38 THE THAMES AND ITS TRIBUTARY RIVERS. CHAP. 



The Ray. At Islip a considerable but variable supply comes 

 into the Cherwell, along the channel of the Ray, which has its 

 head-waters about Bicester and Marsh-Gibbon, and flows through 

 the flat plain of Ottmoor, which in floods resembles a lake. 



The Thames near Oxford. The course of the Thames near Oxford 

 is remarkable on several accounts, but chiefly for the uncommon 

 sweep of its channel round the bold headland of Wytham, by which 

 means it seems rather to enter the valley of the Cherwell than 

 to continue in its own. What makes this the more remarkable 

 is the existence of a great hollow between this Wytham Hill and 

 the equally elevated ridge of Cumnor Hurst. If we could imagine 

 a natural dam to have formerly existed on the course of the river, 

 anywhere below Eynsham Bridge, even as far down as Iffley or 

 Sandford, the course of the current might have been through the 

 transverse hollow alluded to between Eynsham Bridge and Botley. 

 There must have been under these conditions a great lake between 

 Cumnor Hurst and Shotover Hill, spreading all round to Yarnton 

 and Eynsham, and far up the Valley of the Thames. A lake only 

 sixty feet higher than the Thames at Eynsham Bridge, would 

 separate Wytham Hill from Cumnor Hurst by a broad, navigable 

 strait. (See Diagram No. XII. p. 43.) 



Now, as the contracted passages referred to have been cut 

 through and cut down by water, this lowering of a great natural 

 dam may be in fact the true explanation of the desertion of the 

 old watercourse between the Cumnor and Wytham hills, and of 

 the exposure of the broad level gravel terraces on which Oxford 

 and Yarnton stand. In agreement with this is the manner in which 

 the sandy and gravelly deposits referred to are arranged, for they 

 speak unmistakeably of successive depositions under such conditions 

 as expanded water might occasion, and the occurrence of land and 

 fluviatile shells in the midst of the layers, and in some special nests 

 as it were among them. Far more abundant waters than our occa- 

 sional floods have at some former time filled the broad spaces where 

 now the Cherwell and Evenlode and Windrush rivers flow into 

 the Thames. 



Carrying our thoughts backward in time, it is conceivable that 

 the old Botley valley between the hills of Cumnor and Wytham 

 may have been influenced in its origin and excavation by the same 

 causes as the valley of the Windrush. That valley directs itself 



