The Owl Creek Mountains are a direct westward continuation of the Bridger Range, being separated from the latter by the great canyon of Bighorn River. The entire range is in reality a prolongation of the Bighorn Mountains, from which it is separated by a low saddle on Bridger Creek, 25 miles southeast of Thermopolis. To the west the Owl Creek Mountains merge into the Shoshone Mountains, which extend northward to and along the east side of the Yellowstone Park.

The Owl Creek Mountains vary from about 7,000 to 9,500 feet in altitude. They are about 7,000 feet high at the edge of the canyon of Bighorn River, which is a steep-sided trench 2,250 feet deep, cut across the axis of the range. The altitude continues to be about 7,000 feet westward from the canyon to the pass through which the road from Thermopolis to Lander crosses, but there drops rapidly to 6,244 feet, as indicated by a bench mark on the divide. To the west of this pass the range rises rapidly, and south of Embar it attains an altitude of somewhat over 9,000 feet. The highest point in this vicinity is Phlox Mountain, which, by barometric determination, is about 9,300 feet above sea level. This peak is a prominent knob of limestone, presenting high cliffs to the west and south. In this vicinity the southern slope of the mountain is steep, as there is a rapid fall to an altitude of about 6,500 feet. A short distance west of Phlox Mountain the range is crossed by a low pass traversed by the road from Anchor to Hollands, on the summit of which the altitude is 8,099 feet, as determined by United States Geological Survey levels. To the west of this pass the mountains consist of an irregular series of high limestone ridges varying from 8,300 to somewhat over 9,000 feet and merging into the Shoshone Mountains, in which the altitudes rise rapidly to considerably over 11,000 feet. The highest peaks of that range in this vicinity are the Washakie Needles, which reach 12,496 feet. Black Mountain, an outlying peak, has an altitude of 10,165 feet. South of the Owl Creek Mountains there are broad plains, in part bad lands, which extend to Wind River. On the north lies the Bighorn basin.

DRAINAGE.

The greater part of the run-off in the Owl Creek Mountains flows into Owl Creek, a branch of Bighorn River, which it joins 5 miles below Thermopolis. It flows along the north side of the range and has two principal forks-North Fork and South Forkwhich rise in the south end of Shoshone Mountains, in the vicinity of the Washakie Needles. South Fork is much the larger stream. It passes through a deep canyon in the west end of the Owl Creek Mountains and then flows through a valley on the north side of the range. This stream receives the drainage of numerous tributaries on the north slopes, of which the largest is Mud Creek. Near the east end of the mountain the north slope is drained by Red Creek, which empties into Bighorn River 4 miles above Thermopolis. The south side of the Owl Creek Mountains is drained by Muddy Creek and its many branches on the east and by Dry, Red, and Crow creeks toward their western end. The three latter streams are affluents of Wind River. This river rises on the east slope of the Wind River Mountains and the south slope of the Shoshone Mountains. It flows southeastward to its confluence with Popo Agie River, with which it forms Bighorn River, a great stream flowing northeastward to the Yellowstone, a branch of the Missouri.

GEOLOGY.

DESCRIPTION OF THE ROCKS.

GENERAL FEATURES.

The Owl Creek Mountains are due to an anticlinal uplift of many thousand feet which has brought a thick series of Paleozoic and Mesozoic sedimentary rocks high above the adjoining plains. The crest of the uplift has been extensively eroded and in its central portion there appear at intervals areas of pre-Cambrian granites and

associated rocks which underlie the sedimentary beds. The general stratigraphic features are very similar to those presented in the adjoining Bighorn and Wind River uplifts, with certain local variations. Exposures of the rocks are extensive, especially in the many deep canyons of streams which flow out of the range at intervals. The structure presented on either side of the mountains is that of a monocline dipping to the south on the south slopes and to the north on the north slopes, but both of these sides have many variations in dip and are extensively corrugated by local flexures. The cross sections given in Pl. XI show the principal structural features. To the west the uplift passes beneath a thick series of overlapping volcanic and sedimentary rocks of Tertiary age, and in the plains to the south there is more or less extensive overlapping of the "Bridger" Tertiary beds. In the cross sections above referred to it will be seen that the pre-Tertiary formations consist of a series of thick sheets of sandstones, limestones, and shales, all conformable in structure, though lacking some members of the geologic succession. The following is a list of the formations exhibited in the uplift, with a generalized statement as to thickness, characteristics, and age:

Generalized section in the Owl Creek Mountains.

Pierre shale
Colorado formation ...

harder, darker concretions.
Dark-gray shale, with concretions
Gray shales; thin brown sandstones
below; hard, fine gray sandstones
and shales (Mowry beds) in middle
part; concretions with Prionocyclus,
etc., and massive buff sandstones at
top.

