THE GEOLOGY OF THE PTJRBECK HILLS. 157 



as it now exists north of the hill. The question then arises as 

 to what caused the separation of the streams ? In a paper 

 entitled " Dorset during the Glacial Period," recently read 

 by Dr. Colley March before the Geological Section of the 

 Bournemouth Natural Science Society, this subject is briefly 

 referred to, and the suggestion is advanced that the course of 

 the joint river was for a time (i.e., during or at the close of the 

 glacial period) blocked by sedentary ice, which piled up 

 in the only gorge, and compelled the rising water to cut a new 

 channel. Two difficulties present themselves to the accept 

 ance of this view, one being the length of time during which 

 the ice block must have continued to effect so great a change 

 in the course of the streams, and the second that the time 

 usually supposed to have elapsed from the close of the glacial 

 period would hardly permit of the cutting out of the double 

 gap through the chalk to a depth of 200ft. as it now exists. 

 We have, therefore, ventured to seek some other explanation, 

 and think one may be found in a consideration of the manner 

 and progress of the denudation of the great Purbeck 

 anticline. 



A reference to Plate I., Fig. 1, will show that after upheaval 

 of the Purbeck anticline, the trend of drainage would naturally 

 proceed from south to north, and as the chalk was gradually 

 denuded off, the stream flowing over what is now Corfe Gap 

 would, so soon as the chalk was removed and lower Cretaceous 

 strata reached, continue to cut down a channel across the 

 present ridge. The denudation of the softer Greensand and 

 Wealden strata would then begin to form the east and west 

 valley which now exists, and the two streams, the Byle 

 from the east and the Steeple from the west, would carry 

 the debris of this vast mass of strata through the gap in a 

 single stream to form the Corfe river, which flowed (as it does 

 now) into the Frome. In all probability this gap was greatly 

 widened for a time by estuarine or marine denudation, during a 

 subsequent submergence of the land when the Higher Tertiary 

 beds and some of the Oligocene strata were deposited some 

 of which are of estuarine and others of marine origin remains 



