THE GEOLOGY OF THE PURBECK HILLS. 147 



GEOLOGICAL RELATIONSHIPS or THE PURBECK 

 HILLS. 



The relationship, geologically, of the hills with the strata 

 of the South of England demands attention. A study of 

 the geological map of England shows that the chalk forms four 

 anticlinal lines from W. to E. and N.E. These four in order 

 from N. to S. form each the escarpment of four series of 

 hills, namely the Chiltern Hills, the North Downs, the 

 South Downs, and the Brixton anticline of the Isle of 

 Wight. This latter was formerly continuous with the 

 Purbeck Hills before the formation of Bournemouth Bay, 

 the position of the central chalk ridge, with its adjacent 

 strata being almost identical in each case. Between the 

 Chiltern Hills and the North Downs lies the Tertiary basin 

 of the Thames valley, and between the South Downs 

 (with their prolongation westward as the Wiltshire Downs) 

 and the Southern chalk of the Isle of Wight with the 

 Purbeck Hills, lies a similar Tertiary basin, the trough 

 of which is now occupied by the Solent, Bournemouth Bay, 

 and the River Fro me. In addition to these four main 

 anticlines, there are in Hampshire two subsidiary chalk ridges 

 parallel with the southern anticline, those of Guildford and 

 Portsdown. These chalk hills, together with their inter- 

 vening valleys or synclines, are observed to lie en echelon along 

 a line from W. to E., or to E.N.E., coinciding in direction 

 somewhat with the chalk ridges between Norfolk and Dorset. 

 With one important exception, each of these anticlinal 

 ridges is formed of an axis of chalk, the intervening valleys 

 forming basins of softer Tertiary strata, or, between the North 

 and South Downs, by Wealden and Lower Cretaceous beds. 

 The exception to this rule is the case of the Purbeck Hills, and 

 a reference to Plate I., Fig. 1, will show that they do not form 

 the axis of the southern anticline, but rather consist of the 

 northern monoclinal edge of the upheaved mass whose axis 

 is really the Kimmeridge clay, some three miles south of the 



