Читать книгу Mountains and Moorlands - W. Pearsall H. - Страница 9

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 of form and composition such as is illustrated in Fig. 8. The upper part is usually the steepest, and consists of coarser detritus, while the lower part shows finer detritus and gentler slope. As on any steep slope, rainwash leads to the accumulation of the finest materials on the lower parts.

The downward movement of material continues long after an angle of primary stability (usually between 30° and 40°) is reached; and there are numerous interesting manifestations of this movement. The larger stones, in particular, are usually persistent “creepers,” expanding more on the lower side when the temperature rises and contracting more on the upper margin when cooling takes place. They often continue to move downwards long after the rest of the surface has been stabilised by vegetation. When such stones are elongated in shape they generally tend to progress with their long axes more or less parallel to the slope, as may be seen in Pl. 25. Although the larger boulders move most persistently on partly stabilised screes, they usually move more slowly than the finer material on loose scree slopes, where the finer materials often accumulate around the upper side of the boulders, giving a step-like arrangement. Obstacles such as tufts of grass lead to a similar effect, so that some form of terracing is particularly characteristic of steep mountain slopes, even after they have been partly stabilised by vegetation, and one has only to look down on a steep grassy slope under suitable lighting conditions to see what are apparently innumerable more or less parallel “sheep-track” terraces, due mainly to the agencies of soil-creep and rain-wash, though nowadays much accentuated by the movements of grazing animals.

While the characteristic features of crag and scree may occur at almost any level, there are other types of instability which are particularly characteristic of the higher altitudes above 2,000 ft., and generally most clearly shown on the high summits. The high mountains are generally but little affected by the action of running water, and their erosion is due far more to the effects of frost and snow, sometimes collectively distinguished as nivation.

The surface of the higher and steeper summits is commonly covered with rock detritus, sometimes to a depth of several feet (see Pl. III). This material, often called mountain-top detritus, is formed by the disintegration of the native rock by the action of frost. The size of the individual fragments, as in the case of screes, depends largely upon the hardness and the physical character of the underlying rocks.

The frost detritus or mountain-top detritus is the most characteristic of summit surfaces. Its appearance is well illustrated in a number of the plates included here: Pl. 11a Pl. 12 Pl. 25 and its loose surface indicates the constant struggle between the stabilising effect of vegetation and the instability due to wind exposure and the action of frost, snow and gravity. In the plates given here, the striking instability of very slight slopes at high levels is clearly shown. In the examples pictured in Pl. 11a and Pl. 25 the slopes have an inclination of only about 10° to 15°, although the surfaces show little tendency to be fixed by vegetation. A slope of 30° at lower altitudes would quickly become completely covered by vegetation and hence more thoroughly stabilised.

The instability of the surfaces at high altitudes is not confined to those that are predominantly or wholly stony. It is equally evident on many of the more rounded mountains (“moels”) and on those on which the friable nature of the underlying rock has permitted some soil formation. Here solifluction effects may become extremely marked. When soil highly charged with water first freezes and then melts, the expansion accompanying freezing makes the soil very unstable when it thaws, so that downward movement on even the gentlest of slopes becomes possible, the semi-fluid surface slipping easily over the frozen sub-soil. On steeper slopes, large volumes of muddy detritus may be stripped off the flank of a mountain through this agency, and at high levels soil-covered slopes, however slight, almost invariably show signs of movement produced in this way (see Pl. XIb). The most frequent signs are different forms of terracing, and these occur on quite gentle slopes and where vegetation is present. The swollen soil behaves almost as a series of fluid drops, each partly restrained by the turf, which prevents complete movement, bounding the whole on the lower side in the form of a step, the earth being exposed on the upper flatter part.

Of the reality and importance of these influences, no one who has frequented mountain summits in spring can have any doubt. The mountain soils at that time are “puffed up,” as it were, so that the foot sinks deeply into them. The frequent freezing and thawing has the effect of mixing the soil surface, and, in particular, it causes frost-heaving by which the stones present are extruded, so that the surface is commonly more stony than the material beneath.

