GEOLOGY OF THE BEARTOOTH MOUNTAINS, MONTANA 453 



group in Clark Fork Valley,' but the shales become distinctly more 

 arenaceous and in part grade laterally into sandstone as the moun- 

 tain front is approached. The Eagle formation contains workable 

 coal in Clark Fork and Stillwater valleys.^ Along the northern 

 third of the range the Eagle is overlain by the thick tuffaceous beds 

 of the Livingston formation, which apparently accumulated during 

 the early Montana to Fort Union interval.^ This formation consists 

 of andesitic sandstones with a thick areally extensive lens of 

 agglomerate as the middle portion. 



The Lance formation has not been certainly identified in this 

 region, and as typically developed on the Great Plains may be lack- 

 ing. Detailed work in this part of the state will probably show that 

 it is represented in the basal portion of the extraordinarily thick 

 "Fort Union" section in Carbon County. Some of the tuffaceous 

 sandstones in the upper part of the Livingston formation may be in 

 part equivalent to the Lance. 



The Fort Union formation occupies a broad area in the vicinity 

 of Red Lodge and extends south along the mountain front toward 

 the Wyoming line. It consists of a thick series of interbedded sand- 

 stone, shale, and clay, with numerous beds of coal in the middle por- 

 tion. Several of these beds supply most of the coal produced in 

 Carbon County. The entire formation is of continental origin. 



Although the Fort Union formation in Montana is generally 

 considered to be of early Eocene (Paleocene) age, there is a growing 

 tendency to refer it with the Lance to the uppermost Cretaceous.'' 

 This is due in part to the apparent establishment of the Cretaceous 

 age of the Lance by the discovery elsewhere of a member with a 



' C. A. Fisher, op. cit., pp. 77-99. 



^ Students of this region should note that the Laramie as shown on the Livingston 

 areal map has been determined to be lower Montana, including the Eagle. See W. R. 

 Calvert, U .S. Geol. Survey Bull. 471, 191 2, pp. 386-89. 



sR. W. Stone and W. R. Calvert, " Stratigraphic Relations of the Livingston 

 Formation of Montana," Econ. Geol., V (1910), 751-52. 



4 C. H. Clapp, "Cretaceous and Tertiary Continental Formations (of Central 

 and Eastern Montana)," Mont. Bur. of Mines and Metal. Bull. No. 4, 1921, pp. 25-26. 



W. D. Matthew, "Fossil Vertebrates and the Cretaceous-Tertiary Problem," 

 Am. Jour. Sci., 5th Ser., II (1921), 205-27. 



Charles Schuchert, "Are the Lance and Fort Union Formations of Mesozoic 

 Time?" Science, LIII (192 1), 45-47. 



