136 ANNALS NEW YORK ACADEMY OF SCIENCES 



From southeastern Idaho and northern Utah, Richards and Mansfield'** 

 have described a great overthrust which involves strata of late Cretaceous 

 age. The oldest rocks which have been found concealing its trace are 

 the early Eocene conglomerate of the Almy formation/^ making the 

 possible range of age from late Cretaceous to early Eocene. This agrees 

 closely with Blackwelder's determination for the Willard overthrust near 

 Ogden, Utah. 



The latest beds involved in the Alta overthrust are Pennsylvanian 

 within the area studied, but from the general fact that overthrusting 

 accompanies or follows strong folding, ihe overthrusts of the central 

 Wasatch must belong to the late Mesozoic and are probably of the same 

 age as the great Willard and Bannock thrusts. 



The trace of the Alta overthrust has a trend north-northwest, while 

 the thrust surface dips strongly to the east with the general monoclinal 

 structure of the region. This leads to the belief that the movement was 

 from east to west, though this is only tentative. The overthrust block 

 seems to be continuous for eight or ten miles to the east, where it disap- 

 pears below the quaternary beds of Kamas and Weber valleys. More 

 extended work will be needed, however, to show definitely that the direc- 

 tion of thrusting is as above indicated. 



Blackwelder thinks the overthrusting near Ogden came from the east, 

 but Richards and Mansfield have questioned the correctness of this de- 

 termination, as they believe it came from the west. There is thus a 

 difference of opinion in a region perhaps better adapted to the determina- 

 tion of this question. It might be said, however, that the unsymmetrical 

 anticlines of the Cottonwood region are steepest on the west, and in one 

 or two cases seem to be overturned in that direction, suggesting strong 

 lateral pressure from the east. 



The structural relations along the trace of the Alta overthrust are 

 shown in the structure sections accompanying the geologic map. 



A Minor Overthrust 



Immediately south of the town of Alta there is a mass of limestone, 

 shale and quartzite which stands nearly vertical, dipping slightly to the 

 west. In Collins's Gulch, the strata dip eastward at an angle of about 25 

 degrees. Across the ridge to the east of the Albion tunnel, the quartzites 

 appear again vrith an eastern dip. There is thus between Collins's Gulch 

 and the great cirque south of Alta, a mass of limestone, shale and quartzite 



. *> R. W. Richards and G. R. Mansfield : "The Bannock Overthrust," Jour, of Geol., 

 Vol. 20, No. 8. 1912. 



*i U. S. Geol. Surv. Prof. Paper No. 56, p. 89. 



