110 REPORT UNITED STATES GEOLOGICAL SUEVEY. 



CEISTOZOIC FOEMATIONS. 



TERTIARY. 



On the southern drainage of the Sweetwater we find a number of 

 Tertiary groups. They shall not be considered in this chapter, however, 

 as they form but a portion of a very extended series. To this will be 

 devoted the greater part of the fourth chapter. For tliis reason I shall 

 not attempt a discussion of any of these groups here, as it would neces- 

 sarily be but fragmentary. Two groups, however, occupy very extensive 

 areas in this region — the Sweetwater Group and some younger beds — 

 which will be fully treated of. Although interesting to the palaeontolo- 

 gist, and in some of its features to the geologist, this series nevertheless 

 is a great aggravation. Its great horizontal extent obscures many re- 

 gions of the utmost interest as regards stratigraphy, consequently a large 

 portion of the structure of the country adjoining the Sweetwater can be 

 established merely by insufficient data. Fortunately, the Tertiary beds 

 have been removed at some points, and these afford information without 

 which the correlations of older formations would remain very obscure. 



SWEETWATER GROUP. 



A very striking aspect is presented in the vicinity of the Sweetwater by 

 the enormous deposits of clays, sands, marls, and conglomerates. Fremont 

 in j>assing through that region noticed them and comments upon them. 

 Dr. Hayden visited this region in 1870, and describes the Sweetwater 

 Group as "brown, indurated sands"; * * * " among them are impure 

 seams of lignite or carbonaceous clay, with layers of coarse sandstone or 

 an aggregate of particles of quartz." Evidently that section of country 

 running parallel with the river was at one time the object of very exten- 

 sive erosion. Into the basin formed thereby a series of Tertiary beds 

 was deposited. Subsequent removal along nearly the same hue afforded 

 an opportunity for still younger beds to be deposited. Finally, or rather 

 during the latter process of sedimentation, large masses of irregular drift 

 were scattered all along the borders of this second lake. Near the Wind 

 Eiver Mountains this is of such a character that it can scarcely have 

 been dex>osited in any other manner than by the action of moving ice. 



On the northern edge of the Sweetwater Plateau we have occasion to 

 observe the best outcrops of the Sweetwater Group. Overlying the 

 beds which I have described as Wasatch we find an extensive series of 

 brown, yellow, and white arenaceous marls and clays. Some sandstones, 

 though not showing clearly defined stratification, complete the series. 

 Higher up, toward the summit of the plateau edge, the sands and some 

 conglomerates of yellow color become prevalent. On this side the 

 loosely cemented material is very easily eroded, and forms steep, deeply 

 furrowed slopes. All along the northern edge of the plateau the same 

 beds can be traced, dipping very gently toward the east. This inclina- 

 tion is so small that it is not perceptible unless a large face be exposed. 

 We find on the same line the contact of the Sweetwater Group with 

 the older sedimentary groups, and observe the two to be unconformable. 

 Beaching the Wasatch, which is the youngest Tertiary deposit I recog- 

 nized on the Wind Eiver drainage, we find that this shows a steady dip 

 of about 2° to the northeastward. Noticing other formations, older, we find 

 that this dip is due to the same influences which caused the elevation of 

 the Wind Eiver Eange and the subsequent formation of the Sheep Mount- 

 ain anticlinal. The angle of unconformability between the Wasatch 



