158 THE GEOLOGY OF THE PURBECK HILLS. 



now/f Jill] Nine Bamnv Down. 



vr. 



\ 



\ / Corft Hill \ 

 R Steeple K Siyle 



Diagram to Illustrate formation, of G>r/e 



of which are still found at Creech Barrow.* When the level 

 of the land became higher again, and denudation had pro- 

 ceeded so far as the level of the summit of Corfe Hill, the 

 present wide gap between Nine Barrow Down on the east and 

 Flowers Hill on the \vest, would exist as indicated in the 

 diagram above. This is now 170ft. below the summit of 

 the chalk ridge. The channel of the then river is shown at 

 AB just level with the top of the monticle. At this stage 

 the stream became divided, and the cause was probably a 

 sudden diminution in the water supply, which Avould have been 

 produced by a change in the drainage area south of the chalk 

 ridge. It must be remembered that at this period, land ex- 

 tended far to the east of SAvanage Bay, and far to the west 

 of Worbarrow Bay, and that the sea had not yet reached the 

 chalk hills which then were continuous between Ballard Head 

 and the Needles. Both the Byle and the Steeple were then 

 moderately sized rivers, and each brought down perhaps 

 50 or 100 per cent, more water than at present from this 

 drainage area, most of which has since been destroyed by the 

 sea. But up to this time another and rival drainage had been 

 maintained by the ancestor of the present little Swanage river. 

 This, originally commencing in a small stream immediately 



* A comparison with the Tertiary and Oligocene strata tilted up against 

 the chalk in Alum Bay makes it probable that vast beds formerly existed 

 north of the Purbeck range. 



