The Thames. 515 



easily wasted than that of the Oolite, its recession east- 

 ward was more rapid, and this process having gone on 

 from that day to this, the two escarpments in the region 

 across which the Thames runs are far distant from each 

 other. 



All this time the Thames was cutting a valley for 

 itself in the Chalk, and by-and-by, when the escarp- 

 ment had receded to a certain point, its base became in 

 part lower than the edge of the Oolitic escarpment 

 that then, as now, overlooked the valley of the Severn, 

 only at that time the valley was narrower. While this 

 point was being gradually reached, the Thames by 

 degrees was joined by the growing tributary waters 

 that drained part of the surface of the eastward slope of 

 the Oolitic strata, the western escarpment of which was 

 still receding ; and thus was brought about, what at first 

 sight seems the unnatural breaking of the river through 

 the high escarpment of Chalk between Wallingford and 

 Reading. 



From the foregoing remarks it will be understood 

 why the sources of the Thames, the Seven Springs and 

 others, rise so close to the great escarpment of the 

 Inferior Oolite, east of Gloucester and Cheltenham. 

 But just as in times long gone, the sources of the 

 Thames once rose westward of the Seven Springs, 

 so well known on the Cotswolds, so the sources of 

 the river now, are not more stationary than those 

 that preceded. The escarpments, both of Chalk and 

 Oolite, are still slowly changing and receding east- 

 ward ; and as that of the Oolite recedes the area of 

 drainage will diminish and the Thames decrease in 

 volume. This is a geological fact, however distant it 

 may appear to persons unaccustomed to deal with geo- 

 logical time 



L L 2 



