554 D. F. HEWETT 



9,500 feet on the divide between Rattlesnake and Dead Indian 

 Creeks. The base of the Heart Mountain remnant stands at about 

 7,200 feet and the McCuUock Peak remnants from 6,100 to 6,200 

 feet. Rattlesnake Mountain, a persistent ridge which attains an 

 elevation of 9,100 feet and Hes between these two groups of rem- 

 nants, was examined but does not appear to retain any remnants 

 of the overthrust block, although it once overlay the mountain. 

 It cannot be stated assuredly yet whether the differences in elevation 

 here indicated; represent the form of the original surface of over- 

 thrust or whether the surface has been subsequently warped. 

 Although parts of the region have been warped since the deposition 

 of the bedded tuff near Owl Creek, tentatively correlated with the 

 "early acid breccias" (lower Eocene), the writer believes that a 

 large part of the noted differences represent the form of the surface 

 over which the block was thrust. 



3. The structure and attitude of the remnants of the over- 

 thrust block indicate that it was a relatively simple, unfolded layer 

 of rock. The thickness can only be conjectured. The entire 

 Paleozoic, Mesozoic, and Fort Union sections above the Deadwood 

 shale (Cambrian) are about 17,000 feet thick. In attempting to 

 estimate the probable thickness of the block, it must be borne in 

 mind that it was largely removed by erosion before the outburst 

 of "early basic breccias" (upper Miocene). The writer would 

 tentatively estimate the thickness near the mountains at 15,000 

 feet, but the eastern edge was probably much thinner. 



4. The surface upon which the overthrust moved is cut across 

 a sharply folded belt of rocks that range from the pre-Cambrian 

 granite to beds that appear to represent horizon A of the Bridger 

 formation. No part of this surface that has been studied appears 

 to be a fracture across the beds, but it is probably a surface of 

 erosion. To this extent it resembles Willis' interpretation of the 

 Lewis overthrust (17). 



5. Dake mapped the overthrust in an area thirty miles long 

 and the McCullock Peak exposures indicate a minimum thrust of 

 twenty-eight miles. Only casual consideration of the regional 

 geology is needed to convince one that the overthrust must have 

 involved a much larger area, over which it can probably be traced. 



