A. J. Julces-Broicue — The Vale of Manhicood. 167 



throngh the lower claj's for a space, would again enter the Marl stone 

 Sand and still further south would again enter the Upper Lias and 

 JViidford Sand, as shown in the diagram (Fig. 2). 



Now, where the sides of a valley consist of clay, they are rapidly 

 acted on by rain and fi'ost and are made to recede by frequent 

 landslips, but where they consist of firm and dry sand there is very 

 little slipping and the valleys remain comparatively narrow. Thus 

 it came to pass that a wide tract of clay was gradually exposed over 

 the western part of the periclinal area, while to the southward the 

 rivers pass through valleys with steep slopes on each side, the inter- 

 vening tracts rising into a succession of hills, some of which are 

 capped by patches of Inferior Oolite and others by remnants of the 

 original covering of Greensand. 



These southern hills are well seen by anyone standing at the foot 

 of Pilsdon Pen, and they look as if they would present an impassable 

 barrier to any river running southward from the watershed on which 

 the observer stands. 



The rivers which now drain the district are the Char and the 

 Simene, while the Brit drains the eastern part of the periclinal area, 

 and they all make their way through gaps in the southern hills. 

 But, besides the valleys of these rivers, there is a wide gap at the 

 head of the valley of the little river Chid, which runs through 

 Chideock, and I think it probable that this gap was part of the 

 valley of a river which had a more northern source. There is little 

 doubt that in some cases one river-system extended itself at the 

 expense of another, the lateral tributaries of the one encroaching on 

 the area drained by the other, and sometimes entirely cutting off or 

 capturing the headwaters of the adjacent river.^ 



The present course of the Char is so different from the com- 

 paratively straight courses of the Simene and the Brit, that it 

 suggests the idea of its having absorbed the tributaries of an eastern 

 neighbour. The col at the present head of the Chideock valley does 

 not rise above 260 feet, the hills on each side being double that 

 height, and I am inclined to think that there was a time, before the 

 valleys were carved out to their present depth, when three rivers 

 traversed the Vale of Marshwood, and that the ancestor of the Chid 

 was one of them. The final sculpturing of the country took place 

 during and soon after the close of the Glacial Period, and it was 

 probably then that the capture by the Char of the upper tributaries 

 of the Chid was accomplished. 



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 



' For a case in Lincolnsliire described by the author, see Quart. Jouru. Geol. Soc, 

 vol. xxxix, p. 596, 1883. 



