CHAP. X.] CRETACEOUS PERIOD. 181 



been pointed out (p. 160) that in Purbeck times the valleys 

 of the English and Bristol Channels were in all probability 

 occupied by rivers of considerable size, but that on the first 

 emergence of the land the amount of mechanically-trans- 

 ported detritus which they carried seaward seems to have 

 been very small. Not only was the land at a comparatively 

 low elevation, for we must remember that during the 

 Jurassic period the height of the Welsh hills must have 

 been diminished by the accumulation of at least 2,000 feet 

 of strata round their flanks ; but it is also very likely that 

 small lakes existed in their upper reaches, which would in- 

 tercept and retain the detritus carried by the stream, just 

 as the mud of the Rhone is deposited in the Lake of G-eneva. 

 As, however, elevation proceeded and the land rose higher 

 above the sea, rainfall would be increased, erosion would 

 be accelerated, the load carried by the streams would be 

 larger, and the volume and velocity of the currents would 

 be greater, any lakes that existed would be rapidly filled up ; 

 under such conditions a large quantity of detritus would be 

 carried to the mouth of the rivers and poured into the Weal- 

 den basin. This will sufficiently account for the differences 

 observable in passing from the Purbeck to the Wealden 

 beds. 



If, however, these western rivers were the only streams 

 that ran into the lake, we should expect the sandstones to 

 diminish eastward, but this is not the case, the Lower 

 Wealden of Sussex being more sandy than that of Dorset ; 

 and these Sussex sands must, I think, have been brought 

 by minor streams which drained the land on the north side 

 of the lake a supposition which finds confirmation in the 

 pebble bed of Lindfield (p. 165). It is also possible that a 

 stream may have entered the eastern end of the lake, for, as 

 we shall presently see, its outlet is not likely to have been 

 in that direction. 



We must not forget the possibility of a great river 



