GEOLOGY OF THE BEARTOOTH MOUNTAINS, MONTANA 445 

 SALIENT TOPOGRAPHIC FEATURES 



The plains-ward front. — ^The Beartooth Range rises abruptly 

 to a height of a few thousand feet above the Great Plains on the 

 northeast. The 6,000-foot contour almost everywhere follows the 

 base of the range, and the plains-ward crest has an elevation of 

 9,000 to 10,000 feet. In the southern part the frontal slope is 

 uncommonly steep, but to the north it is neither so steep nor so 

 uniform nor everywhere so sharply separated from the bordering 

 plains. 



The sub-summit plateau. — ^The remarkably even outline of the 

 crest for considerable distances, especially in the southern half of 

 the range, is an impressive feature (Fig. 3). It marks the plains- 

 ward rim of a broad flattish plateau that extends along the eastern 

 portion of the range for most of its length. This extensive surface 

 is herewith designated, from its form and topographic positiooi, 

 the "sub-summit plateau." It consists of a series of gently undulat- 

 ing upland fiats, each containing several square miles, that are the 

 remnants of a once continuous erosional plain (Fig. 2). The 

 largest tracts are about 10 miles wide and have an area of 10 to 

 20 square miles. Extensive portions of some of these plateau 

 remnants are strikingly fiat (Fig. 4) . 



In its longest dimension, parallel to the axis of the range, this 

 plateau varies in altitude from slightly less than 10,000 feet, in the 

 vicinity of Boulder River, to more than 11,000 feet in the central 

 portion, then drops a few hundred feet toward its southern extrem- 

 ity. Its surface rises gradually toward the interior of the range, 

 but becomes increasingly steep toward the main axial divide. 



Sharp valleys 3,000 to 4,000 or more feet deep have been carved 

 in this plateau by the several northeasterly flowing streams and 

 their numerous tributaries. The dissection is least advanced in the 

 southern half of the range and has progressed farthest in the north- 

 western portion, but even here the old surface is partially preserved 

 in flattish tracts of considerable size. East Boulder and West 

 Boulder plateaus, which are shown on the topographic map of the 

 Livingston, Montana, quadrangle, are fairly t3^ical remnants, but 

 the best examples are in the south-central part of the range, just 

 north of the Wyoming boundary (Figs. 2 and 4). 



