Mesozoic and Ccenozoic Geology and Palaeontology. 233 



a partial witness to the extent of the denudation. A little south oi 

 west of Fort Bi'idger, is an isolated butte called Bridger's Butte, which 

 forms a prominent land mark to the traveler, and according to the 

 barometer, rises 750 feet above the valley of Black's Fork,, at the fort. 

 The summit appears perfectly level, and was estimated to be about two 

 miles in length, from north to south, and about a fourth of a mile in 

 width, from east to west. The upper portion of the butte is composed 

 of the somber, brown, indurated, arenaceous clays, gray and rusty 

 brown sandstones of the Bridger Group, passing down into limestones 

 and marls of the Green river beds. In the brown clays are abund- 

 ant remains of turtles, with a few fragments of other vertebrate re- 

 mains. The terraces along the valley of Black's Fork, are composed of 

 yellowish and whiteish gray marls, and chalky limestones, some of the 

 laj'ers mostl}' formed of Unio, and other fresh-water shells. A few 

 plants were found in the valley of Smith's Fork, in thin black, flinty 

 layers, mostly ferns and leaves of deciduous trees. Between Fort 

 Bridger and Henry's Fork, the indurated, arenaceous claj^s, of the 

 Bridger Group, are weathered into remarkably unique forms. The 

 absence of harder layers of sandstone did not admit of the weathering 

 into pinnacles, turrets, steeples, domes, etc., as observed near Church 

 Buttes. The surface, though ver3' rugged and almost impassable, ex- 

 cept along the valleys of the streams, is much more rounded ; the hills 

 are more dome or pyramid shaped, and entirely' destitute of vegetation, 

 except the sage, and several varieties of chenopodiaceous shrubs. 

 Passing up the Cottonwood Fork, the marls and limestones make their 

 appearance, for a short distance, in the bluff's. The divide between 

 the drainage of Smith's Fork and Henry's Fork, is a high ridge of the 

 leaden-brown clays of the Bridger Group, which extends up and juts 

 against the base of the Uinta mountains. 



From this ridge to Green river, the valley of Henry's Fork forms a 

 remarkable line of separation between the Bridger Group and the lower 

 beds. This line of separation is somewhat of a surface one, yet it is so 

 marked as to atti'act the attention of the commonest observer. The 

 valley is quite broad, and on* the south side the surface of the country 

 to the summits of the mountains appear smoothed downward, in part 

 grassed over. A close examination will detect some thin remnants of 

 the Bridger Group underlaid by lower Tertiary beds, which have a 

 tendency to weather into rounded, gently-sloping hills. On the north 

 side, the arid, rugged, " bad lands'' are very conspicuous, and rise up 

 somewhat abruptly like a high wall. On the north side of the creek, 

 there is a great thickness of the indurated cla3's of the Bridger Group. 



