178 THE ORIGIN OF THE VALE OP MARSHWOOD. 



that there was a time when the Chalk and Greensand formed a 

 continuous mantle over the rocks \vhich now occur in West 

 Dorset. Let us next consider how this mantle of Cretaceous 

 material has been so largely removed from the district in question. 



When the country was raised above the level of the sea at the 

 close of the Oligocene period it must have undergone considerable 

 erosion from the planing action of the sea waves, and if the flexures 

 were commenced at that time the anticlines would suffer most. We 

 know very little about the history of this part of England during 

 the Miocene and Pliocene times, but the final result of the 

 successive upheavals and denudations was to leave a surface of 

 erosion which was planed across the flexures, and both upheaval 

 and denudation had been carried on to such an extent that the 

 Chalk had been either entirely or almost entirely removed from 

 the central parts of the anticlinal areas. 



This surface of erosion was what our American cousins call a 

 peneplain, that is to say, it was not a level plain or plateau, but 

 had its slight irregularities and slopes and had, moreover, a 

 summit elevation from which it sloped in more than one 

 direction. A consideration of the present watersheds and of 

 the river courses in Dorset and the adjacent counties leads us to 

 infer that the original watershed of this peneplain lay to the 

 north and west of the line now occupied by the Chalk escarpment.* 

 It probably trended from somewhere in the neighbourhood of 

 Wincanton at a high level above Sherborne and Yetminster to 

 Beaminster Down, and thence over Lewesdon and Pilsdori to th, 

 hills between Axminster and Lyme. The western part of this 

 line, from Beaminster Down along the ridge on which Lewesdon 

 and Pilsdon stand, is still the watershed between the streams 

 which run southward and those which drain into the rivers 

 Parret and Axe. 



It will be noticed that this watershed does not coincide with the 

 longer axis of the Marshwood pericline, but lies to the north of it. 



* See "Origin of the Valleys of North Dorset," in Proc. Dorset N.H. 

 and A.F. Club, Vol. xvi., p. 5. 



