A. J. Jukes- Broivne — The Vale of Marshwood. 165 



bring the base to a height of 877 feet. Assuming the thickness of 

 the G-reensand there to have beea 180 feet, the Chalk would have 

 come in at about 1,150 feet. 



The relative levels of sea and land varied, of course, at different 

 epochs of Tertiary time, but we are quite warranted in believing 

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

 tinuous mantle over the rocks which 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 ha^e 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 con- 

 sideration 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 Pilsdon to the 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. 

 In order, therefore, to understand the drainage system of this part 

 of Dorset we must imagine a time when the surface of the land 

 sloped gently both northward and southward from the line above- 

 mentioned. On this surface there was a certain accumulation of 

 clay, pebbles, cherts, and flints, the heavy and insoluble relics of the 

 Eocene, Greensand, and Chalk which had been destroyed ; remnants 

 of this deposit, which is generally called " the clay with flints," 

 still remain on the tops of the higher hills. 



The rain flowing down the southern slope of this surface gathered 

 into streams, which cut channels for themselves through the Chalk 



' See " Origin of the Valleys of North Dorset," in Proc. Dorset N.H. aad A.F. 

 Club, vol. xvi, p. 5. 



