532 The Thames Valley. 



Ouse, and its tributaries in Bedfordshire, and also many 

 other streams flow through areas covered with this clay, 

 and have cut themselves channels through it in such a 

 way as to lead to the inference that parts of the valleys 

 in which they run did not exist before the Boulder-bed 

 period, but that they have excavated their courses 

 through it and the underlying Oolitic strata, and thus 

 formed a new system of valleys. These often only apply 

 to parts of their channels. 



Again, with regard to the Thames, I have said that 

 it is remarkable that it rises in the Seven Springs, not 

 far from the edge of the Oolitic escarpment of the 

 Cotswold Hills that overlooks the Severn, which runs in 

 the valley about 1,000 feet below. The infant Thames 



FlG. 106. Thames. 



1. Boulder-clay. 2. London Clay. 3. Chalk. 



thus flows at first across a broad tableland of Oolitic 

 rocks, and by-and-by comes to a second tableland 

 formed of the Chalk, and the wonder is that there its 

 course was not turned aside by that high escarpment. 

 Instead of that being the case, a valley cuts right 

 across the escarpment of Chalk, through which the 

 river flows, and this I have already explained in Chapter 

 XXX. This escarpment dates from long before the de- 

 position of the Boulder-beds, for we find far-transported 

 boulders and Boulder-clay at its base, while in the same 

 neighbourhood the drift has not always been deposited 

 on its slopes, nor yet does it lie on the top. Yet north 

 of the mouth of the estuary of the Thames in Essex we 



