POST-CRETACEOUS HISTORY OF WESTERN WYOMING 109 



The banded clays, usually of pink and gray colors, which 

 constitute the bulk of the Tertiary beds along the head of the Gros 

 Ventre River, throughout the Wind River basin, and in the Bighorn 

 basin, suggest a different mode of origin. Sinclair,^ from a care- 

 ful study of these clays and their associated sandy beds in the Big- 

 horn basin, reaches the conclusion that they are river floodplain 

 deposits laid down under a moderately dry climate. The color 

 banding he ascribes to alternating cycles of moist and dry climate 

 (pp. 1 1 6-1 7) . So regular is the repetition of their red and gray layers 

 that an oscillating control is strongly indicated. Loomis^ also 

 reached the conclusion that the clays and silts of the Wasatch 

 formation in the Bighorn basin were floodplain deposits. In this 

 connection he called attention to the fact that, of the very large 

 number of fossils which had been found in the beds, only 10 per cent 

 are aquatic, whereas the most abundant forms are horses and other 

 animals which frequent the relatively dry plains. Even the aquatic 

 animals were turtles, crocodiles, and fishes, all of which inhabit rivers. 



Such observations as I have made tend to confirm this hypothe- 

 sis of floodplain deposition, under subarid or steppe conditions. 

 The more sandy layers consist of small angular fragments of quartz, 

 prisms and slivers of hornblende, hexagonal plates of mica, and 

 sharp bits of feldspar. No volcanic ash was observed in the typical 

 striped Wind River beds. The undecayed condition of the grains 

 precludes a warm moist climate, and the prevalent reddish or pink- 

 ish color is evidence of thorough oxidation, which would have been 

 inhibited in a cold moist climate. The general lack of abrasion 

 suggests that the particles have not been transported very far, or 

 at least were not worked over continuously for a long space of time. 

 To Loomis and Sinclair's hypothesis I may add the suggestion that 

 the Eocene sediments probably originated as regolith due to rock 

 disintegration and eolian abrasion in a dry climate. Much of the 

 angular quartz and feldspar dust may well be the product of sand- 

 blast action upon rock outcrops. The regolith, once formed, was 



' W. J. Sinclair and W. Granger, "Eocene and Oligocene of the Wind River and 

 Bighorn Basins," Am. Mus. Nat. Hist. Bull., XXX (1911), 83-117. 



^ F. B. Loomis, "Origin of the Wasatch Deposits," Am. Jour. ScL, XXIII 

 (1907), 362. 



