462 ARTHUR BEVAN 



tion is unknown, but the combined evidence afforded by the Bear- 

 tooth and Absaroka ranges clearly indicates that it took place long 

 after the deposition of the Fort Union in the former, and long prior 

 to the accumulation of the widespread "early basic breccias" in 

 the latter. 



The folding of the Fort Union formation during the first orogeny 

 that affected the underlying Mesozoic strata has been previously 

 pointed out in this paper. If the Beartooth overthrust was essen- 

 tially contemporaneous with the Heart Mountain overthrust, and 

 at present there is no reason to doubt this, it occurred not earlier 

 than the middle Eocene and may not have taken place before the 

 early OKgocene.^ This conclusion by Hewett as to the age of the 

 Heart Mountain fault is based upon these facts: (i) the overthrust 

 beds in places rest upon the Bridger formation (middle Eocene), 

 and (2) the "early basic breccias" (upper Miocene) "locally He in 

 channels cut 200 to 300 feet below the overthrust surface."^ It is 

 significant that the overthrust mass is tentatively estimated to have 

 been about 15,000 feet thick near the mountain front. In view of 

 the complete removal of this thickness prior to the ejection of the 

 upper Miocene volcanics it is hardly probable that the faulting 

 took place later than the Oligocene. 



Epochs of Peneplanation.^ — Prolonged erosion initiated by this 

 uplift reduced the ancestral Beartooth Mountains to the surface 

 of low reHef that is partially preserved in the flattish summits 

 along the crest of the present range. The conclusion that these 

 summit tracts are remnants of an ancient peneplain rests not alone 

 upon the fact of their accordant levels, but is substantiated by their 

 flattishness over a considerable area of diverse rocks in the central 

 part of the range. 



The age of this summit peneplain can not be closely determined 

 with certainty on the basis of the available evidence, but its Hmits 

 can be fairly well established. Inasmuch as the Beartooth over- 



' D. F. Hewett, "The Heart Mountain Overthrust, Wyoming," Jour. Geol., 



xxvm (1920), 537. 



^ Ibid., p. 555. 



3 For a more detailed discussion of this topic see a forthcoming article by the 

 writer, "Rocky Mountain Peneplains Northeast of Yellowstone National Park." 



