Page:Quarterly Journal of the Geological Society of London, vol. 31.djvu/94

48 48 J. PRESTWICH ON THE QUATERNARY PHENOMENA a tergo propelling forward the debris beyond and out of reach of any overhanging cliff. We may, I think, consider that the following points are now established : — 1. That the debris is entirely angular, and is composed, if not altogether of local materials, at all events of such as come within the area of drainage. 2. That the finer material is more frequently found in largest proportion at the base of the deposit, but that it recurs irregu- larly all through the mass of the deposit from bottom to top. 3. That the deposit has a rough stratification caused by the in- tercalation of irregular lenticular masses of fine and coarse portions, the whole having a prolonged and gradually decreasing slope seaward at angles far smaller than could possibly be taken by materials falling over any cliff or a sloping escarpment. 4. That it is thickest at the base of valleys or of cliffs where they debouch on the coast-line. 5. That it contains the bones of land animals, together with land and freshwater shells. These points afford definite grounds for speculation. The mate- rials are evidently of origin which, if not altogether local, never extends beyond the drainage-area of the spot ; and it is equally clear from their arrangement and the distance they subtend the cliff, that no mere subaerial action of weathering could have produced such a deposit. Nor can I conceive the presence in any subaerial talus of delicate land shells and of detached and broken bones of land animals ; for the former are of species which would not frequent dry and stony surfaces, while the skeletons of any of those which might happen to have been destroyed by a fall, would hardly have their parts so dispersed and reduced to so fragmentary state as we find here. On the supposition of Mr. Godwin-Austen that the degradation of the surface took place at a high altitude in regions of excessive cold, we might obtain a greater propulsive power by the action of snow or ice ; but still I doubt whether it would be sufficient to give rise to the prolonged slopes we have noticed, while the presence of land and freshwater shells, of bones of the horse, elephant, rhinoceros, &c. in some places, seems irreconcilable with such conditions of land. On the other hand it is evident that we have in this deposit a surface-wash composed of the loose debris of the rocks of the vicinity, of the shells and slugs of a land surface, and of the remains of animals which might have frequented the same. It seems to me that the results may be ascribed to one of three causes. Either, first, a mass of ice passing over the surface of the land may have pushed forward the debris and thrown it seaward down valleys and over cliffs, turning over their edges ; or, secondly, the accumulations of winter snow may, on sloping surfaces, have tended in a lesser degree to the same result ; or, thirdly, the transient passage of a body of water may have swept the land-debris down to