HINTZE, GEOLOGY OF WASATCH MOUNTAINS, UTAH 139 



evidence of faulting could be found; but on the north wall of Big Cot- 

 tonwood Canyon opposite South Fork, faulting is clearly shown. Here 

 the west block has gone down instead of up. If this fault has anything 

 to do with the Superior fault, it must be in the nature of a pivotal fault 

 with the fulcrum somewhere between the Cardiff mine and the mouth 

 of South Fork. 



Silver Fork fault. — At the head of Silver Fork of Big Cottonwood 

 Canyon, on the ridge north of Alta, there is a wall of limestone breccia 

 which stands up from 10 to 20 feet above the crest of the ridge, having a 

 direction nearly north and south. On both sides of it are limestones, but 

 their metamorphic condition prevents close observation as to the strati- 

 graphic displacement because of the difficulty of identifying a suitable 

 datum plane on both sides. Farther to the south in the gulch leading 

 from Alta to the City Eocks and Alta Consolidated mines, the quartzite 

 and shale of Cambrian age are faulted up on the east so that they are in 

 contact with the limestones which normally overlie them. The displace- 

 ment is estimated to be between 500 and 600 feet, though the exact 

 amount of movement could not be readily determined. It is, however, a 

 fault of considerable magnitude. The fault surface seems to be vertical, 

 and it is therefore impossible to say whether it is of the normal or the 

 reversed type. Minor parallel faults may easily be detected to the west 

 along the top of Emma Hill and Flagstaff Mountain, but on account of 

 the strongly metamorphosed condition of the limestones, the throws have 

 not been determined. They are, however, thought to be only slight. It 

 might be said by way of generalization that the block between the Su- 

 perior and Silver Fork faults has gone down and that the west end ap- 

 pears to have been most depressed. The parallel fractures between them, 

 therefore, may show that the west side has gone down in most cases. 

 This, however, is merely a suggestion and may not be true in all cases. 



Minor faults. — In the various mines of Alta, minor faults are known 

 to be of frequent occurrence. They conform generally to the main direc- 

 tions of fracturing already referred to as northeast-southwest and south- 

 east-northwest. The latter are invariably found to be younger than the 

 northeast-southwest series of faults. The Columbus Extension tunnel 

 has been driven northwest for a considerable distance along one of these 

 breaks. Near the mouth of the tunnel, a displacement of 90 feet has 

 been observed, but farther to the north it is probably less. On the divide 

 between South Fork and Alta, a fault with a throw of about 30 feet is 

 clearly shown on the surface. The west side has been depressed. This 

 fault is shown in the structure sections B-B^ and C-C^ on the map. 



