HINTZE, GEOLOGY OF WASATCH MOUNTAINS, UTAH 133 



thickness and exposure there are hardly more favorable for observation. 

 The strong contrast between the black shale and the almost white quartzite 

 makes the presence of the shale easy to recognize and renders it one of the 

 best guides to the surface geology of the region (see Plate III, A). 



Alta Overthrust 



As already stated above, there is complete stratigraphic evidence of a 

 large overthrust in the vicinity of Alta, for which the name Alta over- 

 thrust is proposed. It has been traced north from the locality where it 

 w^as first discovered northw^est of Alta into Big Cottonwood Canyon and 

 south into American Fork. There can be little doubt, however, that it 

 extends much farther in both directions. The dip of the overthrust beds 

 is not very different from that of the strata upon which they rest, so that 

 the attitude of the beds above the thrust surface furnished no clue to the 

 relationship. The strong contrast in color and lithologic characters be- 

 tween the various stratigraphic members soon led to the recognition of a 

 complete duplication of beds. The other factors were then soon discov- 

 ered. Evidence of intense dynamic action was found in the highly folded 

 and contorted conditions of the weaker strata. Eapid variation in the 

 thickness of the beds, and the complete disappearance of some of them 

 above and below the thrust surface were noted. 



The accompanying diagram (Fig. 5) shows the relation of the beds 

 above and below the thrust surface as they occur between Alta and 

 Argenta, a distance of about four miles. The succession at the right is 

 the same as that seen in the photograph (Plate IV, A). As we go north- 

 west, the lowet members of the series above the thrust line T T^, as w^ell 

 as the limestones and shale below it, disappear, so that when Argenta is 

 reached these beds are missing. The Cambrian quartzite has apparently 

 become much thicker, being nearly twice as thick as it is in the two ex- 

 posures near Alta and at the head of South Fork. The only duplication 

 of strata shown in Big Cottonwood is the Cambrian quartzite, and that 

 shows itself in the increased thickness of the beds, the exact line of sepa- 

 ration not having been observed. On the north slopes of Kessler's Peak 

 coming around from Mineral Fork, the thrust surface disappears beneath 

 a heavy mantle of debris, and where it emerges on the north slopes of 

 Big Cottonw^ood, it has not been found again. 



From Alta southward, the thrust surface is more easily traced. The 

 lower limestones outcrop all along the east wall of Peruvian Gulch to 

 the Bullion Divide, where they cross over in a low saddle and fonn the 

 floor of the great cirque at the head of American Fork, known as Min- 



