142 THE GEOLOGY OF THE PURBECK HILLS. 



PART I. 

 INTRODUCTORY. 



The Purbeck Hills traverse the Isle of Purbeck from east 

 to west, commencing at the chalk cliffs of Handfast Point 

 and terminating at the chalk cliff in Worbarrow Bay. 

 Geologically their chalk formation is continued west of 

 Worbarrow ; but since the Isle of Purbeck terminates 

 geographically at Arishmell Gap, this paper does not consider 

 the further prolongation of their structure west of that point. 

 The Purbeck Hills divide the Isle of Purbeck geographically 

 into two portions, namely a northern half which extends to the 

 River Frome, and consists of Tertiary strata, chiefly Bagshot 

 sands and clays, and a southern portion which forms a 

 syncline of Wealden Clay merging southwards into the 

 northern slope of an anticline of Upper, Middle, and Lower 

 Purbeck beds ending at a coastline of high cliffs of 

 Portland stone and sand resting on Kimmeridge clay, which 

 forms the axis of the anticline. The line of Purbeck Hills thus 

 dividing the Isle of Purbeck is formed of an elevated mono- 

 clinal ridge of highly inclined, often vertical, chalk, flanked 

 on the north by Tertiary beds, and on the south by a narrow 

 strip of Greensand, the beds of which are conformable with the 

 almost vertical chalk, and hence are exposed at the surface 

 on edge. These comprise Punfield beds, Atherfield clay, and 

 Upper and Lower Greensand, which, however, can seldom 

 be distinguished geologically. The Gault is not here repre- 

 sented as distinct from the Upper Greensand.* The three 

 diagrams on Plate I. give an illustration of the relationship 

 of the strata in three transverse sections I. near the eastern 

 extremity of the hills ; II., about their centre ; and III., near 

 their western termination. 



* Strahan, Memoirs Geol. Survey, Isle of Purbeck, p. 143. 



