biological type.
The chief contrast in the Grampians is, however, between the eastern and western halves of the country. The former, exemplified particularly by the Cairngorms, is mainly a high though deeply dissected plateau, which constitutes the greatest continuous area of high ground in the British Isles. The Cairngorms are evidently in the early stages of a new erosion cycle, and their typical outlines, already discussed in connection with Pl. 5, contrast remarkably with those of Dartmoor, for example, also a granite mass, but one characterised by land-forms indicating far advanced weathering and erosion (see Pl. XXXI).
In the western part of the Highlands, erosion and dissection have proceeded far more effectively, so that more often the mountains are partly isolated peaks or broken ridges. The change has undoubtedly been hastened not only by greater precipitation and glacial erosion in these areas, but also by the presence of numerous faults, running roughly from north-east to south-west, which have offered full play to eroding influences and have given us a series of loch-filled valleys. The most notable of these fault-lines is that of the Great Glen. Nevertheless, in spite of the much greater amount of erosion, the general level of the summits among the western mountains is very uniform and is indicative of that of the original plateau from which they must have been derived.
The Scottish Highlands illustrate very well a point that was emphasised a long time ago by the late Professor J. E. Marr. In general, as upland surfaces recover from disturbances, they will tend to develop systems of gentle slopes and to approach, as Dartmoor is doing, characteristic forms of “subdued relief.” Among the upper levels of our British mountain regions it is possible to see a large proportion of land forms which are predominantly those of subdued relief. This implies that these forms must be of great age, for on account of the great hardness of the rocks, it must have taken an enormous time for the outlines to have “softened” in such an extreme manner. From arguments such as these, it may be assumed that the general form of our mountain regions is often ancient, and this usually applies particularly to the positions of the main summits and the river valleys. Superimposed on these ancient features we have also features which are the result of comparatively recent agencies. Foremost among these are the effects of ice and of glaciation.
Much British mountain scenery is that characteristic of a glaciated and ice-eroded country. That there is a marked contrast with other regions will be at once apparent if one compares a typical British upland scene with one, for example, from the Grand Canyon of Colorado.
FIG. 7.—Ice movements in the British Isles. GLACIATION
The most striking feature in the recent geological history of the British Isles was the series of great Quaternary Glaciations, which terminated only some 10,000 years ago. For biologists this is a convenient starting-point for recent biological history, but it was scenically of equal or greater importance. In order to obtain a picture of what Britain was like during the Glacial period, we should have to try to imagine it buried beneath a great ice-sheet many hundreds of feet thick, and covering, at its maximum extent, almost the whole of these islands north of the River Thames. The centres of ice formation were the areas with greatest precipitation (then snowfall, now rain), particularly the greater area of the Highlands of Scotland, centring on Rannoch Moor, to a less extent the Southern Uplands from Merrick outwards and the smaller Lake District, and also, but still less, Snowdonia. From these and other smaller centres the ice flowed outwards, though very slowly. A huge existing ice-sheet, that in Antarctica, to-day is still moving at the rate of a yard and a half a day when it reaches the sea as the Ross Barrier, hundreds of miles from its source.
We can trace the main directions in which our British ice-sheets moved, because they carried with them all the soil and rock detritus that had accumulated on the surface of the land in the preceding ages. Any unusual types of rock are readily recognised and, because they have characteristic fossils or special mineral constituents, limestone and igneous or volcanic rocks are especially useful for this purpose. Rocks thus found far from their place of origin are termed erratics, and the photograph in Pl. VI, shows a well-known example, one of the Norber boulders in West Yorkshire, slate rocks carried by ice from an adjacent valley and left on top of the Carboniferous Limestone which normally overlies the slates (see Fig. 2). In England, erratics of the Shap granite, coming from a small area in the eastern Lake District, have been particularly valuable in tracing the movements of Lake District ice. A magnificent boulder of this rock some ten feet in cube, standing in the main quadrangle of the University of Manchester, illustrates the fact that ice from the Lake District left debris as far south as Cheshire. Farther west there was an ice-flow carrying Galloway granite to Flint and Shropshire. Similar evidence shows that some Lake District ice went east over Stainmoor, leaving boulders of Shap granite as far away as the Yorkshire coast. The accompanying map (Fig. 7), constructed mainly from evidence of this type, shows the main lines of ice movement in Britain during this period. It will be noticed from this map that the ice movements did not always follow the obvious lines of outward radiation. In Lancashire and Wales, for example, the ice was deflected southwards and eastwards by Scottish and Irish sea-borne ice. In Scotland particularly, and to some extent in Yorkshire and Northumbria, the outward-moving ice was dammed up and deflected by Scandinavian ice coming across the North Sea. Moreover, in the partial northerly deflection of the northern ice there is evidence that it overrode mountains 3,000 ft. high. On the west coast of the Highlands, the ice-marks not only reach this altitude, but, allowing for the depth of adjacent lochs, it can be estimated that the ice-field must at times have been some 4,000 ft. thick. Similarly in the Lake District, where the area of high precipitation is much smaller, the ice-fields were some 2,000 ft. thick. Indeed, on Scafell and Helvellyn the marks of glaciation may be seen up to a height of 2,500 ft. It is not easy to imagine the scale of this ice-covering. The nearest thing to it at present may be the Greenland ice-cap; that in Antarctica is apparently larger.
Even at its maximum extent, it did not wholly cover the country (see Fig. 7). There could not have been much ice south of the Thames, and Dartmoor seems to have been quite unglaciated. Moreover, the highest mountains, and, indeed, many of the lower outlying ones, projected through the ice as nunataks. They can often be recognised by their greater altitude and bolder shape, which contrasts markedly with that of the lower, rounded and glaciated hills. In the later stages of the Ice Age, at least, considerable areas of the Southern Pennines may have been generally ice-free, though no doubt supplied with local snowfields. The main ice-flow at this time seems to have been deflected by the Howgill and Bowland Fells, or westward down into the Cheshire Plain. The existence of ice-free areas makes the comparison with Greenland more valuable and it allows us to assume that there were probably at least some plants and animals there.
In their movements, the ice-sheets not only scoured away existing soils and rock debris, but they also scraped away rock. Thus in glaciated regions, every projecting rock tends to be smoothed and scratched on the exposed side, even if it retains rough surfaces on the lee side. Such rocks are termed “roches moutonnées,” and often they allow us to infer the direction of local ice movements even better than do erratics. Although it did not invariably do so, the moving ice tended to follow existing valley lines and hence these were scoured out and deepened, particularly towards the valley heads where the ice was normally deeper. Often rock basins were formed which now contain lakes (see Pl. 8). The form of these glaciated valleys (and of the lakes) is very characteristic: they tend to be “canal-sided” in plan and U-shaped in section. The effect of these great ice-sheets is not only to deepen and broaden the main valleys but also in doing so to remove the lower and gentler slopes on each side. Thus spurs are cut off and lateral valleys are cut short, while the lateral streams they contain now tend to enter the main valley by sudden rapids or waterfalls. “Hanging valleys” of this type and “truncated spurs” are a characteristic feature of British mountain scenery. The photograph of Loch Avon in
