88 GEOLOGICAL SURVEY OF THE TEREITORIES. 



the mouth of Eocb Creek is about two miles in width, and beautifully 

 terraced. The river splits into numerous branches, inclosing islets on 

 which are groves of cottonwoodSr The terrace bluffs on the north side 

 are about one hundred feet in height. JKock Creek enters the valley, 

 emerging from the Elk Mountains, about three miles above its mouth. 

 After the junction of the two streams, Eock Creek turns and flows north 

 25<^ west. Just before it turns, it flows by bluffs of red sandstone, out- 

 cropxjing on the north side. They dip 10° to 20° a few degrees east of 

 north. A short distance back of the river they are capped with basalt, 

 and covered with a growth of low cedars. The hills continue on the 

 north or northeast side of the river until we reach the Grand. There is 

 some slight folding at right angles to the river, allowing the gypsiferous 

 beds to outcrop as we go down. It is only a minor fold, for Roaring 

 Fork occupies the axis of the main fold, which is an anticlinal. On 

 the opposite side of the river there are well-marked hog-backs, in which 

 the strike is parallel to the course of the river, as is shown also in the 

 parallel course of the streams draining them. A section through these 

 hog-backs and across Eoaring E'ork is shown in section F, Plate 

 IV. In tbe hills, farther back, there is basalt on the summits. The 

 hog-backs extend to within about four miles of the Grand, when the 

 basaltic capping comes so near the river as to preserve the edge of the 

 hills and prevent the underlying beds from being eroded. The line of 

 outcrop also curves slightly to the westward, under the capping, so 

 that the Cretaceous No. 1, which forms the best defined hog back, is 

 considerably farther from Eoaring Fork, near the Grand, than it is 

 near Eock Creek. At this point, then, the Eed Beds are seen on 

 top, and beneath a considerable thickness of the gypsiferous strata 

 outcrops on both sides of the river. Those on the south side have 

 weathered into curious pinnacle and tower like forms of pink, ,^ el- 

 low, gray, and creamy colors. The valley of Eoaring Fork is 

 here comparatively narrow. The river and local drift is terraced, 

 the terraces in the lower part of the valley being covered with bowld- ' 

 ers of black basalt, from the hills back of the hog-backs. About three 

 miles and a half above the mouth, Eoaring Fork is joined by the creek 

 draining the hog-backs, which here cuts across them and flows along 

 the upiier edge of a terrace that is 200 feet above the level of the river. 

 The course of the river here is almost due north until it joins the Grand, 

 just as the latter emerges from the caiion that commences below the 

 mouth of the Eagle. The exit is through a narrow gorge in the lime- 

 stones. These limestones continue along the upper side of the river, 

 dipping generally toward the south, at an angle of 20° to 30°. The 

 inclination is more abrupt near the river, decreasing as we go back, un- 

 til, on the summit of the hills, the beds are almost horizontal. On the 

 south side the Eed Beds outcrop, forming bluffs that at some points are 

 five hundred feet in height, rising in sheer precipices. There are proba- 

 bly gypsiferous beds beneath, but they are concealed by the local debris. 

 A short distance back the edge of the basaltic mass capping the hills 

 appears. Its outlines are somewhat difScult to trace in places, as it is 

 covered with a growth of scrub oak, and scattered groves of cotton- 

 woods. The line of outcrop of the Dakota group appears from beneath 

 the basaltic area, and keeps along the lower side of the river, the strike 

 being north 75° west. The beds form a t^eries of hog-backs, the contin- 

 uation of those on Eoaring Fork. They extend along the Grand on the 

 lower side for eight miles, when they cross and stretch away to the north- 

 west, in an isolated range which gradually dies out in a plateau. A sec- 

 tion across these hog-backs west of station 17 is shown in Fig. 2, Plate 



