GEOLOGY OF THE BEARTOOTH MOUNTAINS, MONTANA 451 



The Flathead formation (middle Cambrian) and the Gallatin 

 formation (middle and upper Cambrian) cannot be readily sub- 

 divided in this part of the state into the several formations that 

 constitute the Cambrian system in the Little Belt Mountains^ and 

 northwest along the Rocky Mountain Front. The Bighorn dolo- 

 mite apparently marks in this range the northern-most extent of 

 the Ordovician in Montana. Although the formation is a litho- 

 logic unit with no evidence of a break in it yet discovered on the 

 northeast side of the range, " a conspicuous surface of disconformity, 

 with a basal breccia" exists in it on the extreme southwest slope.^ 

 It may be noted further that the Devonian of Montana apparently 

 does not extend many miles south of the Beartooth Range in Wyo- 

 ming. Hence this range contains one of the most complete Paleozoic 

 sections in the northern Rocky Mountains. The Madison lime- 

 stone is the bold cliff -making limestone (Fig. 7) which is so conspicu- 

 ous throughout the northern Rockies. The Quadrant formation 

 in the northern part of the range is roughly equivalent to the 

 Amsden and Tensleep formations in Clark Fork Canyon at the 

 southern end of the range. D. D. Condit has recently differentiated 

 the Phosphoria formation in this region, and has indicated its 

 distribution along the east base of the range. ^ 



A narrow belt of the Chugwater "Red Beds," presumably in 

 the main of Triassic age, is present along the base of the range in 

 northwestern Wyoming, but disappears a short distance north of 

 the state line. The Chugwater and Quadrant formations are 

 overlain respectively by the Sundance formation in Wyoming and 

 the Ellis formation in Montana — two marine formations that are 

 equivalent in the main and of middle to late Jurassic age."* They 

 are composed chiefly of interbedded shale and limestone in propor- 

 tions which vary from place to place. Between the mountain front 

 and Clark Fork River in the plains, the Morrison and Kootenai 



^ W. H. Weed, "Geology of the Little Belt Mountains, Montana," U.S. Geol. Sur- 

 vey Ann. Rept. XX, 1900, Pt. 3, pp. 284-87. 



^ C. W. Tomlinson, "The Middle Paleozoic Stratigraphy of the Central Rocky 

 Mountain Region," Jour. Geol., XXV (1917), 35. 



3 U.S. Geol. Survey, Prof. Paper i2a-F, PL IX, 1918. 



■^ Charles Schuchert, Bull. Geol. Sec. Am., XXIX (1918), 246; 