Cloverly formation.... Coarse, massive buff sandstone below,
with purplish and gray shales and
some sandstone above.

Morrison formation... Massive shale, greenish gray, buff, and maroon, with thin sandstones.

Sundance formation... Soft sandstones, overlain by greenishgray shales; several hard fossiliferous layers near top and bottom.

Chugwater formation. Red shales and soft sandstone; thin limestone layers near top and bottom and gypsum deposits near top.

Embar formation...... Gray limestone, with cherty beds........

Feet.

2,250

Upper Cretaceous (and
Eocene 2).

Upper Cretaceous.

225

a The geology of the Bighorn Mountains is described by N. H. Darton in Professional Paper No. 51, U. S. Geol. Survey, now in press.

A.

Granite at fault in foreground. D, Black schist; B, Bighorn limestone; L, Middle limestone in Deadwood formation; M, Madison limestone; A, Amsden formation.

B. VIEW NORTHWARD INTO NARROWS OF UPPER CANYON OF BIGHORN RIVER, AT EAST END OF OWL CREEK MOUNTAINS.

In the foreground are diorites brought up by a great fault. In the distance are overlying formations to Tensleep sandstone

The granites are mostly red and massive, but associated with them are extensive bodies of dark-colored, basic rocks which, with the granite, constituted a floor for sediments of middle Cambrian age. These Cambrian rocks are sandstones, shales, and limestones about 900 feet thick and apparently do not include sediments of upper Cambrian age. The Ordovician is represented by massive limestone of Trenton age, while the uppermost Ordovician and the Silurian and Devonian are absent. The Carboniferous presents about 1,250 feet of beds, of which the lower half is Mississippian and the upper half Pennsylvanian. The top member is a limestone (Embar), which often constitutes long slopes on the mountain sides. Next follow the "Red Beds," which extend along the foot of the mountain, as in most other uplifts in the Rocky Mountain province. They attain a thickness of about 800 feet and perhaps are all of Permian age, though possibly the Triassic may be represented by the uppermost beds. The marine Jurassic which follows is similar to that of the Black Hills and Bighorn Mountains, and contains an abundant upper Jurassic fauna It is overlain by the Morrison shales and sandstone, only about 200 feet thick but presenting the characteristics which are so general throughout the Rocky Mountain province.

Representatives of the "Dakota" sandstone appear in the uplift, but with greatly diminished development as compared with the Black Hills and other regions to the south. As in the Bighorn Mountains, the basal, hard sandstone, supposed to represent the Lakota formation, is the most conspicuous feature. The great series of upper Cretaceous shales attains a thickness of somewhat over 2,000 feet in the lowlands north of the Owl Creek Mountains-very much less than in the region adjoining the Bighorn uplift on the east. At the base is the Benton formation, which, as in the Bighorn uplift, does not present the characteristic middle limestone (Greenhorn) which is so prominent in the Black Hills uplift, eastern Colorado, and farther east. The Niobrara presents but little evidence of the chalky element which is so characteristic farther east and south, and also lacks the characteristic fossil Ostrea congesta. Its presence is indicated, however, by the apparently unbroken sedimentation from the Benton to the Pierre. At or near this horizon occurs a prominent bed of sandstone, containing large numbers of fossils which may represent a special Niobrara fauna. The Pierre shale, with a thickness of 1,000 feet on the north and somewhat over 2,000 feet on the south, presents the usual monotonous succession of gray shales with fossil-bearing concretions. It is terminated by a sandstone supposed to represent the Fox Hills, and this in turn is succeeded by a great development of sandstones and carbonaceous shales supposed to be of Laramie age. The Tertiary is represented by the "Bridger" deposits in the plains south of the mountains, with outliers overlapping onto the mountain slopes at some points, and to the northwest by a great. succession of volcanic and sedimentary rocks constituting the Shoshone Mountains. The Quaternary deposits consist of high terraces and old fans along the lower slopes of the mountains and alluvial plains along the streams, which merge into flood plains of the present period. But little evidence of glaciation has been found. The hot springs at Thermopolis are depositing travertine along the banks of Bighorn River, and deposits of this material of considerable antiquity cap some of the buttes north of the town.

PRE-CAMBRIAN ROCKS.

GENERAL RELATIONS.

The largest exposure of granites and associated rocks in the central portion of the Owl Creek Mountains occupies an area of about 100 square miles. Its form is elliptical, about 5 miles wide at its greatest breadth and extending about 25 miles from northwest to southeast. It constitutes a prominent ridge, or series of ridges, bordered by slopes and knobs of the Deadwood sandstone and overlying rocks. At a few