PEALE.] DESCRIPTIVE GEOLOGY — BEAR RIVER VALLEY. 589 



occupied by lake deposits. This valley was described by Dr. Hayden 

 in 1871, and I quote from his report : * 



About three miles above Soda Springs, on the margin of Bear River, there is a bed 

 of black slaty clay underneath the superficial deposits of drift, "which contains a seam 

 of impure coal, visible only when the water is low in autumn. The slate above the 

 coal is literally crowded with fresh-water shells, as Planorbis, &c. 



A little farther up the river, on the ojjposite side there are hills, cut by the river, 

 showing about 200 feet of gray indurated sandstones, with beds of pudding-stones 

 and light gray and whitish marly sand and clay, a very modern deposit, but attaining 

 such a thickness and giving form to the high hills bordering the river, as to be regarded 

 as worthy of attention in describing the geological features of this valley. I may state, 

 in short, that for 10 miles the valley and the foothills on either side exhibit an exten- 

 sive deposit, gradually passing ujJ into the Quaternary or drift, and over the drift is 

 here and there a crust of basalt. There are also old. spring deposits in the form of 

 rather compact tufa. 



About fifteen miles above Soda Springs the river cuts through a vast thickness of 

 thin shales, varying in thickness from one-twentieth of an inch to an inch, averaging 

 about one-eighth of an inch, thick, resembling the Green River shales on the Union 

 Pacific Railroad. They are mostly horizontal, but occasionally incline 3° to 5°. They 

 reach a thickness of 500 to 800 feet and a]ipear to pass up into variegated beds of light 

 gray and pink sands and clays in this valley, resembling those of the Wahsatch Group 

 west of Fort Bridger. 



The appearance of the large mass of shales in the valley of Bear River is not easily 

 accounted for, and they do not appear to conform to the older rocks. No fossils could 

 be found in the shales, and all I can say of them is that they api^ear to be of Modern 

 Tertiary age, and that in the scooping out of the valley they seem to have escaped 

 the general erosion. 



These beds last described are probably of Pliocene age and equivalent 

 to those of Cache Valley and the Malade Valley. What their extent is 

 on the west side of the river I am unable to say. They probably extend 

 northward from Station 115, forming the lower hills west of the river, but 

 north of Georgetown, I have no data as to their limit westward. I have 

 therefore colored them only as bordering the valley along the foot-hills of 

 the Bear Eiver Eange. 



The coaly layer noted three miles above Soda Springs is probably of 

 the same age as a bed noted in Gentile Valley, with which Planorhis^ 

 Sphcerium, Limncea, and other fresh-water forms were associated. The 

 beds in the latter case passed under the basalt, as do those near Soda 

 Springs. It is altogether probable, therefore, that the lake of Gentile 

 Valley and Basalt Valley connected by a narrow inlet with the lake 

 that filled the Bear Lake Valley and the valley of Bear Eiver, north of it. 



We can, therefore, with considerable accuracy state that the Bear 

 Eiver Valley is a basin of at least three lakes. First, the Pliocene Ter- 

 tiary Lake ; second, the lake (probably Early Quaternary) which depos- 

 ited the coaly strata ; and thirdly, the late Quaternary Lake, of which 

 a remnant still exists in Bear Lake and Bear Marsh. Whether the lake 

 was continuous from Pliocene time is difficult to say, but it probably 

 was not. At the end of the Pliocene deposition there must have been 

 orographical disturbance, as we note dips in the strata at all localities 

 where the beds have been seen. These dips at some places, as in Bear 

 Lake Valley, are only a few degrees, but in others they were as high as 

 60°. This subject will be referred to again in the resume of the forma- 

 tions. 



PREUSS RANGE. 



The main portion of the Preuss Eange was described in the preceding 

 chapter under the head of the Blackfoot Eiver, which drains the greater 

 portion of the mountains. The southern spm\s, however, form the east 

 side of the Bear La^'e Valley, and the Aspen Eidge is the boundary 



* Report U. S. Geol. Survey, 1871, 1872, pp. 154, 155. 



