WHITE.] FLEXUEES. 49 



Although the Great Hogback and Midland flexures are practically one 

 and the same, their separate lines of hogbacks approach each other at 

 an obtuse angle about eight miles north of White River, at the east side 

 of Hogback Valley. Here the continuity of the two great flexures is 

 unbroken, but at the point of meeting of the upturned strata of the hog- 

 back lines there branches off the shallow synclinal axis which sweeps 

 around the southeastern end of the Danforth Hills Uplift, and thence 

 extends along its southern side. 



The Grand Hogback Flexure is nearly a, true monoclinal one. The 

 strata that lie to the eastward of it dip gently to the westward from 

 the foot-hills of the Park Range to the flexure proper, where they have a 

 maximum dip of nearly or quite 75 degrees, and then immediately stretch 

 out nearly horizontally. The strata originally involved in this flexure, 

 as they now appear at the surface, are those of the Colorado Group to 

 the Green Group, inclusive, the maximum dip now appearing in the 

 comparatively soft strata of the Wasatch Group. Subsequent erosion 

 has so far removed portions of these strata that only those of the Green 

 Eiver and Wasatch groups now occupy the surface west of the line of 

 maximum flexure. Those of the former group, that now remain, are 

 nearly horizontal, and have evidently been little if any affected by that 

 great flexure, although it is so near their present escarpments. 



The Midland Flexure. — This flexure is, in some respects, the most in- 

 teresting displacement of strata within the limits of this district, espe- 

 cially as regards its great length and unbroken continuity from one 

 mountain s^'stem to the other. It is continuous with, and in fact a i)art 

 of the Great Hogback Flexure, which, as before shown, is so intimately 

 connected with the Elk Mountain Uplift. From the point of its nominal 

 connection with that flexure in Hogback Valley, Midland Flexure ex- 

 tends in a tortuous course through the whole remaining length of the dis- 

 trict to the flank of the principal uplift of the Uinta Range, with the 

 lateral flexure of which it blends and becomes continuous far beyond 

 the western boundary of this district. The course of Midland Flexure 

 is northwestward about twenty miles up Hogback Valley ; thence due 

 west nearly ten miles ; thence almost directly south nearly fifteen miles 

 to the southern end of Pinon Ridge, around which it bends abruptly ; ' 

 thence in a straight direction, 15° north of west, about fifty miles. At 

 a point about midway of the latter distance it coalesces with the flexure 

 of the south side of Midland Uplift, with which it is in fact a part, all 

 the way westward from the southern end of Piiiou Ridge. From the 

 flank of Midland Uplift, Midland Flexure is continuous to and along the 

 south flank of Section Ridge, where it blends with and becomes a part 

 of the Fox Creek Flexure; thence around the western end of Section 

 Ridge, into the retreating angle formed by the dipping syncliual between 

 Section Ridge and Split Mountain. From here it sweeps around the end 

 of Split Mountain as it did around that of Section Ridge, and into a simi^ 

 lar but much broader retreating angle or notch,, at the apex of which is 

 Island Park ; thence by an abrupt bend to the westward, along the 

 flank of the main uplift of the Uinta Range, and far beyond the limits 

 of this district. It is thus shown that this remarkable flexure, regard- 

 ing both the Great Hogback and Midland flexures as really one, can be 

 traced continuously from the west flank of Elk Mountain Uplift to the 

 south flank of the main Uinta Uplift, a distance of more than two huur 

 dred miles. Throughout the greater part of this distance a large propor- 

 tion of the strata that are involved in this flfexure are exposed in long 

 lines of hogbacks, which constitute more or less conspicuous topograph- 

 ical features. The dip of the strata, in all the long reaches of this flex- 

 4g 



