t)40 REPORT UNITED STATES GEOLOGICAL SURVEY. 



The isolated character of the beds in our district does not allow us to 

 predicate anything in regard to them, and I therefore leave them with 

 this brief reference to their outcrops. 



No fossils were collected. 



PLIOCENE DEPOSITS. 



At the end of the early Tertiary period there appears to have been an 

 elevation which lifted the lake that had continued from Wahsateh time, 

 so that it was completely drained. Accompanying this elevation there 

 may have been a depression to the westward. Whatever the conditions 

 may have been, it is certain that a large area in the western portion of our 

 district was occupied by a lake that did not exist in Bridger time. This 

 lake appears to have occupied depressed areas that were probably being 

 eroded during the earlier Tertiary time. In Bear River Valley, below 

 Bear Lake, as described in Chapter V, there is a series of conglomerates 

 and sandstones, with calcareous layers, which are mainly horizontal in 

 position, resting on Mesozoic and Carboniferous rocks. Near the middle 

 canon of the Bear the Pliocene beds consist of greenish and white shales 

 and sands. They incline about 60°, and rest on Silurian limestones and 

 quartzites, which dip almost in the same direction and at about the same 

 angle as though they had been uplifted at the same time. 



In various portions of Cache Valley, especially on the west and north 

 sides, the beds are seen inclining toward the valley, where they are ob- 

 scured by the Quaternary lacustrine deposits. Wherever they are seen 

 in this region they are disturbed, showing that at the end of the Pliocene 

 there was considerable disturbance. 



I have retained the name of Salt Lake Group, given by Dr. Hayden 

 for these beds. The southern end of Cache Valley is colored on the map 

 published by the Fortieth Parallel Survey as occupied by the Humboldt 

 Group. In our district the central portions are tilled with later beds, to 

 which, for the present, I give the name Cache Valley Group, separating 

 from it the Pliocene Salt Lake Group, which corresponds to King's 

 Humboldt Group. On Franklin Butte there are fragments of lime- 

 stones that are probably Pliocene, but the rock could not be seen in 

 position. Fossils were obtained from the rocks at several localities. 

 The following are the genera recognized : 



Limncea sp. ? 

 Planorlris sp. i 

 Sphcerium sp. $ 

 Valvata sp. % 

 Bythinella sp. ? 



TERTIARY OROGRAPHY. 



In taking up this subject we find that the facts observed in our district 

 are very meagre. During the deposition of the sediments of the Wah- 

 satch Group, the evidence indicates a continued subsidence, which may 

 also have continued during the deposition of the two succeeding groups. 



Along the edge of the Wyoming Mountains the strata of the Wah- 

 satch Group dip toward the Green River Basin beneath the Green River 

 Group, and as far as seen conformably, the angle of inclination being 

 from 4° to 10°. Along the Wind River Mountains they are also seen 

 to rise, so that they have been affected by an elevation which appears 

 to be of later age than that of the deposition of the Green River Group. 



