48 EEPORT UNITED STATES GEOLOGICAL SURVEY. 



daries of this district. Among these is the Dorthern extremity of the 

 Great Elk Mountain Uplift, the principal part of which has been so ably 

 d2scribed and illustrated by Mr. W. H. Holmes in his report for 1874. 

 Although the principal mass of that uplift is more than a hundred miles 

 distant to the southward, its northern vanishing end is in the bluffs that 

 form the northern border of Agency Park, where it is separated from 

 the southeastern end of the Danforth Hills Uplift only by the gentle 

 synclinal that has been before mentioned as a part of the boundary of 

 the latter uplift. The connection of Elk Mountain Uplift will be further 

 referred to under the head of "Flexures." 



Two or three very faint uplifts, or undulations of the strata, cross 

 Green River from the southeastern part of this district. These are 

 doubtless the result of the same force that elevated the Uinta Mountain 

 system, as it diminished in intensity with the distance from the great axis. 



Besides the uplifts the boundaries of which may be defined, there is 

 a general uprising of all the strata, to the eastward, against the western 

 flank of the Park Range system. It is upon this broad, gently -inclined 

 plane of strata that a portion of the minor uplifts alreatly described are 

 defined. Even the eastern termination of the great Axial Uplift of the 

 Uinta system extends upon this elevated inclined plane of strata which 

 has been lifted as a part of the Park Range system. 



FLEXURES. 



An uplifting, of course, always implies the flexing of the strata up- 

 lifted; but in this district the conditions of displacement and subsequent 

 erosion of the strata have been such as to make it desirable to give separate 

 names and descriptions to certain flexures. For example, the principal up- 

 lifts that pertain to the Uinta system, namely, the mountain portion of the 

 great Axial Uplift, including the two upthrusts, and the Plateau Uplift, 

 do not, like some others, consist of a nearly uniform convex flexure, but 

 of two more or less abrupt lateral flexures, one at each side, with a com- 

 paratively slight convexity between them. Others, and this applies to 

 all others in this district except the eastern extension of Axial Uplift, 

 which consists of a gentle, uniform, upward flexure, have one side very 

 much more abruptly flexed than the other. This difference is so great 

 in some cases as to give almost a true monoclinal character to the flexure 

 of the whole uplift. In all cases of this kind in this district the steei)er 

 dip is oh the southern, southwestern, or western side of the uplift, ac- 

 cording to its position ; or, in other words, upon the inner side of the 

 great curve which those displacements form as they reach from one 

 mountain system toward the other. It is these steeper flexures to which 

 I have given distinctive names, and which are briefly described in the 

 following paragraphs. 



The Grand Hogback Flexure. — The Fox Hills strata, that flank the west 

 side of Elk Mountain, Mr. Holmes has shown, extends northward as a 

 continuous hogback, which he has called the Grand Hogback, a hundred 

 miles or more, it being the same one that crosses White River and sepa- 

 rates Agency, from Powell's park. The flexure by which that great line 

 of hogbacks has been produced I have called the Grand Hogback Flexure. 

 It belongs, of course, to the Park Range system ; but, alter crossing 

 White River Valley, which it does in an almost due north and south line, 

 it becomes continuous with the great Midland Flexure, which extends in 

 a tortuous course through this district and blends with the principal 

 Uinta displacements. The latter flexure is, then, as much an integral 

 part of the Uinta system as the former is of the Park Range system. 



