248 Geological Society. 



anticline, is much less conspicuous. Besides these main faults, 

 there are at least three other big faults parallel to them, which 

 produce minor scarps within the valley ; the valley-floor thus 

 descends in terraces towards the south-west, besides having a 

 general south-eastward inclination of some 10°. Uphill, towards 

 the crest of the mountain, the downthrow of the limiting faults 

 diminishes gradually to zero, and the valley dies out on the broad 

 top of the anticline. Downhill, towards the plunging nose of the 

 anticline, the trough is closed abruptly by a cross-fault nearly at 

 right angles to the anticlinal axis. The length of the whole trough 

 is 2 1 miles, and its width half a mile. 



The northern boundary-fault at its maximum has a downthrow 

 of about 500 feet, the parallel faults range from 150 to 200 feet, 

 and the cross-fault throws about 100 feet. 



The gypsiferous beds which once completely filled the trough 

 have been partly removed by erosion, clearly revealing the fault- 

 walls in the lower part of the valley. These fault-scarps in their 

 lower portions are remarkably fresh, and show the smoothed and 

 fluted surfaces produced by the friction of the sliding faces. 



The drainage of the faulted region is curious in several respects. 

 The only perennial stream that traverses the valley cuts sheer 

 across it from side to side. Rising in the gypsum-beds on the 

 north-eastern flank of the anticline, the stream turns south- 

 westwards, and cuts right through the limestone in a deep canon ; 

 the latter breaches the northern fault-scarp where the downthrow 

 is greatest, and from here the bed of the stream lies in the gypsum 

 of the valley-bottom, except for a short stretch between two 

 faults, where the limestone-floor is again exposed to view. The 

 stream finally breaks through the southern limestone-wall, and so 

 makes its escape from the valley. 



These conditions imply that the river assumed approximately its 

 present direction when there was a cover of gypsiferous beds over 

 the whole area, and that, although younger than the initiation 

 of the faulting, it is older than the sculpturing of the inside of the 

 fault-trough. The eastern end of the trough is again tapped by 

 another and smaller stream with a precisely similar development. 



It might be surmised from the excellent preservation of the 

 fluted surfaces on the fault- scarps that the last movement took 

 place at a very recent date, but the behaviour of the river in 

 crossing the scarps does not favour this supposition. The succession 

 of events appears to have been as follows : — 



(1) Formation of the trough with gyp sum -infilling- by partial collapse of 



the anticline. 



(2) Levelling of the gypsum surface and development of a stream across 



the position of the buried trough. 



(3) The stream, cutting down through gypsum, discovers the faults, 



etches them out, develops subsequents along them, and thus 

 gradually eats out the gypsum filling the rift, until the present 

 topography results. 



