href="#litres_trial_promo"> Pl. 8, shows a fine example of a hanging valley, while truncated spurs can be seen in Pl. 10.
Additional results of this form of erosion are, first, that vast quantities of detritus are removed and scattered over the adjacent country; and, secondly, that the existing forms of low relief are “sharpened” as it were and made more mountainous in aspect (see Pl. 23). The gentle plateau-like profiles of some of our mountain areas, as seen from a distance, give no hint as to the steepness and wildness of the ridges and valleys they prove to contain. At the same time, the removal of pre-existing soils and gentle slopes has generally left the valleys in what is essentially a “montane” condition of bare rock or rock detritus in marked contrast to the deep and long-established soils that must have prevailed in pre-glacial conditions.
As to the materials left behind by the ice, the most obvious are usually coarse rock-waste in the valleys, often material which had fallen from adjacent hills on to the ice during the last stages of its retreat. The effects of these last remaining valley glaciers were comparable indeed to those observed in existing high alpine regions, but they were insignificant as compared with those of the great inland ice-sheets. The detritus accumulated by these sheets in their ground moraines and left behind when they finally melted, has largely determined the appearance of the existing British lowlands and indeed of the lower valleys, obliterating all pre-existing soils or underlying rock, and often smothering the pre-glacial features under layers of “drift” 30 or 40 ft. thick (see Pl. V). Although it may contain rock-waste of almost every type, the drift usually consists of some form of boulder clay in which ice-worn and scratched boulders are mixed with what is really rock-flour. The prominent clay fraction of this drift, whatever its general texture, usually suffices to make it “set” readily, and accordingly it often tends to be impervious to water. Not only has this type of material been scattered to great depths all over the lowlands adjacent to our mountain areas, but, in thinner sheets, it will also often be found to be plastered over almost any area of low relief in the mountains themselves. Consequently most gentle slopes in the upland districts, if not left scoured clean by ice action, are covered with drift which is often of a clayey nature.
These drifts and morainic deposits usually contain material carried from a distance and, like the erratics, unrelated in characters to the locally underlying rock. Pl. 9, for example, shows some glacial ground moraines of a non-calcareous nature bearing moorland vegetation, although the underlying rock is limestone, on which such vegetation does not normally occur. A similar effect is clearly shown in Pl. 17, where drift overlying limestone has blocked up the drainage so thoroughly that peat, bearing moorland vegetation, has developed over the limestone. Of course the reverse can also take place. In many places, calcareous clays have been distributed over non-calcareous strata, and thus as a general rule considerable caution is necessary in glaciated regions in attempting to relate soil or vegetation types to underlying rock.
Of course, at the close of the glacial period, there was a great deal of resorting of the rubbish left behind by the melting ice. The vast quantities of moving water which must have resulted during the melting process obviously redistributed the morainic materials on an enormous scale. Consequently to-day we often see streams running down valleys that now seem far too large for them or issuing across deltas which, quite obviously, they could never have produced in their present condition. A particularly striking condition at that time must have been the numerous large lakes held up among the ice-sheets, often in what are now quite unexpected places. At least one of the high limestone hills in Yorkshire, Moughton Fell, has lake silts on its summit, and such sediments or delta cones deposited in water are not uncommon on the flanks of the wider valleys. A classical British example, Lake Pickering, lying to the south of the Cleveland Hills in North Yorkshire, is associated with the name of the late Professor P. F. Kendall, who showed how the existence of the ice-dammed lake could be recognised. While we are not concerned here with the detailed history of these bodies of water, it should be recognised that they left behind various sediments, including impervious ones, that helped to create local stagnation of the drainage system; and, as we shall see, so gave a definite character to the sites they had occupied.
The great glaciation merits more attention than we can really give it, and this not only because it moulded our scenery. It is far more important because it represents a characteristic phase in the history of our mountains and moorlands as we now know them, as well as the agency which has perhaps more than anything else determined their present biological character. Whatever may ultimately prove to be the underlying cause of such an ice-age, it cannot effectively develop, as Sir George Simpson has emphasised, without the heavy precipitatation (then as snow, now as rain) that characterises British mountains. From this point of view, the Ice Age seems to be as characteristic of British highlands as is the present climate. From the historical point of view, the Ice Age is important because it means that the starting-point from which our present fauna and flora is derived must have been largely an arctic-alpine group of organisms. From still a third point of view—of particular interest to the botanist—the glacial epoch left behind it an upland country largely sterilised by the removal of existing soils and fertile deposits. Some areas, such as large parts of the Hebrides and of Sutherland, were in fact left sterile, and even to-day remain as an almost bare and undulating rock surface occupied only by small tarns and moorland of the bleakest type. This is a common condition among the mountains. But speaking generally, even among the mountains and particularly among the foothills round each mountain group, areas of gentle slope were usually plastered over with clayey drifts or sediments. The result was a great deterioration of the natural drainage in many areas. Even the limestone hills, as we have seen above, Plates 9 and 17, though naturally extremely porous, often suffered in this way, and in many cases cannot be superficially distinguished from the non-calcareous rocks around them in this respect. Elsewhere, and particularly in the valleys, the erosion of the lower slopes by ice and by glacial streams left behind a sharpened relief of a much more montane character and, incidentally, often paved the way for a new cycle of erosion.
RECENT EROSION
The forms of the mountains we see to-day are clearly the results of three agencies — of the original rock structure as modified by large-scale earth movements, of the long continued erosion which, acting on the original structure, has fixed the positions and outlines of the main valleys and summits, and, lastly, of the sharpening of land forms and the removal of pre-glacial screes and soils by ice action. As a habitat for living organisms, the surface of a mountain is as important as its skeleton, and this is affected not only by the legacy of slope and structure already described, but still more by the recent or post-glacial effects of erosion. Speaking generally, the upland surfaces are either physically stable or unstable, and it is the large proportion of unstable surfaces which is particularly characteristic of upland areas. Nevertheless, even the stable surfaces, those more comparable in slope and form with lowland areas, show peculiarities, for they are often rock surfaces, either scraped clean during glaciation (as in the Northern Scottish examples just mentioned) or sometimes, like limestone pavements, composed of rock which yields little or no soil on weathering. Even the soil-covered stable surfaces are often areas covered by poor glacial drifts or with impervious rock strata beneath, and now mostly peat-covered.
FIG. 8.—Arrangement of main zones below a rock outcrop.
The unstable surfaces naturally tend to lie along the main lines of erosion, and they include both the places over-steepened by ice action as well as those showing the immediate effects of stream erosion. The most widespread type of unstable surface is the steep scree-slope with its capping of crag. This generally shows a gradation
