A. Tylor — Formation of Deltas. 395 



Mammalian remains have been found at the Brighton raised sea- 

 beach within the 50 feet level. The shells in the raised beaches in 

 the English Channel not being of Arctic forms, is presumptive 

 evidence that the Straits of Dover were not open at the raised beach 

 period. 



No one can see the great valley of the Somme, or the Dover 

 Valley, without being convinced that in the Quaternary Period 

 these wide and deep valleys, excavated out of solid chalk, were 

 filled by large rivers. 



I would propose to continue Godwin-Austen's great river up to 

 the Somme, and the Dover river down the space now occupied by 

 the English Channel, at the period when the Atlantic only reached 

 the present 100 fathom line on our coasts, and when the spaces now 

 occupied by the German Ocean and the English Channel were dry 

 land. During the Glacial Period of 160,000 years (?), there would 

 have been ample time for the river Somme and the Dover river, 

 draining land relatively 600 feet higher than at present, to have ex- 

 cavated wide valleys where the Straits of Dover are now situated. 

 Under a cold climate, such as the South-east of England would have 

 been, removed so far from the Gulf-Stream, and reaching an eleva- 

 tion of 1400 feet above the sea at the close of the cold period, after 

 a very extensive denudation, we might expect the whole of the South 

 of England to have been denuded very rapidly. The chalk is now 

 covered by vegetation, and the atmosphere acts slowly upon it ; but 

 wherever it is suddenly exposed to the air and cold, it crumbles 

 away with great rapidity. The chalk hills resting upon beds of 

 sand and loose clay, exposed to a very wet and cold climate, at ele- 

 vations between 600 feet and 2000 feet above the sea, would be 

 rapidly denuded. The abruptness of the escarpment of the North 

 and South Downs, and the steepness of the curves of denudation, 

 give an exact measure of the force of the cold and pluvial action 

 upon strata (of different powers of resistance and elevation), 

 situated on either side of the watershed, the apex of the curves. 

 Cold and rain, those active agencies of denudation, have operated 

 upon strata differing in their pervious or impervious qualities in re- 

 lation to lines of drainage in mineral character, in inclination to the 

 horizon, and in distance from the points of drainage to the highest 

 point of land (determining all intermediate levels), and have given 

 these escarpments their present outline. 



The Dover river has its source in a valley commencing within a 

 few yards of the steep escarpment of the chalk, only separated by a 

 few yards from the edge of the Chalk hills down which water flows 

 in the opposite direction, following a binomial rather than a para- 

 bolic curve. 



In passing from a watershed in any direction to the sea, or to the 

 bottom of a valley, the slope of the ground gradually increases and 

 then gradually decreases until it reaches the still water. This form 

 or outline at the surface can be expressed by a binomial curve of 

 some kind on any line of section. This is therefore the theoretical 

 curve of denudation of which the author has already spoken. See 



