THE ORfGlN OP THE VALE OF MARSHWOOD. 



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landslips would take place, as is always the 

 case where springs issue from sand overlying a 

 clay. The valleys would be rapidly widened, 

 and during periods of upheaval they would 

 be deepened also. Much of this work was 

 probably done during the Glacial Period and 

 was finally completed during the time when 

 the raised beaches of the south coast were 

 being raised to their present position. Over 

 the western part of the pericline the Midford 

 Sands and Upper Lias are absent ; that is to 

 say, they were planed off before the Greensand 

 was deposited, and the latter rests directly on 

 the Marlstone Sands. Here the process of 

 valley erosion would continue till the base of 

 these sands was reached, when strong springs 

 would be thrown out by the underlying tnar- 

 gariiatus clays, and these clays would be for 

 a certain distance exposed along the valley 

 bottoms. 



We must remember that all this time the 

 slope of the valley-ways was less than the 

 southerly inclination of the beds on the 

 southern curve of the pericline ; hence the 

 rivers, after cutting through the lower clays 

 for a space, would again enter the Marlstone 

 Sand and still further south would again 

 enter the Upper Lias and Midford Sand, as 

 shown in the accompanying diagram. 



Now, where the sides of a valley consist 

 of clay, they are rapidly acted on by rain and 

 frost 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 



