54 REPORT UNITED STATES GEOLOGICAL SURVEY. 



with the flexed borders of the outcrop of the different groups of strata 

 in the neighborhood of the river. Concerning this portion of the valley, 

 upward of fortj^ miles in length, there is nothing especially remarkable 

 in the relation of the river to the displacements of the strata over which 

 it flows; but in the lower portion of its valley the case is very different. 



The isolated position of Yampa Mountain in the comparatively low 

 land of Axial Basin, and its character as a sharply- defined upthrust of 

 Palseozic rocks through those of Mesozoic age, has been already explaiued. 

 Yampa River, after entering Axial Basin, which it does four or five miles 

 eastward from Yampa Mountain, instead of bending around the mount- 

 ain in the lower ground at its base, cuts, by a narrow carton, through 

 the northern portion of the mountain. The walls of this canon are not 

 only several hundred feet higher than the low ground at the base of the 

 mountain, but the strata through which it has been cut are composed of 

 hard limestone, while those underlying the low ground are of soft sand 

 stones and clayey shales. 



Below Yampa Mountain, the river runs for a distance of fifteen or six- 

 teen miles, to Junction Mountain, along the low grounds of the western 

 portion of Axial Basin, and approximately upon the main axis of the 

 Uinta Mountain Uplift. Reaching Junction Mountain, which is another 

 isolated mountain upthrust almost identical in character and surround- 

 ings with Yampa Mountain, the river cuts through it in the same manner 

 as through the latter mountain, but by a deeper and longer caiion. The 

 length of this caiion through Junction Mountain is about three-quarters 

 of a mile in a straight line, from which its course varies a little, and its 

 greatest depth is about 1,200 feet, the walls being almost perpendicular. 

 As in the case of Yampa Mountain, the strata of the low grounds that sur- 

 round Junction Mountain are soft and easily eroded ; while the caBon 

 is cut through not only the hard limestone and other strata of Car- 

 boniferous age, but also through several hundred feet in thickness of 

 the still harder Weber quartzite beneath them. 



After leaving Junction Mountain, Yampa Eiver flows across Lily's 

 Park directly to the eastern end of the high range of paleozoic rocks 

 that have been brought up by the Uinta Mountain Uplift. It then 

 flows more than 30 miles through a tortuous and continuous caiion 

 which it has cut in the carboniferous strata which form that portion of 

 the southern side of the main fold of the great Uinta Uplift, and empties 

 into Green River where itself is passing through a deep canon in the 

 Uinta Mountains. The walls of the Yampa Canon are almost everywhere 

 nearly perpendicular, varying in height above the stream from 1,000 to 

 1,500 feet. Along a great part of its course Yampa Caiion lies approxi- 

 mately parallel with a valley that lies in the synclinal flexure between 

 the main fold of the Uinta Uplift and the accessory one of the Yampa pla- 

 teau. Red Rock Basin constitutes the western end of this synclinal 

 valley, the bottom of which is 1,600 or 1,800 feet lower than the brink of 

 that portion of the caiion which lies opposite to it. 



Viewing this region, according to its present topography and the sus- 

 ceptibility to erosion of the rocks that occupy the surface, the most 

 natural course for Yampa River to have pursued to form a junction with 

 the Green would seem to be around the north side of Yampa Mountain, 

 thence past the southern end of Junction Mountain through Midland 

 Basin, and thence down the dry valley that lies along the southern side 

 of Midland and Section Ridges to the valley of Green River. On the 

 contrary Yampa River traverses the most mountainous and difficult por- 

 tion of the district after having cut through, instead of around, two 

 isolated mountains- composed of hard and compact strata. 



