GEOLOGICAL MAPS 171 



such as to leave portions of the old valleys dry and function- 

 less, and in others to leave mere streamlets to occupy important 

 valleys whose size and depth were the work of larger streams now 

 beheaded and captured by the stronger streams. There is plenty 

 of evidence of this work in the past in almost every river system, 

 and the process is still going on, so that no drainage system can 

 be said to be stable or completed. It is all part of the life process 



FIG. 63. The Severn and Avon, by widening their valley, are causing the 

 retreat of the Cotswold escarpment, and thus beheading the tributaries 

 of the Thames. 



of the landscape ; but it will be observed that it is all in the 

 direction of adjustment of drainage to rock structure and history. 

 The Severn in Gloucestershire, and the Avon in Worcestershire and 

 Warwickshire, are longitudinal streams flowing at the foot of the 

 great escarpment of the Cotswolds and Edge Hills (Fig. 63). The 

 dip-slopes of these wolds are the head waters of the chief tributaries 

 of the Thames. But the Severn and Avon are under-cutting 



