184 The Geology of the Berks 8f Hants Extension, 



Between the two valleys, forming a step from the higher to the 

 lower, is an escarpment of Upper-green-sand, which may be traced 

 from Earlestoke, round to Stert, Potterne, Devizes, &c. This step 

 seems to indicate that between two periods of comparative rest, 

 during which the upper and lower valleys were excavated, with 

 their bottoms sloping gently seaward from the enclosing cliffs, 

 an interval occurred in which the rate of uprise was greater. 



We should naturally expect to find the waters from Pewsey Yale 

 flowing with a general westerly course into the lower valley. 

 They have this westerly course from the watershed at Burbage, as 

 far as Newington, but there they turn southward, and soon fall 

 into the Avon at Rushall. The Avon, which drains all the country 

 as far as Bishops Cannings, Echilhampton, and Lydeway, has up 

 to this point been flowing up the vale. It now turns south- 

 ward, and flows in a narrow valley completely through Salisbury 

 Plain. 



It is curious to note how completely the whole drainage of the 

 country, up to the very edge of the lower valley, is turned back. 

 Standing near the " Clock," on the Turnpike-road between Fullaway 

 and Lydeway, a quarter of a mile south of the railway bridge, one sees 

 close by on the west, a rapid fall leading down into Urchfont 

 bottom ; yet the water from the road finds its way back up the 

 valley to the Avon. 



We find that in a smilar way the waters of the Wiley, and of 

 the Nadder, are turned back at the mouths of valleys of elevation 

 opening out westward ; and in fact until we come to the Lias plain 

 of Somersetshire, there seems to be a general tendency in the 

 streams to flow backwards and reach the sea by some unexpected 

 channel. 



We must look to some general cause for this tendency ; and the 

 cause probably is, that combined with upheaval, there was a tilting 

 up from the northwestward, by which the sea bottom, after being 

 laid dry, received an inclination away from the sea, sufficient to turn 

 the drainage backwards. There are evidences remaining of such 

 a tendency to a greater relative upheaval towards the north-west. 

 The general dip of the strata is from the north-west to the south-east, 



