326 . PEOf . J. PRESTWICH ON THE RAISED 



underground as now, ran off immediately and formed violent transi- 

 tory mountain-torrents, which tore up the surface, and carried the 

 Chalk-rubble to the mouth of the valleys. He is of the same opinion 

 as other writers that the loam and flints spread over the plain between 

 Chichester and the sea are due to the same cause as that which 

 formed the Coombe Rock, and contests the erosion and wearing back 

 of those Chalk valleys by ordinary running water — a view which, 

 by mistake, he ascribes to me. 



Other geologists would attribute certain portions of the Kubble 

 to waste left by the recession of the hills. 



The Rubble-drift or Head has thus been referred to — 



1st. The wash of the surface-debris over the edges of the old 

 cliflfs by an excessive annual rainfall, and during a period of 

 great cold either caused by an elevation of the land or due 

 to glacial conditions. 



2nd. The agency of ice and snow sliding down the hill-slopes, 

 aided by the running off of the water resulting from the 

 melting of the ice and snow. 



3rd. A wave of translation or a cataclysm caused by earthquake- 

 movements in the central area. 



4th. Pluviatile and torrential action, during a period of great 

 cold, aided by floating ice. 



5th. Subaerial action in part. 



The idea that the Head is a mere talus caused by ordinary 

 weathering of the cliff may be discarded at once. The angles of 

 slope are too small and too extended from the base of the cliff, and 

 the mass is too compact, for an ordinary subaerial talus, while the 

 presence of debris and occasionally of lalocks foreign to the imme- 

 diate spot indicates a transport of the material from some point 

 other than the face of the cliffs. 



1. To the first of the above explanations there is the objection 

 that the rain would at once run off along the lines of lowest level, 

 and, in the course of time, these channels would be worn into 

 gullies more or less deep, and the debris carried down through them 

 would be spread out fan-shaped at their end, as — to compare small 

 with great things — in the case of the great cones of debris in the 

 Upper Indus basin described by Mr. F. Drew.^ The edge of the 

 cliffs and the heights above would in such a case be scored by these 

 gullies, and ' cones of dejection ' be formed at given intervals ; but 

 no such water-channel and no such ' cones of dejection ' exist in the 

 face of these old cliffs. The drift follows the line of pre-existing 

 slopes and coombes, which it may have deepened, but in which it 

 has not eroded special channels ; the edges of the cliffs present, so' 

 far as can be seen, one unbroken line, and the rubble extends in 

 a mass of uniform character the whole length of that line, — results 



i Quart. Journ. Geol. Soc. vol. xxix, (1873) p. 441. 



