THE ORIGIN OF THE VALE OP MARSflWOOD. 183 



In conclusion, I may briefly call attention to the points of 

 resemblance and difference between the Vale of Marshwood and 

 the Weald of South-eastern England. Both are elliptical periclinal 

 areas, both have been truncated by planes of (presumably marine) 

 erosion, and both have rivers which, after traversing the inner 

 plain, pass through gaps in the southern escarpment to reach the 

 sea. In the Weald, however, the watershed coincides roughly 

 with the longer axis of the pericline, and the streams run both 

 northward and southward, so that both lines of escarpment are 

 trenched by river valleys. In the case of the Dorsetshire Weald 

 the original watershed was outside and north of the central axis, so 

 that all the streams ran southward and only the southern border is 

 trenched by river-valleys. 



The features of the northern range of hills have been produced 

 by the detritive agencies of rain and frost, heat and cold. The 

 greater height of the northern range of hills as compared with the 

 summits of the southern range, is due to a combination of two 

 causes : in the first place they are nearer to the central axis of the 

 pericline, and in the second place they form part of a watershed 

 which was the summit of a peneplain sloping southwards over the 

 southern hills into the valley of the English Channel. It is, in 

 fact, on this particular combination of a pericline truncated by a 

 southward sloping plain that the physical geography of West 

 Dorset primarily depends. 



With respect to the isolation of Pilsdon and Lewesdon Hills, 

 this has been effected by the excavation of the intervening spaces ; 

 in technical language they are " hills of circum-denudation." The 

 interspaces are the heads of the valleys formed by the action of 

 rain and springs on the slopes of the old watershed. The 

 tributaries of the Axe have trenched it on the north, while on the. 

 south side the strong springs thrown out at the base of the 

 Marlstone Sands have eaten backward some little way into the 

 ridge of the original watershed, causing the actual water-parting to 

 retreat northward. This recession has taken place principally 

 near the villages of Pilsdon and Bettiscombe, while Lewesdon 



