THE HEART MOUNTAIN OVERTHRUST, WYOMING 553 



near Sheep Mountain between the north and south forks of Sho- 

 shone River, 28 miles west. It is impossible to imagine that the 

 McCullock Peaks blocks are somehow related to another over- 

 thrust or to some obscure and complex structure, for the structure 

 of the beds between the peaks and Sheep Mountain is completely- 

 shown in the canyon of Shoshone River (14). 



The recognition of mammalian fossils characteristic of the 

 Bridger formation of western Wyoming under the blocks of Madison 

 limestone shows conclusively that the period of overthrust was no 

 older than these beds. 



PHYSICAL FEATURES OF THE OVERTHRUST 



Sufficient information is not yet available to make a compre- 

 hensive statement concerning those features of the overthrust that 

 are needed to interpret the conditions under which it developed. 

 It may be that the thick masses of "early basic breccias" cover so 

 much of the mountainward portion that a satisfactory explanation 

 can never be given. The known exposures permit the following 

 summary of its features : 



1. The lithology of the beds and few fossils collected in the 

 mountainward area indicate that the base of the overthrust is 

 uniformly the base of the Madison limestone. Dake states that 

 the base of the Heart Mountain block is Madison limestone. On 

 the other hand, although the few bowlders on Middle Peak yield 

 Madison fossils, the lithologic features of the East Peak remnants 

 resemble the Bighorn limestone. The base of the overthrust is, 

 therefore, probably not the same horizon throughout. A limestone 

 breccia is present at the base of all of the blocks that were examined. 

 In the few days available in the mountain region, the writer was 

 unable to confirm Dake's conclusion that there are two surfaces 

 of overthrust. Although the presence of Sundance fossils under 

 the Chalk Mountain Block appears to be evidence that such is 

 ithe case, the exposures along South Fork, where a lower surface 



s also mapped, do not appear to demand this interpretation. 



2. The elevation of the base of the remnants of the overthrust 

 block decreases from about 7,200 feet on Carter Mountain to, 

 about 6,800 feet on Sheep Mountain, then increases to about 



