From The Warminster Official Guide And Souvenir 1928 (penned by Victor Strode Manley):
Geology. Cley Hill, at an elevation of over 800 feet, indicates the position of the dome formed when the sea bed rose as dry land, a resulting crack weathering away into the Wylye Valley, with its streams from this hill and the Deverill Valley going to swell the Avon at Salisbury. The tilt of the land sends the Whitbourn streams from the hill in the opposite direction, flowing into the Bath Avon. Not less than 1,000 feet of rocks have been denuded from above the site occupied by the Minster. All the local downs are of chalk averaging 650 feet, the town being built on a terrace of greensand at 400 feet above sea level, and the Shearwater Valley provides an outcrop of gault clay for brickmaking.