50 REPORT UNITED STATES GEOLOGICAL SURVEY. 



lire, is away from the axes of the two mountain-ranges, respectively, 

 which the flexure connects ; for its position there is upon the distal side 

 of each of the uplifts which it successively flanks. In all these places 

 the dip is very abrupt, but it is much less in all the partiversal sweeps 

 around the end of the dippiu^j anticlinals and in the sag of the dipping 

 synclinals than elsewhere. The continuity of the narrow outcrop of the 

 strata of the Dakota and Jurassic groups around the end of the mount- 

 ain portion of Axial, Plateau and Midland uplifts would, at first sight, 

 appear to indicate a separate flexure there; but this apparently continu- 

 ous outcrop belongs in fact to the three different displacements just 

 named, and portions of it are continuous respectively with the Yampa, 

 Fox Creekj and Midland flexures. The character of the latter flexure, 

 as well as the strata involved in it at various points, is shown in the 

 sections at the bottom of the accompanying geological map. 



The Fox Creek Flexure. — From the point where this flexure blends 

 with Midland Flexure at the south side of Section Ridge, it passes east- 

 ward in a gently sinuous direction, forming the dividing displacement 

 between Midland and Plateau uplifts. Then, sweeping around the east- 

 ern end of the latter uplift, it ends against the Yampa Flexure, which 

 in turn sweeps around the eastern end of the terminal mountain-mass 

 of the Great Axial Uplift. 



Fox Creek Flexure is a very abrupt one, the maximum dip being nearly 

 vertical, and it embraces a maximum displacement of about 3,000 feet. 

 The strata involved in it, which appear at the surface, are those of the 

 Jura-Trias and Carboniferous. Its general character is shown in the 

 sections at the bottom of the accompanying geological map. A marked 

 peculiarity of the flexure is the apparent dragging of its strata against 

 the side of the Plateau Uplift, which has before been mentioned. 



The Yampa Flexure. — From its sweep around the terminal mountain- 

 mass of the Axial Uplift this flexure extends westward, and constitutes 

 the dividing displacement between the Plateau and Axial ui)lifts. Its 

 general direction is approximately parallel with that of the Midland 

 Flexure, except that toward its western end it bends away to the north- 

 westward. It ends abruptly, at least in part, against a great north and 

 south fault at the. western end of Eed Eock Basin, the western wall of 

 which fault also constitutes the end of the basin. 



The flexures that have been herein described are all nearly true 

 mouoclinal flexures, at least in their more abrupt and characteristic 

 portions. But Yampa Flexure is different in this respect, since it forms 

 an abrupt synclinal with the almost immediate rise of the strata which 

 form the southern side of the great Uinta Uplift. This flexure itself 

 has a nearly true mouoclinal character, similar to that of the Fox Creek 

 Flexure at the other side of the Plateau Uplift. Contrary to the general 

 rule, however, in the case of the secondary uplifts. Plateau Uplift has 

 its strata sharply flexed upon both sides, instead of one only, the distal 

 side. In this respect Plateau Uplift possesses a peculiarity that has been 

 shown to characterize the main uplift of the Uinta Mountain system as 

 well as some of its accessory uplifts. 



Within the southern border of Eed Eock Basin, and some five or sis 

 miles from its western end, Yampa flexure divides into two branches, 

 both of which are mouoclinal, the northernmost or lower one having 400 

 or 500 feet greater displacement than the other. The aggregate dis- 

 placement of strata at the western end of the flexure is not far from 

 3,000 feet, which amount gradually lessens to the eastward, where also 

 the synclinal becomes narrower and shallower. Circumstances did not 

 permit a careful examination of the mouatain-mass at the western end 



