138 Geological Society : — 



line of a partially buried sea-cliff before passing under the low-lying 

 drift areas. Subsequent to the formation of this sea-cliff a mass of 

 angular flint and chalk detritus spread out from the Downs over 

 the low lands, being seldom found far up the valleys. This is the 

 Coombe Rock, which passes further on into a worthless mixture of 

 angular flint and loam, and at a still greater distance into almost 

 clean brick-earth. It is not of glacial origin, neither is it marine, 

 nor is it a gravel formed by ordinary fluviatile action. A study of 

 the contours of the Downs may give us, the Author thinks, a key to 

 the mode of its formation. 



The rolling outline of the Downs, and the steep-sided dry valleys 

 point to conditions which have passed away. However much rain 

 may fall, the upper parts of these valleys are always dry, and no 

 running water can be found where the incline of the bottom of the 

 valley exceeds the slope of the plane of saturation — never more than 

 60 feet per mile. Three explanations have been offered : — 



(1 ) Former submergence and rise in level of the plane of saturation. 



(2) Former higher level of the plane of saturation before the 

 valleys were cut down to their present depth. 



(3) Increase in the rainfall. 



None of these theories is sufficient to account for the origin of 

 Coombes and the transport of Coombe Rock. There is no evidence 

 of submergence whilst the Coombes were being eroded ; on the con- 

 trary, the descent of the Coombes to the sea-level near Rottingdean 

 and elsewhere, is suggestive of a slight elevation. The deep trench- 

 ing of the Downs by valleys, and the consequent lowering of the 

 plane of saturation is applicable to many of the slightly inclined 

 Coombes, but the whole structure of the country shows that the 

 outlet for the water must have been as clear then as now. Since 

 the dry chalk valleys play no part in the present superficial drainage, 

 it would make but little difference in the plane of saturation if they 

 were filled up again. If springs had formerly existed in the higher 

 valleys, their gradual failure would have left evidence in the shape 

 of gravel deposits and terraces. Moreover, as an objection, both to 

 the first and second theories, it is urged that if valleys had been 

 cut back by springs, some of them should fall to the north, where 

 most of the springs occur, whereas the Coombes open to the south. 

 Lastly he finds no traces of the "hypothetical pluvial period." 



In suggesting an origin for the dry valleys and Coombe Rock, he 

 considers that the fauna and flora, both at Fipherton and Bovey 

 Tracey, point to a great degree of cold, from 20° to 30° lower than 

 what now prevails in the South of England. The ground would 

 thus be frozen to the depth of several hundred feet, and the drain- 

 gge system of the chalk entirely modified. There would be no 

 underground circulation. The summer rains would immediately 

 run off any steep slope, often in violent torrents. These would tear 

 up the layer of rubble already loosened by the frost, carrying down 

 masses of unthawed chalk too rapidly for solvents to have much 

 effect. No Coombe Rock is found in valleys that have a greater 

 slope than 100 feet per mile. There is no need of excessive rainfall ; 

 it might have been a dry period corresponding to that of the Loss. 



