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ME. C. REID ON THE ORIGIN OF DRY CHALK 



27. On the Origin of Dry Chalk Valleys and of Coombe Rock *. 

 By Clement Eeid, Esq., F.G.S. (Eead February 23, 1887.) 



For many years the singular mass of angular flints and Chalk, 

 known as the Brighton Elephant-bed, has been familiar to geolo- 

 gists f. This deposit I had not seen till 1884, when I was in- 

 structed to examine, for the Geological Survey, the Pleistocene 

 deposits of Sussex between the escarpment of the South Downs 

 and the sea. 



Coming from eight years' work in strongly glaciated districts, I 

 was at once struck by the appearance of the Elephant-bed — or, as it 

 is called in the district, the " Coombe Bock." It is a very different 

 deposit from anything commonly seen in the Yorkshire or Lincoln- 

 shire "Wolds, and different, though not so markedly different, from 

 anything found in Norfolk. 



This occurrence in a non-glaciated district of a type of gravel 

 unlike anything of ordinary occurrence in glaciated districts of 

 similar configuration aroused my interest. After two years' study 

 of the beds in the field, I venture to bring forward my views as to 

 the mode of formation of Coombe Bock and as to the origin of dry 

 Chalk Valleys — two subjects intimately connected. 



The configuration of the surface beneath the drift on the seaward 

 side of the South Downs is identical with that found in the Chalk 

 districts of Yorkshire and Lincolnshire. In each of these districts 

 we have a dip-slope from the edge of the escarpment seaward. But 

 this slope does not pass under the low-lying drift areas ; it ends 

 abruptly in a cliff, now much degraded, but still recognizable as a 

 sea-cliff both by the marine deposits banked against it and by 

 its independence of the line of strike. This cliff is well seen 

 at Black Bock, near Brighton, and passes also through Goodwood 

 Park. 



The result of this ancient marine erosion is, that we have in the 

 South Downs a moderate southern slope from the escarpment, then 

 a sudden drop at the partially buried cliff, and then a plain sloping 

 very gently seaward. 



It is needless to say more about this period of marine erosion, for 

 the structure described is not directly connected with the subject of 

 this paper. It is important, however, to realize the general contour 

 of the country before the Coombe Bock was deposited, or one cannot 

 understand the distribution of this later gravel. 



Subsequent to the formation of the ancient sea-cliff an enormous 

 mass of angular flint and chalk detritus was swept from the Downs, 

 and spread far and wide in a continuous sheet over the low lands. 



* The facts obtained during the work of the Geological Survey are communi- 

 cated by permission of the Director General. 



t See MantelL ' Geology of Sussex/ 1822, p. 277. 



