THE GEOLOGY OF THE PURBECK HILLS. 149 



PART II. 



SPECIAL POINTS IN GEOLOGY OF DISTRICT. 

 THE GREAT PURBECK THRUST FAULT. 



In the cliffs below Ballard Down an almost transverse 

 section of this well-known fault is magnificently exposed. 

 The fault runs due east and west, and its effect has been to 

 thrust a huge sheet of upper chalk strata on the northern sides 

 of the Purbeck Hills for probably their whole extent 10 

 miles, and also for an equal distance beyond the district 

 upwards and southwards for probably 300 yards or 

 more over the upturned edges of the strata south of the line 

 of fracture (Plate III., Fig. 3). From the exposure in 

 Ballard Cliffs, the fault runs in the axis of Ballard Down, 

 for nearly two miles to a gap in the hills between that and 

 Nine Barrow Down, through which the road from Swanage to 

 Studland passes, and where the Ulwell spring is thrown out. 

 It continues thence along the line of the hills, and can be 

 clearly traced beyond the Isle of Purbeck in several 

 cliff sections. The enormous forces involved in this 

 great earth movement, a fracture at least a quarter 

 of a mile across, and extending for over twenty miles, 

 are shown in the changes in the chalk contiguous 

 to the line of motion. These at Ballard Cliff have pul- 

 verised the flints and drawn them out into streaks of black 

 dust, the rock between the bedding planes being polished into 

 slickenslides. The fracture is seen (Plate II., Fig. 2. and Plate 

 III., Fig. 1), to have taken place along one of the bedding 

 planes, while the upturned edges of the nearly vertical beds 

 on the southern side of the fault have been sheared off by 

 the forcing of the upper beds over them. So much is obvious ; 

 but the exact means by which the present effect was brought 

 about is less easily to be traced. Many geologists have 

 endeavoured to explain this, but the latest and most thorough 

 account is that given by Dr. Strahan in " Memoirs of the 



