Dr. PiTTON on the Strata below the Chalk, 243 



(124.) Blackdown to Shaftesburi/. — The a^reat range of the chalk escarp- 

 ment in the interior of England, which stretches, like the shore of a sea or 

 lake, from Crewkerne in Dorsetshire to the north-east of Dunstable in Bed- 

 fordshire, is perfectly analogous in structure and appearance to the downs 

 of Surrey and Sussex. It is interrupted by three or four indentations or 

 gulfs ; — one of great width, opening towards the west, between Crewkerne 

 and the heights about Stour-head in South Wiltshire ; another expanding 

 to the north-west and terminating in the defile where the Thames cuts 

 through the chalk, in its course to the south-east from Buckinghamshire and 

 Oxfordshire. The vales of Pewsy and of Warminster are intermediate 

 bays of the same general structure but of smaller dimensions; and all these 

 valleys are apparently the result of denudation, aided by previous disturbance 

 of the strata, — which has carried away the chalk, and laid bare to various 

 depths the strata beneath it*. 



The Green-sand hills range nearly from west to east for about ten miles on 

 the south of Wellington and Taunton, and are then broken through irre- 

 gularly, retiring towards the south to join the north-western extremity of the 

 Dorsetshire chalk, of which some large outliers exist in the vicinity of Chard 

 and Crewkerne. The chalk escarpment is thence continued in a direction 

 parallel to the northern range of the Blackdown hills, to an opening on the 

 north of Blandford, through which the Stour makes its way to the sea at 

 Christchurch ; — the tributary branches of that river, before it cuts through the 

 chalk, winding extensively over the country around Sturminster, from whence 

 the waters of the whole bay are conducted to the south-eastern coast. From 

 the opening of the valley of the Stour the escarpment turns nearly north 

 towards Shaftesbury, between which place, or rather between the promi- 

 nence of the Upper green-sand on the north of it, and Mere, is the opening 

 of the vale of Wardour. The beds immediately below the chalk from Crew- 

 kerne to Shaftesbury, have not yet been examined in detail, but the general 

 distribution of the strata is represented in Mr. Greenough's Map. 



South Wiltshire. 

 (125.) Vale of Wardour. — This tract may be considered as an eastern 

 prolongation of the first of the great gulfs above mentioned, which being 

 suddenly reduced in width between Shaftesbury and Mere, is continued east- 



* The principles of structure in these tracts, are ably discussed in Dr. Buckland's well'known 

 paper "On the Formation of Valleys, by Elevation of the Strata that inclose them," (Geol. 

 Trans., 2nd Series, vol. ii. p, 119, &c.,) — where the vales of Wardour, Warminster, and Pewsy 

 are expressly mentioned in illustration of the author's views. 



