and Marlborough Railways. 185 



and besides the fault at Stert already noticed, there are others laid 

 down on the Geological Survey Maps, running in a parallel direc- 

 tion from north-east to south-west, in which the beds to the north- 

 western side of the fault are raised. There is such a fault between 

 Westbury Field and Reydown Common, bringing up the Oxford- 

 clay and Calcareous-grit to the Kimmeridge-clay ; another running 

 from Elm, to Trowbridge and Semington, bringing up the Corn- 

 brash to the Oxford-clay ; another between Atford and Laycock, 

 bringing up the Forest- marble to the Oxford-clay. 



The same general tendency to a greater upheaval to the north- 

 west, which gave rise to these and other faults to the westward, 

 may have still been in action when the land was emerging from 

 the sea. 



The inclination from the head of the valley seaward, as left by 

 marine denudation may have been but slight ; still the first direction 

 of the drainage of the valley after it emerged from the sea was 

 probably westward. We may suppose however that as the tilting 

 up of the mouth of the valley went on faster than the stream 

 flowing seaward could lower its course, the drainage would be 

 first stopped and then turned backwards up the valley. 



Having the fact before us that the drainage does not flow west- 

 ward, but through the Avon valley, let us now see what light can 

 be thrown on the origin of the latter. 



A glance at the ordnance map will shew that there are a succes- 

 sion of dry valleys, or coombs, originating at the very edge of the 

 chalk escarpment, and falling away from the vale on either side. 

 They join one with another like the branches of a river, as they 

 recede from the valley, and their course in its lower part is occupied 

 by a stream. There is a group of these coombs ramifying up from 

 Tilshead towards Redhorn, and others behind TJpavon Hill, Pewsey 

 Hill, Easton Hill, «&c. All these sooner or later, join the Avon 

 valley. Behind the northern escarpment are similar coombs, 

 falling away from Saint Ann's Hill to Beckhampton, and to West 

 Kennet ; from Milk Hill to East Kennet ; from Golden Ball Hill, 

 and from Hewish Hill, to Clatford. 



If we go beyond the district now under consideration, similar 



