144 THE GEOLOGY OF THE PURBECK HILLS. 



GEOLOGICAL STRUCTURE or THE PURBECK HILLS. 



The geological structure of the Purbeck Hills can best be 

 studied at Ballard Cliffs, and especially the adjacent coast 

 of Studland Bay. The great cliff section at Ballard Downs 

 shows the structure to consist of a central axis of nearly 

 vertical chalk strata which projects upwards, so that its 

 edges form an escarpment near the summit of the hills. 

 On the northern slopes this vertical edge is flanked by 

 a sheet of chalk beds which incline rapidly from the 

 vertical to the horizontal, where they are overlain by 

 Tertiary strata. On the southern slopes of the chalk ridge 

 is a thin strip of the edges of Greensand strata comprising 

 Upper and Lower Greensand, and Punfield beds, which strata 

 are almost vertical, conformably with the chalk. These are 

 succeeded by Wealden clays, which, being highly inclined 

 against the Greensand at first, become gradually horizontal 

 southward, and form the lowest slopes of the .outhern flanks of 

 the hills. This structure is shown in each of the three sections 

 in Plate I. 



The disposition of strata on the northern slopes of the hills 

 can best be studied in Studland Bay, from the opening of the 

 little dell, whence Studland village is approached, to near -the 

 Foreland. From the former spot Bagshot sands occupy the 

 coast line. These are the lower beds of the Bagshot series, and 

 are of fluviatile origin, although here unfossiliferous. They 

 consist of variegated and coloured sands, with thin partings of 

 clay, dipping at a slight angle of 5 to 7 N.E., and are con- 

 tinued to a point about 400 yards beyond the little coombe 

 leading to the village, where they are replaced by London Clay. 

 This in a short distance gives place to Woolwich and Reading 

 beds, which here consist chiefly of white sand, some 40 ft. 

 thick, with a bed of flint gravel 6 to 10 ft. thick between it and 

 the London clay. The Woolwich and Reading beds occupy 

 some 100 yards of the foreshore, which here curves round to 

 the east. At a point half a mile from the Foreland the chalk 

 appears at the shore level. Its upper surface, on which the 



