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
