112 EEPORT UNITED STATES GEOLOGICAL SUEVEY. 



in tlie isolated ridges. At tliat time there was a fresh-water lake which occupied the 

 entire valley, mnch as Salt Lake once occupied the great basin, concealing most of the 

 granite ridges, while others rose above the water like islands. Then was deiiosited 

 what might be called the Sweetwater Group, or ijerhaps a series of beds identical 

 with the upper portion of the Wind River deposits. These were scooi^ed out again in 

 time, and the Pliocene marls and sands were deposited ; and then again there was 

 another scooping out of the valley, and iinally a covering the hills with drift." 



As regards the geological position of this local deposit, there may 

 appear to be some doubt. From what I have seen of it, I consider it 

 younger than the Green River Group. This agrees with the Aiews held 

 by Dr. Hayden, Cope, and Comstock. At this place no more shall be 

 said on the subject, because the relative positions of Tertiary groups will 

 be more fully discussed in Chapter IV. 



Pliocene. 



Filling the excavation produced by the second period of erosion of the 

 Sweetwater beds we find an extensive series of Pliocene deposits. Their 

 greatest development was observed near the eastern termination of our 

 district, decreasing as we proceed to the westward. Near the base of 

 this Pliocene deposit we find a very loosely aggregated sandstone, almost 

 partaking of the character of a conglomerate. It is light gray and yel- 

 lowish, easily decomposing. Locally a few thin strata of yellow or gray 

 more compact sandstone may be observed. Above this follows a succes- 

 sion of light marls and indurated clays. Usually these are either very 

 light yellow or white, but j^ink and greenish beds are not wanting. 

 Toward the eastern termination of the group the strata become highly 

 siliceous. Thoroughly j)ermeated by silica, the clays become very hard 

 and brittle. The former occurs also in the shape of narrow veins, con- 

 cretions, and even strata. Chalcedony and flint are the two main vari- 

 eties. Near the northern edge of the Sweetwater Plateau we perceive 

 isolated, table-shaped bluffs rising above the general level. They are 

 of dazzling white color, and are composed of soft., partly arenaceous 

 marls. From this edge very even ridges slope gently down toward the 

 Sweetwater. On the south side the same feature may be observed. 

 Starting from the Sweetwater and Seminole Hills the ridges trend north 

 and south, are very regular in form, and are composed of very nearly 

 horizontal strata. Between Sandy Creek and the Sweetwater we find 

 the Pliocene Group cut into sharp, prominent forms, due in part to the 

 large amount of silica it contains. This produces angular forms upon 

 weathering, while the soft marls and sands show gently -rounded slopes. 

 Near the northern slope of the hills extensive dei)Osits of loose drift 

 cover the strata, totally obscuring all junction-lines. 



For a considerable distance the Pliocene beds form the northern rim 

 of the plateau, until they are crowded from this position by the remain- 

 ing portions of the Sweetwater Group toward the west. Along a north 

 and soutli line drawn about four or five miles east of Saint Mary's Sta- 

 tion we may regard their western limits as being located. Within the 

 granitic area south of the river we find them directly superimposed in 

 snow-white masses lipon the Carboniferous Umestones. From there 

 eastward they are confined within the Sweetwater Valley. Their hori- 

 zontal distribution is entirely dependent upon the area of erosion \^ithin 

 the Sweetwater Group. Along the slope of the plateau the Pliocene 

 strata x^articiijate in the slight southerly dip which has been recorded as 

 existing in the SAveetwater Group. It amounts from 1° to 4P. 



In some of the upper beds of this series Dr. Hayden has found some 

 very iuteresting mammalian remains, which place them on a parallel 



