574 KEPORT UNITED STATES GEOLOGICAL SURVEY. 



■existing west of the gap. The river, liowever, soon turns to the west- 

 ward and cuts a deep canon in the basalt, wliich, continuing northward, 

 forms the floor of Basalt Yalley. The. caiion is about 300 feet deep on 

 the west side of the valley, and the river flows through it, broken by falls 

 and rapids. . At the western side of the basalt-field the river turns to the 

 south, making a right angle in its course, and soon flows out of the canon, 

 skirting the western edge of the basalt, into the northern end of Grentile 

 Valley. The general course of the river in Gentile Yalley is south. It 

 is broad and flows sluggishly between broad meadows that are bounded 

 by soft lacustrine deposits. Leaving this valley the Bear enters a deep 

 canon cut in Silurian limestones. This caiion, which I shall call the 

 ''middle canon of Bear Eiver," is nine miles in length and divides Gen- 

 tile Valley from the upper end of Cache Valley. On entering Cache 

 Valley, the river turns to the westward, keeping a course a little south 

 of west for nine miles. Again it turns southward, a course which it 

 keeps until it approaches Cache Butte, when it turns to the southeast, 

 flowing sluggishly with inclosed islands and broad meadows. It keeps 

 this direction for only three or four miles, once more resuming its south- 

 ern course. About two miles above the mo^th of Logan Eiver it turns 

 to the northwest, and about five miles below the mouth of the latter 

 stream it enters the "gates," from which it flows out into the lower end 

 of the Malade Valley, and leaves our district in longitude 112° 8', to 

 flow southward to Bear Eiver Bay of the Great Salt Lake. The eccen- 

 tric course of Bear Eiver will be better appreciated by a glance at the 

 map. The river is seen to have two general courses, one northern and 

 the other southern. The most northern point reached is at Soda Springs. 

 The Bear Eiver Eange separates the two portions of Bear Eiver as thus 

 described. I shall now take up the diflerent portions of the Bear, con- 

 sidering the general geological structure of its various valleys and the 

 geology of the bounding ranges. 



UPPER BEAR RIVER VALLEY. 



The Upper Bear Eiver Valley, as I liave already defined it, extends 

 from the mouth of Smith's Fork, southward, to the limits of our dis- 

 trict. Beyond this it reaches nearly 30 miles to the southward. Its 

 length in our district is 25 miles in an air-line direction. It is from three 

 to four miles in width, and of rather uniform surface, covered with 

 abundant good grass. The general elevation is 6,250 feet. The Bear 

 flows through this broad bottom in curves, with an extremely sluggish 

 current, over a soft, sandy, and muddy bed, which is so treacherous as 

 to afford few good fords, although the river is neither very wide nor 

 deep. Alluvial deposits and local drift from, the bounding hills conceal 

 the strata underlying the valley. The two principal streams joining the 

 Bear from the west side in this upper valley are Twin Creek and Sub- 

 lette's Creek. 



Ticin Greek. — Twin Creek joins the Bear a few miles above our south 

 line. The upperportion has been considered under the head of "the Ham's 

 Fork Plateau." Leaving the upper vaUey, which is cut in the varie- 

 gated beds of the Wahsatch Group, Twin Creek enters a caiion cutting 

 across a high ridge of Carboniferous and Jurassic rocks. Soon after 

 entering the caiion it is joined by its northern branch, Eock Creek, 

 which is the x>rincipal stream. Eock Creek occupies a narrow valley at 

 the west side of the ridge that forms the west boundary of the Ham's Fork 

 Plateau and Basin. This ridge is a sharp antichnal of Carboniferous 

 limestone, which continues northward west of Smith's Fork, where it 



