58 C. R. Long well — Geology of 



fault, the net result being a monoclinal dip southeast by 

 east, although strong dips to the northeast occur locally. 

 Near the line of greatest disturbance dips of 20° are com- 

 mon and inclinations as high as 40° were observed. This 

 folding probably occurred in connection with recurrent 

 movement on the Rogers Spring fault within compara- 

 tively recent time. 



An anticline following the course of Virgin River for 

 several miles south of Aluddy Creek is indicated by per- 

 sistent dips ranging from 20° to 35°, westward on the 

 west side of the stream and eastward on the east side. 

 Four miles south of St. Thomas a broad, gentle swell 

 intersects the steeper north-south fold, producing a 

 dome. A more pronounced east and west anticline affects 

 the clays on both sides of the river immediately north of 

 Bitter Wash, where the underlying Horse Spring beds 

 have been exposed by erosion. Between Callville Moun- 

 tain and Virgin River the clays have a basin-like struc- 

 ture, produced by combination of the anticlines men- 

 tioned and the monoclinal dip eastward from the Rogers 

 Spring fault. 



Structure of the Virgin Mountains. 



Only a small part of the Virgin Mountains was studied 

 in connection with this report, and therefore the major 

 framework cannot be fully described. Northeast of St. 

 Thomas the end of the principal range is a large anticline 

 with a metamorphic core, the thick sediments dipping 

 northwest and southeast from the axis, which extends 

 generally northeast. South of St. Thomas Gap the rocks 

 dip steeply eastward in a great tilted block in which both 

 faulting and folding have complicated the details of struc- 

 ture. One system of faults, trending nearly north and 

 south, is responsible for a number of parallel, finger-like 

 ridges. Faults of another important svstem extend N. 

 60° to 70° E. St. Thomas Gap has resulted from the 

 dropping of a large irregular block or a number of ad- 

 jacent blocks, and on the floor of the gap the rocks have 

 been tilted by folding and faulting. Displacements on 

 faults bounding the gap and affecting the adjacent high- 

 lands range from hundreds of feet to several thousand 

 feet. In a plunging anticline near Mud Well the Overton 

 and Horse Spring formations appear to be almost con- 



