and Marlborough Railways. 



183 



It is one of three creeks in the ancient coast line represented by the 

 chalk escarpment, the others being the Yale of the Wily near 

 Warminster, and the Yale of Wardour. All three valleys or 

 creeks are similar in origin. Along the centre of each runs an 

 anticlinal axis from which the strata dip outwards in opposite 

 directions ; indicating that a much greater upheaval has taken place 

 along that line, than in the surrounding table land of Chalk. Marine 

 denudation, then, acting among the cracks and fissures, produced by 

 elevation along an anticlinal line, removed the chalk once continuous 

 across these valleys, and exposed the inferior formations. 



The Geological date of this ancient coast line can be approxima- 

 tely fixed. It is after the Lower Eocene period ; as we find the 

 remains of those beds along the edge of the escarpment, while there 

 is no trace of them on the lower ground ; and it is before the 

 Glacial period, for we find Boulder clay in places at the base of the 

 escarpment, and not deposited in the same neighbourhood up 

 the sides, or on the top of it. 1 The marine denudation to 

 which the formation of this ancient coast line and its creeks is 

 due, must have acted during a slow upheaval of the land. The 

 upheaval may have been intermittent, or varied by downward 

 movements, but the broad general result must have been, that the 

 Tertiary beds covering the chalk table land were denuded, and the 

 surface of the downs was moulded, (so far as it was moulded by 

 marine denudation), before the formation of the chalk escarpment 

 and its creeks ; and that the chalk escarpment and higher valleys, 

 like Pewsey Yale, were eroded to something like their present form, 

 before a lower valley, such as Urchfont bottom, and the lower plain 

 stretching below Devizes, were denuded. 



In the Yale of Pewsey with which we are more immediately 

 concerned, we have a valley extending from Martin to Devizes, 

 walled in on the north and south by chalk escarpments, rising 

 boldly above the Upper-green-sand, and opening out westward. 

 Within the mouth of this valley, between Stert and Urchfont, is 

 the upper end of another valley at a much lower level, also opening 

 out westward. 



1 Ramsay's Lectures on the Physical Geology of Great Britian, p. 107. 



