540 D. F. HEWETT 



1,500 feet above the nearby streams; Squaw Buttes (6,200 feet), 

 Tatman Mountain (5,800 feet), and McCullock Peaks,^ three in 

 number and approximately equal in elevation (6,200 feet). Heart 

 Mountain (8,080 feet) is a conspicuous peak that lies west of the 

 central part of the basin, where it merges with the border belt. 



The border belt is largely made up of long stretches of flat 

 upland terraces that rise gently toward the mountains. Most of 

 the streams have cut broad terraced valleys below the uplands. 

 The rocks exposed in this belt range from the Chugwater formation 

 ("Red Beds") to the Fort Union formation (lower Eocene), and 

 attain a maximum thickness of about 15,000 feet on the west side 

 of the basin. The successive formations are brought to the surface 

 in a series of pronounced folds whose axes are roughly parallel to a 

 median trough in the central part of the basin. Dips that range 

 from 15° to 60° are common along the flanks of the folds (14a). 



In a broad way, the mountains that limit the basin on the east, 

 southeast, and south, the Bighorn (4), Bridger (4), and Owl Creek (3) 

 ranges respectively, are rather simple smooth ridges that coincide 

 with extensive anticlines. The mountains west of the basin (10) 

 are high and rugged and present an imposing front toward the 

 basin. They coincide roughly with an area of volcanic tuffs, 

 breccias, and flows that are a part of the extensive field of vol- 

 canic rocks which covers northwestern Wyoming and eastern Idaho. 



STRATIGRAPHY 



It will be sufficient at this place to state briefly the general 

 features of the Paleozoic and Mesozoic sections and such details 

 of the Fort Union, Wasatch, and younger rocks as bear upon the 

 age of the overthrust and the physical conditions surrounding the 

 process. The commonest underlying pre-Cambrian rock in this 

 region is a rather homogeneous red granite which is locally cut by 

 diabase dikes. 



The Paleozoic and Mesozoic sections are separable into three 

 groups on the basis of lithology and degree of induration, which 

 measure their strength. The first and strongest group includes 

 the Paleozoic limestones and associated sandstones and quartzites, 



* Commonly referred to as McCullock Peak. 