The processes seen at work on the higher mountain summits bear a considerable resemblance to those observed in arctic regions. On flat or nearly flat surfaces in the Arctic, solifluction effects are associated with the production of curious “stone polygons” in which a central area of mud, often associated with smaller rock detritus, is surrounded by a polygonal boundary of larger stones. Possibly because of the prevalent slopes, polygons of this sort are not very common on British mountains, although they have been recorded by Professor J. W. Gregory from Merrick in the Southern Uplands and by Dr. J. B. Simpson from Ben Iadain in Morven. An interesting area may be seen at about 3,100 ft. on the broad saddle connecting Foel Grach with Carnedd Llewellyn. This shows that the polygons are found only on a flat surface, giving way to “stone stripes” as soon as the

FIG. 9.—Distribution of materials below “stone stripes.” (Diagrammatic.)

surface acquires an appreciable slope. Stone stripes, or the somewhat similar “striped screes” which appear in coarser and more sloping material, were first described in this country by Professor S. E. Hollingworth from examples in the Lake District, where, once one learns to look for them, they are not uncommon. The larger stones collect in rows parallel to the slope as is shown in Fig. 9. The stone stripes, like polygons, overlie soil, and presumably the stones have been extruded from the soil by the movements due to freezing and thawing. Apparently both polygons and stripes occur where frozen layers of soil persist below the thawed surface. The British mountain polygons and stone stripes are often quite small. Those shown in Pl. XI, were only about a foot apart, though where the movements are on a larger scale they may be three or four times this size. There are especially striking ones on the eastern face of Yr Elen in Snowdonia which can easily be seen from a distance of over a mile.

Many of these solifluction areas illustrate the general feature that the unstable areas on high mountains are often as characteristic of gentle slopes as of steep slopes. Thus the average angle at which an equivalent degree of stability is reached seems to be much less at 2,500 ft. and upwards than at, say, 1,000 ft. No doubt the slower growth of vegetation at higher altitudes also contributes to this condition. Nevertheless, if it is invariably the case, the difference must have a considerable influence on the shape of a mountain. Wherever the rock structure allows comparable rates of weathering, we might perhaps expect to get a shape of the type illustrated in Fig. 4, rather than the simple cone. The preliminary steepening of the lower slopes must, of course, be due to more remote causes.

There are probably other processes by which rock and soil movement may be brought about at high levels, though they do not seem to have been much studied in this country. Some are undoubtedly associated with places where snow lies long. Where such a slope persists below a region of surface instability, rock-waste may move rapidly downward across the snow surface, collecting in a band at its base. Further, long-persistent snow-banks almost always terminate below in erosion channels, which, at the higher levels, may give permanent drainage channels cutting back towards the mountain crest. The interest of these features is not only biological (see here), but it lies also in their possible bearing on the origin of the high-level corries (or cwms or cirques) so often found in the larger British mountains. In the extreme form these are rock basins and undoubtedly relics of the small high-level glaciers and nevé which must have lingered on for long after the main ice-sheets had passed away. Corries seem to be most frequent on the east of a main summit or ridge, and it may be that in the first place their position was the result of a semi-permanent snow-bank which started an erosion system. In the later stages it has been supposed that the upper nevé exerts a plucking action on the frost-shattered mountain face, through the periodic filling and downward contraction of the bergschrund, if one may use this term in such a case. In this way continuous over-steepening of the head of the erosion system may have resulted in the formation of the crags encircling the corrie. It seems probable that corrie-formation was most vigorous during and just after the Ice Age, but as it usually lies above the other main erosion effects of the ice-sheets it may be appropriate to regard it as an extreme effect of persistent snow-lie.

In attempting to summarise what has been obtained from this survey, it becomes clear that physical instability is the most noticeable feature of upland surfaces, and it is equally evidently a chief characteristic of the high-level or montane region—although it also accompanies any steep slope as well as the borders of active erosion systems such as streams. Physically stable areas in the uplands differ little from lowland areas, except in other features such as those of climatic origin.

We also see that British mountains are often likely to show an upper zone of comparatively gentle slopes, representing the ancient land forms, moulded long ago, but often kept alive or unstable through the agencies we call nivation. The lower slopes have often been over-steepened in comparatively recent times as a result of glaciation or of the extensive erosion which must have been associated with the melting of the ice. This common plan, if we may call it so, results in the appearance of numerous rather round-topped mountains, although it is modified in innumerable ways as a result of the varieties of rock which make up the mountain blocks and of the different sorts of bedding planes which may be found in different areas.

There is still another way of looking at these matters. The present cycle of erosion as it affects the upland surfaces may be considered to have started at the end of the Ice Age. The upland surface at that time, except where covered by drifts or morainic materials, must have been very different from what it is to-day. It must have been mostly exposed rock which, presumably under the sub-Arctic post-glacial conditions, quickly developed frost-shattering and the characteristic erosion forms found to-day in the Arctic and at high altitudes. To-day much of the corresponding surface is soil- or peat-covered, and only the montane or unstable areas preserve what must have been a widespread condition in the immediate post-glacial period. It will be seen, therefore, if this argument is correct, that the study of the montane areas is likely to be of especial interest. We shall expect to find that the biological character of the unstable areas is widely different from that found elsewhere and perhaps in some respects reminiscent of a condition that was more widespread in post-glacial times.