THE GLACIAL PERIOD. 



225 



Mr. Prestwich, who has also furnished geologists with an 

 admirable account of the " head " as seen in the neighbourhood 

 of Weymouth, 1 does not agree with " those who consider this an 

 old talus, or with Mr. Godwin- Austen, who regards it as a talus 

 formed at high altitudes under great cold." He points out that 

 " if the deposit were a mere subaerial accumulation it would in 

 all cases be in close connection 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 Brighton, although the layers 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." 

 Thus at Chesilton, where the escarpment is 400 feet high, the 

 angular ddbris spreads over the ground to a distance from the 

 base of the cliff of 1600 feet, which, as Mr. Prestwich remarks, 

 is " very far beyond that to which any materials falling from the 

 cliff, had it even been originally double the height, could possibly 

 have extended." At Portland Bill this appearance is still more 

 striking, for the angular debris in that neighbourhood extends 

 south for a distance of nearly a quarter of a mile at the small 

 angle of 4° or 5°. Such inclinations are so slight that we 

 cannot but admit with Mr. Prestwich that they are quite insuffi- 

 cient to keep loose rubbish and rocks in motion ; something 

 more than the mere gravity of the stones was required to cause 

 them to travel down slopes so gentle. Accordingly he tries to 

 account for the phenomena by supposing that after the land had 

 been subjected for some considerable time to the ordinary action 

 of the weather, the low grounds bordering on the Channel were 

 temporarily submerged, and that the " head *' probably owes its 

 origin to the inundations brought about during the subsequent 

 more or less sudden emergence of the land. He infers, to use his 

 own words, " that the emergence, at first gradual, was marked by 

 short oscillations, which, according to their relative force and 

 duration, swept down the soil with its land-shells and softer 

 beds, alternately with the coarser materials and the bones of 



1 Quart. Journ. Geol. Soc, 1875, p. 29. 



