionably coalesced with the mer de glace of the British Islands. Its bottom-moraine and the associated fluvio-glacial detritus are known in Germany as “Lower Diluvium,” and the various phenomena connected with it clearly show that the inland-ice radiated outwards from the high-grounds of Scandinavia. The terminal front of that vast mer de glace is roughly indicated by a line drawn from the south coast of Belgium round the north base of the Harz, and by Leipzig and Dresden to Krakow, thence north-east to Nijnii Novgorod, and further north to the head-waters of the Dvina and the shores of the Arctic Sea near the Tcheskaia Gulf.
The lower diluvium is covered in certain places by interglacial deposits and an overlying upper diluvium—a succession clearly indicative of climatic changes. In the interglacial beds occur remains of Elephas antiquus and other Pleistocene mammals, and a flora which denotes a genial temperate climate. One of the latest discoveries of interglacial remains is that of two peat-beds lying between the lower and upper diluvium near Grünenthal in Holstein.[CA] Among the abundant plant-relics are pines and firs (no longer indigenous to Schleswig-Holstein), aspen, willow, white birch, hazel, hornbeam, oak, and juniper. Associated with these are Ilex and Trapa natans, the presence of which, as Dr. Weber remarks, betokens a climate like that of western middle Germany. Amongst the plants is a water-lily, which occurs also in the interglacial beds of Switzerland, but is not now found in Europe. The evidence furnished by this and other interglacial deposits in north Germany shows that, after the ice-sheet of the lower diluvium had melted away, the climate became as temperate as that now experienced in Europe. Another recent find of the same kind[CB] is the “diluvial” peat, etc., of Klinge, in Brandenburg, described by Professor Nehring. These beds have yielded remains of elk (Cervus alces), rhinoceros (species not determined), a small fox (?), and Megaceros. This latter is not the typical great Irish deer, but a variety (C. megaceros, var. Ruffii, Nehring). The plant-remains include pine, fir (Picea excelsa), hornbeam, warty birch (Betula verrucosa), various willows (Salix repens, S. aurita, S. caprea [?], S. cinerea), hazel, poplar (?), common holly, etc. It is worthy of note that here also the interglacial water-lily (Cratopleura) of Schleswig-Holstein and Switzerland makes its appearance. Dr. Weber writes me that the facies of this flora implies a well-marked temperate insular climate (Seeklima). The occurrence of holly in the heart of the Continent, where it no longer grows wild, is particularly noteworthy. The evidence furnished by such a flora leads one to conclude that at the climax of the genial interglacial epoch, the Scandinavian snow-fields and glaciers were not more extensive than they are at present.
[CA] Neues Jahrbuch f. Min. Geol. u. Palæont., 1891, ii., pp. 62, 228; Ibid., 1892, i., p. 114.