and Marlborough Railways. 



191 



period ? They are these : — in the Avon valley there are patches 

 and terraces of gravel at Enford, Knighton, Bulford, &c, at consi- 

 derable heights above the present level of the river. At Bulford 

 remains of the Mammoth and Tichorhine Rhinoceros have been found, 

 proving that at the same period when the streams were flowing at 

 a high level in Pevvsey Yale, a river was flowing at a corresponding 

 level in the Avon valley. The connection of the gravels in the 

 latter with Pewsey Yale has not } r et however been fully traced. 



It can be proved in many cases, in fact generally, that the 

 systems of river drainage were the same before the Glacial-period 

 as now, and if this case be an exception, we are bound to suppose 

 that the tilting up of the mouth of the valley to a higher level 

 than the " col " at Rushall, and the subsequent lowering of the 

 ground by 90 feet at least, have both taken place since the post- 

 pliocene period. 



It is more reasonable to suppose that the reversal of the drainage 

 took place earlier, perhaps during the period of quicker upheaval 

 which is marked by the step between the valley and Urchfont bot- 

 tom ; and that an oscilliation of level, tending to lower the mouth 

 of the valley, had not, at the time of the deposition of the post- 

 pliocene gravel at Pewsey Station, yet brought down the watershed 

 near Lydevvay to its present level. 



In looking upon the Avon valley in the light of a coomb, we 

 must bear in mind, that it is the river action which has caused the 

 .sudden bends which occur in the course of the Avon. At only 90 

 feet above its present level, the river would flow in a straight line 

 over the high ground near Durrington, avoiding Bulford altogether ; 

 and at much lower level than this, the bends at Amesbury, &c, 

 would be avoided. There is no reason for supposing the original 

 coomb to have been more tortuous than many others. 



On the northern side of the valley, the boundary between the 

 Upper-green -sand and Chalk, forms a very irregular line in the 

 Geological Survey Map. There is a very striking connection 

 between its irregularites and its level. Whenever the line rises in 

 level, there is a corresponding set back in plan, and vice versa. Thus, 

 from Horton to Wilcot the level of the outcrop is about the same 



