160 THE GEOLOGY OF THE PURBECK HILLS. 



hardness* and resistance to denuding action of the chalk 

 at this spot, similar to that which led to the formation of the 

 Old Harry Rock and other chalk pinnacles. We should 

 thus have the former single river which had carved out its 

 one channel A B, dividing into two much smaller streams, 

 each tending to cut down and widen a channel for itself, 

 and leaving between them, as they gradually descended, the 

 Corfe monticle. Its conical shape would be accounted for by 

 the usual aids of aerial and frost denudation on each bank 

 of the two streams as the chalk was cut away and eroded. 



Doubtless when the division of the streams was first 

 effected, the amount of water was greater than now ; it has 

 continuously decreased until the present time, since the sea 

 commenced encroaching upon the land area and carved out 

 Swanage and Worbarrow Bays. It seems probable that 

 the first step in the formation of Bournemouth Bay was the 

 breaking of the sea through the gap in the chalk hills 

 formed where the ancient Swanage river had cut its channel 

 southward to join the Fro me, somewhere east of Handfast 

 Point. 



* This hardness has been observed at Corfe Castle and recorded (v. 

 Strahan, cp. cit. p. 168). 



