190 The Geology of the Berks 8f Hants Extension, 



since its emergence from the sea. The country to the north of the! 

 Thames and Bristol channel, since the commencement of the] 

 Glacial period, has been depressed at least 600 feet below its present] 

 level, and raised at least 600 feet above it. In these oscillations of 

 level; the district under notice must have participated, though 

 probably not to an extent which would have laid Pewsey Yale 

 under water. A difference of level of 90 feet between Lydeway 

 and Rushall is equivalent to 15 feet in a mile, an amount of tilting 

 which we may easily imagine to have taken place during such 

 movements as the above. 



We have fixed the limit of marine denudation, and the height of 

 the gap at Rushall through which the Avon first flowed, by the 

 level of the flint gravel of Pewsey .Common. In the absence of 

 organic remains the age of this is uncertain, but it is as much older 

 than the post-pliocene gravel of Pewsey Station, as is represented 

 by a difference of 38 feet in level. Compared with the difference 

 of level of 67 feet, which we find to exist between the Post-pliocene 

 gravel at Pewsey Station and the present stream, this would give an 

 age about one half greater to the flint gravel of Pewsey Common ; 

 but it is impossible to allow even approximately, for the greater 

 power which the streams must have had in the earlier times, when 

 although the artic severity of the earlier portion of the Glacial 

 period -had passed away, a condition of climate giving a far greater 

 excavating power to the streams must have existed. 



Let us now come down to the time of the deposition of the 

 gravel at Pewsey Station. 



The stream at Pewsey, as before stated, has lowered its course 67 

 feet since the post-pliocene gravel was deposited. The river at 

 Rushall must have lowered its course as much, or more, in the 

 same time. In post-pliocene times then it flowed at least 67 feet 

 above its present level. But its present level at Rushall is 67 feet 

 below the watershed near Lydeway, so that in post-pliocene times 

 it flowed at Rushall at a level not lower than the present watershed 

 near Lydeway. 



It may be asked then, what reasons are there for supposing that 

 the river flowed down the Avon valley, and not westward at that 



