The Existence of Threshold Hillslopes in the Denudation of the Landscape

Abstract

The frequency distribution of angles of straight hillslopes in two areas of strong, well-jointed rock, one on Exmoor and the other in the southern Pennines, has been determined through field survey. These histograms are trimodal and angles of 20?-2I?, 25?-27? and 32?-34? are 'characteristic' of both areas. Analysis of the shear-strength characteristics of the waste mantle on these three slope types indicates that these particular angles are very probably limiting angles of stability for these three types of waste mantle. A model of slope evolution based on the history of disintegration of the weathered mantle is suggested and compared with the qualitative ideas of W. M. Davis on the development of hillslopes. THE CONCEPT of grade has probably been one of the most misunderstood and abused in the whole of geomorphology, and yet its main thesis is extremely simple. This is that most denudational processes operating in the landscape tend to create a threshold angle of slope. At angles above this threshold value there is rapid denudation which must produce a slope of a lower angle. This may be achieved either through the flattening of the existing slope until the angle is at the threshold for the operation of the process, or through the retreat of the initial slope and the emergence of a new slope, at the threshold angle, at the base of the slope. Slopes will not be produced at angles below the threshold, since at gentler angles the process can no longer be operative. The universality of this concept was stressed by W. M. Davis1 who applied it to both stream and valley-side slopes. Davis pointed out that denudation may still take place when a slope is graded. The threshold slope depends as much on the calibre of the material upon the slope as on the energy of the process, so that, as the calibre of the material diminishes during geological time, it becomes possible for the angle of the threshold slope to diminish. The concepts of grade and slope decline are thus not incompatible: slope decline according to Davis is simply a diminishing value of the graded slope. The proponents of the school of parallel retreat2 also recognize the concept of the threshold slope. They argue, however, that the fact that a slope may not be lowered below its threshold angle, assuming that the threshold angle is constant over time, still does not preclude denudation. The transport of debris may still take place on a threshold slope but through slope retreat rather than decline. In this way the slope still exists at its threshold angle. It is one of the paradoxes of geomorphology that, although confronted with this almost universal concept of the threshold slope, no attempts have been made to examine whether or