Rock Floors in Arid and in Humid Climates. I

This paper, after an introduction regarding the fundamental elements of the cycle of erosion, institutes a comparison between the barren mountain sides and the desert rock floors (pediments) of arid regions and the forested mountain slopes and the fertile peneplains of humid regions, with the object of discovering how far these contrasted landscapes are comparable with each other and of denning the various factors on which their contrasts depend. For humid regions the familiar principles of weathering and streams are followed; for arid regions the principles of degradation set forth by Lawson and the examples described by Bryan are accepted as guides. Sequences of forms are sketched for both regions, beginning with those which are largely consequent upon the deformation of a peneplain of massive granite, and ending with those which are wholly consequent upon the far advanced degradation of the deformed mass. It is concluded that a close analogy exists between the two sequences, and that their difference of appearance is due to their difference of climate. The plant cover of humid regions permits the accumulation of abundant soil even on fairly steep slopes; and of still deeper soil on lowlands of degradation; yet under the soil cover of mountain slopes boulders of decomposition are found grading into solid rock, which are closely homologous with the visible boulders resting on the bare rock of arid mountains. Similarly, under the heavy soil cover of a granite peneplain is a buried foundation of decomposed rock which is truly homologous with the gravel-veneered rock floors or pediments of arid degradation.