192 The Geology of the Berks ^ Hants Extension, 



except near All Cannings, when the level falls 50 feet and rises 

 again, and the outcrop line juts out a mile into the valley and 

 recedes again. Between Wilcot and Hewish, the level rises 70 feet 

 and the line falls back IJ miles. From Hewish round Martinsell, 

 there is no great difference in the level of the outcrop ; but between 

 Martinsell and Brimslade, there is a rise of 30 feet accompanied by 

 a set back of f mile near Clench. Near Brimslade the outcrop line 

 reaches its greatest elevation, and from this point to Crofton En- 

 gine, there is a regular fall of 115 feet, with as regular a contraction 

 of the valley. On the high ground above the highest point in the 

 outcrop line, the westernmost outlier of the London Tertiary basin 

 occurs. 



The alterations of level in the outcrop line are too sudden to be 

 due to anything but faults, yet there are no traces of them on the 

 surface ; subsequent denudation has so effectually masked them, that 

 it is useless to speculate on their probable course. 



The connection between the level of the line of outcrop and its 

 position on plan becomes intelligible, when we consider what the 

 effect would be of marine denudation acting sometimes above, and 

 sometimes below the outcrop line. In the former case, the hard 

 Lower-chalk would be eroded, while in the latter case it would be 

 the Upper-green-sand ; with what increased undermining and wide- 

 ning of the sides of the valley may easily be conceived. 



On the southern side of the valley the outcrop line is at a lower 

 level than marine denudation could have reached, and the only 

 irregularities on plan are at the mouth of the Avon valley 



That part of the valley of denudation which lies to the east of 

 Burbage, is drained by a tributary of the Kennet, which traverses 

 the Chalk from Crofton Engine to Hungerford. The upper end of 

 the valley was not deepened by marine agency to so great an extent 

 as the more seaward portions. The lowest point on the watershed 

 which runs from Durley through Burbage to Southgrove, is 30 feet 

 above the line drawn across the section fig. 9, and there are many 

 points about Burbage, where the TJpper-green-sand attains a height 

 of 100 feet above that line, or 550 above the sea level. This is not 

 much lower than the surrounding chalk table land, which is here 



