THE GEOLOGY OF THE PURBECK HILLS. 159 



south of the then existing chalk ridge somewhere east 

 of Handfast Point, had also cut down a channel for itself 

 through the chalk hills which were then continuous to 

 the Isle of Wight and emptied into the Frome river 

 in what is now Bournemouth Bay. As it cut its channel 

 more deeply it would have drained a larger area, and it 

 or a westerly tributary, as the present Swanage stream may 

 perhaps have been would have gradually robbed the Byle 

 of a good portion of its water supply. It is possible that the 

 Swanage stream is really a portion of the upper eastern 

 waters of the Byle, and that at about the period named a 

 reversal took place, the head of the Byle being captured 

 (far out in Swanage Bay) by the Swanage river, and thus the 

 Byle was left a truncated and vastly diminished stream. 

 In all probability at about the same time a similar process 

 occurred as denudation proceeded at the head waters of the 

 Steeple river in the land area which then occupied Worbarrow 

 Bay, and was then continuous towards Weymouth Bay, before 

 the sea had encroached to its present coast line. 



The little stream which now flows west into Worbarrow Bay 

 is probably the head of the ancient Steeple, and that now van- 

 ished land having been drained by the Steeple it flowed east- 

 ward as now, and joining the Byle proceeded through Corfe 

 Gap. If this view be accepted, we see that at about the 

 same time the flow through the combined streams at Corfe 

 was diminished to an amount of perhaps a fifth to a twentieth 

 of its former volume, and that this occurred when denudation 

 had proceeded to the point shown in the diagram on p. 158. 



From this we should find the passage A B, formerly well- 

 filled with water, converted into a swampy morass with a few 

 broken-up remains of the previous river trickling through 

 several irregular channels. As, however, the Byle stream 

 tended from its direction to hug the eastern bank of the former 

 river channel, and the Steeple similarly its western bank, 

 the centre of the channel would tend to dry up and form an 

 island which presently was to form the summit of the Corfe 

 monticle. This may have been assisted by some special 



