263 Dr. PiTTON on the Strata below the Chalk. 



as to furnish me with two additional sections^ Nos. 15. and 16. (Plate X. a.) 

 which may be considered as supplementary to those above mentioned ; the 

 first. No. 15., running east and west, from Etchilhampton Hill south-east of 

 Devizes, to the Kimmeridj^e clay beneath the green-sand which caps the 

 heights of Poulshot Green : the second. No. 16., from the lower chalk at 

 Beacon Down, through Sandridge Hill, which is capped with the Lower 

 green-sand over Kimmeridge clay and Oxford oolite, to the Oxford clay 

 around Melksham, where the canal is about 154 feet above the sea*. The 

 sections themselves sufficiently explain the general relations of the strata; 

 and for an account of their composition I refer to Mr. Lonsdale's paper; 

 regretting only that his colouring of the Ordnance Maps of that part of 

 England still remains unpublished. 



The town of Devizes stands upon a platform of the Upper green-sand, which is there about 

 430 feet f above the sea, and is overlooked by the chalk of Etchilhampton Hill and the 

 adjacent heights. And here, as throughout the tract from hence to Swindon, the contrast be- 

 tween the upper and lower chalk is very remarkable, both in external features and agricultural 

 character. 



The ground near Westbury rises with such rapidity, that the distance between the outcrop of 

 the upper chalk and of the Upper green-sand is less than a mile ; and on the north-east of that 

 town the slope is still more rapid. But, thence towards the north and east, a great superficial 

 extent of the lower beds is exposed in the Vale of Pewsey, which consists of the Upper green- 

 sand surmounted by detached portions of the chalk-marl and lower chalk, the latter, in some 

 cases, rising into summits of considerable elevation. 



The outcrop of the Upper green-sand and Gault has, in general, though irregularly, a northern 

 direction, from about Great Cheverell, through Pottern, — to Heddington, about three miles north 

 of Devizes. On the south of the Vale of Pewsey, the Lower chalk is disclosed by denudation, on 

 a line almost due east from Easton Hill, — through St. Ann's Hill, Milk Hill, Golden Ball, Hewish, 

 Martensell, and Terrace Hills ; and thence westward through Easton, Milson, Pewsey field, Up- 

 avon, Cleeve Hill, and the slope thence towards Urchfont, and southward to the upper part of 

 West Lavington. The outcrop of the Upper chalk, after forming a bold escarpment, between 

 Westbury and Cheverell Hill, is thrown back towards the east, forming the heights, along which 

 the Ridgemay is conducted, between Gincross by Bushton Down to Wivelsford. The Ridgeway, 

 which runs on the verge of the ridge bounding the wide tract of Blackheath, is first directed 



* This and^some other heights mentioned hereafter, are derived from Bradshaw's map of the 

 canals of England, and are counted from low-water mark at Liverpool. (See Appendix C.) 



f 429 feet 6 inches above low-water at Liverpool. At this level the Kennet and Avon canal is 

 continued in a direction nearly parallel to the heights of chalk, as far east as the vicinity of 

 W^ooton Rivers, where it rises to 462 feet 6 inches (the summit level) to surmount a narrow neck 

 of elevated ground on the north of Burbage : — and thereabouts the streams divide ; one group 

 running west, throug Pewsey, to join the Avon near Upavon ; the other north-eastward, along 

 a valley descending rapidly from north to south, through Great and Little Bedwin ; and thence 

 through Salisbury to the sea at Christchurch. 



