312 DESCRIPTIVE GE'JLOGY. 



plateau-like summit, covered by dense growths of pine forest, out of wliich 

 rise bare, steep ridges of horizontally-bedded quartzite. The extreme west- 

 ern end falls off somewhat m height, having an average elevation of less 

 than 10,000 feet west of Bald Mountain, and hence is more densely Avooded 

 than the main crest, but shows the same broad amphitheatre-like basins at 

 the heads of the streams, and similar glacier-cut canons on its flanks. In 

 geological structure, it preserves the same general character of one broad 

 east and west anticlinal fold with axis near the northern edge, the southern 

 member inclined gently over the higher portion, and dipping suddenly 

 down on the southern flanks, while the northern member bends down at 

 steep angles immediately north of the axis. The flanking Mesozoic beds 

 are well exposed on either slope, but at the western end, where the range 

 ends somewhat abruptly, in close proximity to the Wahsatch uplift, their 

 outcrops are masked by a body of trachj^te, which has poured out along 

 the contact-line of the two ranges, and which, together with the Quaternary 

 accumulations of several little mountain- valleys, so completely covers the 

 present surface that but little can be learned of the structure of this inter- 

 esting middle region. 



In resuming the description of the southern flanks of the range, where 

 left in Chapter II, the next cailon-section is found in the valley of the Du- 

 chesne Fork of the Uinta River, one of the most interesting and character- 

 istic canons of the range. This stream takes its rise between Bald Mount- 

 ain (11,977 feet) and Mount Agassiz (13,000 feet), in an almost circular 

 amphitheatre from 4 to 6 miles in diameter, studded with shallow mountain- 

 lakes, which occupy glacier-worn basins in the horizontal quartzites, with 

 an average elevation of 10,000 feet above sea-level. It then flows for about 

 10 miles nearly due south through a deep U-shaped caiion, cut from 2,000 

 to 3,000 feet into the hard quartzite strata, with almost perpendicular walls, 

 which, when seen from below, with the bare summit of Bald Mountain at 

 the head, gives an efi'ect somewhat like that of the Yosemite Valley, so 

 completely does it seem to be shut in. Reaching the steeper-dipping beds, 

 its course is somewhat deflected, and, in the softer Mesozoic rocks, it has 

 cut a broader valley, flowing through a wide alluvial bottom out into the 

 region of Tertiary benches and plains. 



