Page:Quarterly Journal of the Geological Society of London, vol. 31.djvu/92

46 J. PRESTWICH ON THE QUATERNARY PHENOMENA far from the larger fragments being at bottom, my experience leads me to a different conclusion. At Sangatte I found the lower part of the deposit to consist of a chalk rubble, often very fine, and the upper part chiefly of fragments of flints of all sizes. At Brighton, seams of chalk silt are intercalated in the mass from top to bottom ; whilst at Chesilton the section distinctly shows a large preponderance of heavy blocks of Portland flint and stone in the upper part of the cliff. There was not a single large block of stone or flint within reach in the lower part of the cliff; and again at the Bill the superposition of the coarse angular debris on the fine loam and silt is clear. Nor can I agree with those who consider this deposit an old talus, or with Mr. Godwin-Austen, who regards it as a talus formed at high altitudes under great cold ; for in either case the materials would, as pointed out by the latter, have arranged themselves at the angle taken by loose materials falling over steep slopes; and when, at first, the cliffs or slopes were steepest, the greater would be the masses which would fall from them, while with diminishing slopes or angles, the rubble or debris would, as a rule, become of smaller size and of diminished quantity. Also if the deposit were a mere subaerial accumulation, it would in all cases be in close connexion with the slope or cliff supplying the materials — would dip from it at a high angle, and never be carried far beyond the range which that angle would subtend; whereas at Sangatte and at Brighton, although the layers of the deposit are turned up at a high angle against the old cliff, they are prolonged in a gradually diminishing angle to a considerable distance from it. At Chesilton the deposit does not even extend up to that part of the cliff where the Portland Stone is in situ ; and whereas the escarp- ment is 400 feet high, the angular debris, commencing at the height of about 180 feet (see sketch, fig. 8 ; Sect. 5, PI. I.), is prolonged to a distance from it of 1600 feet, or very far beyond that to which any materials falling from the cliff, had it even been originally double the height, could possibly have extended. The recent talus and old debris are in fact perfectly distinct. In the case of this deposit at the Bill of Portland, these features are still more decided. At the base of it is a light loam foreign to the old cliff against which it abuts, while the strata forming the top of the cliff are broken up and thrown over this loam, and the two together extend southward over the raised beach at the slight angle of 4° or 5° for a distance of nearly a quarter of a mile. The old cliff itself, and the surface of the old land above the cliff and of the old shore, are all levelled, and form a uniform and scarcely apparent slope, their original relation and levels being completely obliterated (see fig. 4, PI. I.) I hold that small angles such as these and such a mode of arrange- ment are quite incompatible with any subaerial deposit of local origin formed on the principle of a talus ; and the difficulty becomes greater when we see that in this bed at the Bill there are fragments of strata which not only are absent in the adjacent cliffs, but do not even at present exist on the island. Not only is the fall insufficient to carry the materials to the distance shown, but there is evidence of a vis