370 



ME. C. EEID ON THE ORIGIN OF DET CHALK 



This would modify the entire system of drainage of the country 

 in a way that I do not think has been realized. All rocks would 

 be equally and entirely impervious to water, and all springs would 

 fail. While these conditions lasted, any rain falling in the summer 

 would be unable to penetrate more than a few inches. Instead of 

 sinking into the Chalk, or other pervious rock, and being slowly given 

 out in springs, the whole rainfall would immediately run off any 

 steep slopes like those of the Downs, and form violent and transi- 

 tory mountain-torrents. These would tear up a layer of rubble 

 previously loosened by the frost and unprotected by vegetation. 

 The material carried away would not have the Chalk washed or 

 dissolved out, for a single flood of this description could have little 

 solvent power, and much of the Chalk might not be thoroughly 

 thawed*. 



Each of these floods would have an enormous scouring and trans- 

 porting power ; for the fall in the valleys is very great. It is 

 noticeable that no Coombe Rock is found in valleys that have a 

 greater slope than 100 feet in the mile, and that the main mass is 

 deposited south of the Downs, where the slope is much less. Pro- 

 bably the further transport across the low lands of the unworn Hints 

 scattered through the brick-earth was greatly assisted by " anchor 

 ice." 



On the flat lands any small channels formed by one summer's 

 floods would be filled with ice and frozen gravel next winter, so 

 that subsequent floods would have to cut fresh channels over the 

 plain. This would lead to the formation of flat subaerial deltas, which 

 advanced seaward and became confluent over the whole plain 

 between the old cliff and the sea. Chichester stands on one of 

 these old deltas, which consists of chalky Coombe Rock for 4| miles 

 from the Downs and slopes about 30 feet in a mile ; the loamy 

 southward continuation is, of course, very much flatter. 



The constant excavation of new channels, while these conditions 

 lasted, is probably the cause also of much of the apparently un- 

 systematic grooving so commonly found in rocks underlying gravels 

 of this type ; a frozen gravel is just as hard as, or probably harder 

 than, frozen chalk. 



This, I believe, was the origin of our steep-sided Coombes and 

 of the Coombe Rock. There is no need of any excessive rainfall : 

 in fact the apparent deficiency of snow during this cold period, com- 

 bined with the remarkably Arctic character of the fauna of 

 Eisherton, makes it probable that it was a period of drought, perhaps 

 equivalent to the Loss period in Central Europe. 



A recognition that physical conditions of this peculiar type held 

 in non-glaciated parts of England during the glaciation of other 

 parts seems necessary to a more perfect understanding of the origin 

 of our Pleistocene deposits. In the Arctic regions, beyond the 



* The erosive power of rain falling on porous beds before they are thawed 

 has been incidentally alluded to by several writers ; but apparently this per- 

 manent freezing of the rocks and consequent stoppage of all underground 

 circulation in the south of England has not been taken into account. 



