GEOLOGY OF THE BEARTOOTH MOUNTAINS, MONTANA 455 



and a small amount of limestone. It is a local deposit, covering a 

 few square miles, which is probably much younger than the con- 

 glomerate south of the state line. 



Tertiary igneous rocks. — With the exception of the Livingston 

 formation igneous materials of post- Cambrian age are not common 

 in the Beartooth Mountains. Several dikes, sills, and stocks of 

 Tertiary ( ?) age are present on the lower southwest slope in the 

 Cooke mining district, and in the Haystack Peak area at the head 

 of the main Boulder River. ^ On the east side of the range, south of 

 Red Lodge, the Fort Union formation is invaded by a few narrow 

 vertical dikes that are roughly parallel to the mountain front.^ 

 Some of the dikes and stocks within the pre-Cambrian core are 

 probably of Tertiary age, but their extent remains to be determined. 



The upper part of the imposing front of the adjacent Absaroka 

 Range in northwestern Wyoming has been carved by the Clark 

 Fork from more than 6,000 feet of volcanics that were ejected from 

 the Crandall Basin volcano and associated vents during the Tertiary. 

 The well-known landmarks of Pilot and Index peaks, which tower 

 impressively above the southwest slope of the Beartooth Range, 

 are sharply eroded from these formations. Although these volcanics 

 do not now extend into the Beartooth Mountains, their great thick- 

 ness and proximity indicate that their former eastward extent was 

 much greater. 



Terrace gravels. — UnconsoHdated deposits of Tertiary and 

 Quaternary age are widespread along the western margin of the 

 Great Plains where they extend for many miles beyond the plains- 

 ward front of the Beartooth Mountains. The most common 

 materials are stream gravels and bowlders of diverse constitution, 

 which once probably formed an extensive piedmont plain. They 

 have been worked over and over by the major streams and deposited 

 in part at successively lower levels until now they mantle a series 

 of broad terraces upon the truncated sedimentary formations along 

 and between the principal valleys. Three or four of these terraces 



' W. H. Emmons, "Geology of the Haystack Stock, Cowles, Park County, Mon- 

 tana," Jour. Geol., XVI (1908), 193-229. 



^N. H. Darton, "Coals of Carbon County, Montana," U.S. Geol. Survey Bull. 

 316, 1907, p. 186. 



