284 DESCRIPTIVE GEOLOGY. 



lying these, form the southern slope of the ridge, dipping in general but 

 very little steeper than the slope of the hills themselves ; at Zenobia Peak, 

 their angle is 10°, the same as that of the underlying quartzite. South- 

 ward toward the Yampa River, however, the dip is somewhat steeper. 

 The valley of the Yampa River from Lily's Park to its junction with Green 

 River is a broad synclinal valley, whose northern borders are formed by 

 the gentle sloping beds of the Escalante Hills, and bounded on the south 

 by the more abrupt escarpments of the Yampa Plateau. Through the bot- 

 tom of this valley, the Yampa River has cut a narrow, winding canon 1,000 

 to ] ,500 feet deep, through the beds of the Upper Coal-Measure group, 

 which, in the bottom of the valley, are almost horizontal. The walls of this 

 canon are so steep, and its bed so narrow, that it is hardly visible until one 

 is immediately on its brink. In the western portion of the valley, toward 

 Green River, are some remains of the red sandstones of the Triassic, which 

 have escaped erosion. From Tank Peak, whence was obtained the most 

 comprehensive view of the structure of this valley, since owing to the depth 

 of the canons it was impossible to cross the Yampa River below Lily's 

 Park, these remaining Triassic beds can be very distinctly seen, ending on 

 cither side in prominent red bluffs. The flat- topped ridge, of which Tank Peak 

 forms the eastern extremity, is capped by the drab limestones of the Upper 

 Coal-Measuros, dipping only 5° to the southward. In the valley-bottom, at 

 the base of this ridge, however, the same beds have a steep dip to the north- 

 ward, which is 45^ at the foot of Tank Peak, and increases to 80° near the 

 western end of the Triassic beds. A line of bluffs to the west of the Triassic 

 beds, which forms a continuation of the vf estern wall of the canon of Lodore 

 to the south of the Yampa River, represents a similar line of sharp flexure 

 and dislocation, the summit of the bluffs being formed of horizontal limestone 

 strata, while at its eastern base the beds curve sharply downward at an 

 angle of from 80° to 90°. 



In the valley of Fox Creek, on the south of the Tank Peak Ridge, is a 

 similar sharp flexure, where, on the southern border of the Yampa Plateau, 

 the limestone beds dip again 70° to the southward. On the hills south of 

 Fox Creek, which represent a third broad fold, the Triassic sandstones still 

 cover the surface. East of Section Ridge the southern flanks of the Yampa 

 Plateau were not examined, though the outlines of the formations could be 



