THE HEART MOUNTAIN OVERTHRUST, WYOMING 555 



6. The relations of the border Wasatch and the residuals east 

 of Sunshine indicate that the border belt of folds had assumed' 

 their present form and were truncated by erosion before these 

 beds were laid down. Apparently, therefore, the folds were devel- 

 oped considerably earlier than the overthrust. 



AGE OF THE OVERTHRUST 



As a result of his work Dake concluded that the overthrust was 

 younger than certain beds along the north and south fork of Sho- 

 shone River, tentatively correlated with his Fort Union ( ?) forma- 

 tion. He also concluded that the overthrust was older than the 

 ''early basic breccias." 



The short time available to the writer in this region in 191 9 

 only permitted the following conclusions : 



1. The beds exposed along the north fork of Shoshone River 

 under the "early basic breccias" (13), and locally under remnants 

 of the overthrust block resemble lithologically those 1,000 to 

 1,500 feet above the base of the Fort Union formation in the 

 Bighorn Basin and lack the arkoses which are rather characteristic 

 of the Bighorn Basin Wasatch. On the other hand they yield 

 collections of leaves, among which F. H. Knowlton has recognized 

 "Aralia notata Lesq. " which though considered to be a Fort 

 Union species, has not yet been recognized in the Bighorn Basin 

 Fort Union, but is present in most of the collections from the 

 "early acid breccias" in Yellowstone Park and the tuffs north 

 of Owl Creek. The beds yield numerous bone fragments, largely 

 turtle skutes, not yet recognized in the Bighorn Basin Fort Union, 

 but common in the Bighorn Basin Wasatch. Although existing 

 evidence is conflicting, the writer is inclined to consider that the 

 beds are to be correlated with the border Wasatch. 



2. The overthrust is older than the "early basic breccias" 

 because the breccias locally lie in channels cut 200 to 300 feet 

 below the overthrust surface. 



3. The period of erosion that followed the overthrust and 

 preceded the deposition of the " early basic breccias " was sufBciently 

 long to destroy most of the beds that made up the thrust block, 

 for the thickest remnant, that which makes up the summit of 



