GEOLOGY OF THE BEARTOOTH MOUNTAINS, MONTANA 461 



is indicated on the areal map of the Crandall quadrangle (FoHo 52). 

 Furthermore, the pre-Cambrian granite of which the escarpment is 

 composed rises abruptly more than 2,000 feet above Cambrian strata 

 at its base, which are apparently horizontal, or at most only slightly 

 warped (Fig, 12). This escarpment trends northwest and dis- 

 appears southwest of Beartooth Butte in the crystalline rocks of 

 the sub-summit plateau. The maximum vertical displacement of 

 this fault may be several thousand feet, as all the Paleozoic forma- 

 tions, and an unknown thickness of the pre-Cambrian rocks, have 

 been eroded from the summit of the range. 



HISTORY OF THE RANGE 



Antecedent conditions. — ^The Paleozoic and Mesozoic eras were 

 a time of gradual preparation for the birth of this Rocky Mountain 

 front range. An erosional plain of low rehef that had been devel- 

 oped upon the pre-Cambrian crystalHnes was flooded by the middle 

 Cambrian sea. Repeated incursions in subsequent Paleozoic 

 periods resulted in the deposition of a considerable body of sedi- 

 ments upon the site of the future range. The general region was 

 upwarped somewhat during the closing stages of the Paleozoic era 

 but was peneplaned before the transgression of the middle Jurassic 

 sea.^ Throughout the remainder of the Mesozoic era a huge mass 

 of marine and continental sediments, with a large body of inter- 

 calated eflusives, accumulated over the area. The ejection of the 

 volcanics appears to have been a premonitory symptom of the 

 orogenic revolution which was soon to involve this region. That 

 not remote portions of the province were already being affected 

 by notable uphf t and erosion is attested by pebbles of Carboniferous 

 Hmestones and presumably pre-Cambrian granite in the Fort Union 

 formation near Cody, Wyoming.^ 



Early Terdtiary orogeny. — The climacteric event in the growth 

 of the Beartooth Range was the profound orogeny which resulted 

 in the huge overturned anticline and the great overthrust fault along 

 its northeastern flank. The precise date of this epochal deforma- 



' D. D. Condit, "Relations of Late Paleozoic and Early Mesozoic Formations of 

 Southwestern Montana and Adjacent Parts of Wyoming," U.S. Geol. Survey, Prof. 

 Paper 120-F, 1918. 



^D. F. Hewett, "The Shoshone River Section, Wyoming," U.S. Geol. Survey 

 Bull. ^41, 1914, pp. 105-6. 



