part 4] V BIFT-VALLEY IN WKsTKKN l'ki:si\. 



12. A Rift-Valley in Western Persia. By S. James 



Shand, D.Sc, F.G-.S., Professor of Geology in the University 

 of Stellenbosch, South Africa. (Read December 17th. 1910.) 



[Plates XII-XIV.] 



A Jiii" i -\ alley so small that its whole extent can be surveyed from 

 the top of one of the bounding walls, so free from soil and vege- 

 tation that each fault can be traced by the eye and its hade and 

 displacement measured directly, and so perfectly preserved that the 

 fault-scarps retain the grooves produced by the friction of the 

 sliding rocks, is surely a structure of no little interest. If the 

 valley which I have to describe were in Europe instead of Persia, 

 I do not doubt that it would attract its geological pilgrims and 

 supply illustrations for the text-books for many years to come. 



Asmari Mountain, near the oilfields of Alaidan-i-Naftun in the 

 Bakhtiari country of Western Persia, is an inlier of Oligocene 

 limestone among the beds of the Pars System (Miocene). The 

 only division of the latter system that comes into the question is 

 the lower or gypsum series, consisting of bedded gypsum with 

 intercalated shales and a few thin limestones. These beds are 

 locally conformable to the Asmari Limestone, although the higher 

 members of the system overlap the lower. The mountain is a 

 * whalebaek," 16 miles long and 3 wide at the middle, formed by 

 a simple symmetrical anticline plunging at both ends. The north- 

 western end plunges rather steeply, and shows no abnormal 

 structures ; but the south-eastern end disappears more gradually, the 

 axis dipping some 5° or 6° only, and here for a distance of 3 miles 

 the fold has collapsed along its length, letting the gypsum-beds 

 down into a trough in the limestone. 



This trough is bounded by two main faults hading north-eastwards 

 and Bouth- westwards respectively, with an average hade of 20°, and 

 marked by steep escarpments. The northern scarp, which lies 

 practically along the axis of the anticline, is at one point 500 feet 

 high ; but the southern one, being low down on the Hank of the 

 anticline, is much less conspicuous. Besides the main faults there 

 are at least three other major faults parallel to them which produce 

 smaller scarps within the valley ; the valley-Moor 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 bounding 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 trough is 2! miles, and 

 its width half a mile. 



The downthrow of the most important faults is given below, the 

 Q. J. G. S. Xo. 800. r 



