GEOLOGY OF THE BEARTOOTH MOUNTAINS, MONTANA 449 



in many places. Coarse pegmatite dikes are fairly common in 

 the granite and gneiss. A unique feature is a large dike of pyroxen- 

 ite that extends from Boulder River, below Contact, southeast 

 beyond the head of Little Rocky Creek, southwest of Dean, and 

 contains a large tabular deposit of high-grade chromite.^ 



No sedimentary formations of pre-Cambrian age have been 

 discovered anywhere in this region. 



Sedimentary formations. — Sedimentary formations are restricted 

 to the flanks of the range. The indurated strata range in age from 

 middle Cambrian to late Cretaceous or Paleocene. All the Paleo- 

 zoic systems, except the Silurian which is absent, are represented by 

 extensive deposits of limestone and dolomite with subordinate 

 amounts of more or less calcareous shale and sandstone. The 

 entire sequence is present along the eastern and northern base of 

 the range, but erosion has removed most of the upper Paleozoics on 

 the southwest slope north of the Wyoming boundary, Mesozoic 

 formations are present only along the plains-ward front of the range 

 and in the adjacent plains. All the systems are present. They 

 consist preponderantly of alternating sandstone and shale with 

 some limestone, gypsum, and coal. The upper part of the sequence 

 in the northern part of the area is composed mainly of volcanic 

 materials (Livingston formation). In Carbon County a thick 

 series (Fort Union) of sandstone, shale, and coal lies with apparent 

 conformity upon the youngest beds of undoubted Mesozoic age. 

 Farther south along the mountain front a conspicuous conglomerate 

 may be of later age. 



The maximum thickness of the sedimentary formations on the 

 east side of the Beartooth Range is approximately 18,000 feet. This 

 is exclusive of the Livingston tuffaceous sandstones and agglomer- 

 ates which in places are more than 5,000 feet thick. The Paleozoic 

 systems have a total thickness between 3,500 and 4,000 feet, the 

 undoubted Mesozoic systems about 6,000 feet, and the strata 

 included in the Fort Union formation approximately 8,500 feet.^ 



' L. G. Westgate, "Deposits of Chromite in Stillwater and Sweetgrass Counties, 

 Montana," U.S. Geol. Survey Bull. 725-A, 1921, pp. 67-84. 



» E. G. Woodruff, "The Red Lodge Coal Field, Montana," U.S. Geol. Survey Bull. 

 341, 1908, Pt. II, p. 92. 



