The Thames. 513 



great plain, somewhat higher than the summit level of 

 the Carboniferous Limestone plateau. This plain being 

 slightly inclined to the west at the time the Severn was 

 scooping out its valley, as I have already explained at 

 p. 508, the ancient Avon flowed over the top of the 

 plateau of Clifton and Durdham Downs, through a 

 minor inequality of the surface, and, as rivers do, it 

 steadily worked at the deepening of its own channel. 

 As it did this, so in like proportion the river and its 

 tributaries in the upper part of their courses gradually 

 wasted and lowered the hill-sides and valleys through 

 which they flowed, being aided by rains and snows and 

 all the ordinary agents of atmospheric denudation ; and 

 thus it happens, that what was once a high slightly- 

 inclined tableland, has been converted partly into flat- 

 topped fragments of a high plain, and partly into undu- 

 lating hills and vales ; while in the great Oolitic plateau, 

 that stretches eastward as far as the Chalk escarpment, 

 we have still remaining a large tract of the ancient 

 plain, "with this difference, that the average gentle slope 

 of its surface is now east instead of west. 



This naturally leads to the question, Why is it that 

 the Thames, and some other rivers that flow through 

 the Oolites and Chalk, run eastward? The answer 

 seems to be, that after the original valley of the Severn 

 was well established by its river, a new disturbance of 

 the whole country took place, by which the Cretaceous 

 and other strata were tilted eastward, not suddenly, but 

 by degrees, and thus a second slope was given to the 

 Chalk and Eocene strata, in a direction opposite to the 

 dip, that originally led to the scooping out of the 

 present valley of the Severn. This dip lay east of the 

 comparatively newly-formed escarpment of the Chalk 

 indicated by the dark line in fig. 102 marked e. The 



L L 



