184 THE ORIGIN OF THE VALE OP MARSHWOOD. 



may really be very nearly on the site of "the original ridge of the 

 watershed. 



The same process has taken place near Beaminster, where the 

 spring-heads which furnish the head-waters of the Brit have 

 undoubtedly eaten deep into the Chalk and Greensand area, and 

 there the escarpment is still receding, as the frequent scars of 

 landslips testify. 



It will be seen, therefore, that the history of the evolution of 

 the present physical features of West Dorset involves the con- 

 sideration of many agencies and many conditional phases. Here, 

 as elsewhere, rain, rivers, snow, frost, and heat, have been the 

 principal agents at work, but in order to understand how their 

 operation has resulted in the particular arrangement of hills and 

 valleys which we see around us, we must form some conception of 

 the conditions under which they started to work, and we must 

 remember that their working powers have always been guided and 

 modified by the movements of the land, those slow movements 

 of upheaval and subsidence to which every portion of the earth's 

 crust has been repeatedly subjected. 



