TERTIARY SYSTEM _ 463 



from the lower sandstone of the upper division, 2 miles south of Glen 

 Eock, comprised the following species identified by Mr Stanton : Anomia 

 i-p. Avicula linguiformis, A. nehrascana, Modiola meeki, Lunatia occi- 

 dentalis, Haminea occidentalis, CalUsta deiueyi, and ScapJiitcs modosus, 

 all upper Montana forms. 



Tertiary System 



No detailed examination was made of the Tertiary deposits, but the 

 limits of the larger areas were determined, as shown on the maps. Tlie 

 Wind Eiver basin and broad divide north of Rattlesnake mountains con- 

 tains a very broad belt of sands, sandstones, and clays, belonging mainly 

 to the Wasatch formation (see plate 30). It extends eastward across the 

 Powder River-North Platte divide, and thence southeastward in the high 

 plateau which ends a short distance west of Alcova. To the south, in the 

 Sweetwater plateau, it is overlain by the Sweetwater formation of the 

 Hayden Survey. The latter formation strongly suggests the White River, 

 into which it appears to merge in the high plateau west of Alcova. In 

 the upper part of Wind River valley the Wa'satch is overlapped by a thick 

 series of later Tertiary volcanic rocks with intercalated shales and sand- 

 stones, which are part of the great area extending through Yellowstone 

 park. The wide area of Tertiary constituting the high plateau extending 

 south from Bates creek and into the valley of Little Medicine creek is 

 mainly White River. Tt consists of light colored sandy clays, of whicli 

 the basal member is the Chadron formation, or Titanotherium beds. The 

 latter and the overlying Brule clay also occupy areas west, south, and 

 southeast of Douglas and extend far up some of the mountain valleys 

 south. They are revealed again in the valleys of Crow, Lodgepole, and 

 Horse creek nearly to longitude 105° and down Chugwater creek to a 

 point 8 miles south of AYheatland. They pass beneath the Arikaree for- 

 mation, which consists mostly of sands and soft sandstones with Imsa] 

 conglomerate. This formation thickens rapidly to the east and north and 

 extends far up the Laramie Mountain slopes on the divides and into some 

 of the higher valleys. It is on this formation that the Union Pacific rail- 

 road grade rises nearly to the eastern crest of the Laramie mountains. 

 An area of sandy clay resembling Wasatch in appearance occupies a basin 

 on Dutton-Cooper creeks. Small outlying areas of imknown age lie on 

 the mountain slopes south of Centennial and west of Red mountain. 



