164 REPORT UNITED STATES GEOLOGICAL SURVEY. 



east side of the valley. In this valley, as in all the plains region round 

 about, the exposures of strata, are not only few, but none of them arc 

 extensive. The most southerly exposure is about ten miles from the 

 mouth of the creek, and here I again recognized the oyster horizon which 

 has been mentioned twice before. The speeies mentioned before were 

 found abundantly here, and many other molluscan speeies besides, in 

 associated layers. I traced this fossiliferous horizon northward for a 

 distance of five or six miles above the point where I first discovered it, 

 and found it to occupy nearly a uniform height above the level of the 

 creek. The exposures are in the face of the low sloping - hills that border 

 the east side of the valley, and are distant from the creek only from a 

 few hundred feet to half a. mile. 



The full section of the strata constituting the valley side here was quite 

 clearly ascertained, although the debris which prevails upon the plains 

 has so obscured them in most places, even on the slope, that they were 

 not all observable at any one point. The following is a record of the sec- 

 tion as ascertained by measurements at several different points within the 

 few miles that they were found exposed, as before stated : 



Crow Creelc section. 



Feet. 



1. Sandy soil or debris of the plains . 10 



2. Grayish siliceous marl 5 



3. Sandy and calcareous layers ; with CorMcula, &c 3 



4. Soft, sandy, and argillaceous material ; with Ostrea and Anomia 5 



5. Arenaceous rock, somewhat concretionary; with numerous fresh-water forms. 2 



6. Arenaceous marly strata 20 



7. Carbonaceous shale 6 



8. Gray siliceous marl 6 



9. Carbonaceous shale * . 3 



10. Gray siliceous marl 25 



11. Unexposed to the surface of the creek 5 



No. 1 is the prevailing debris of the plains, which at top constitutes 

 the sandy soil. 



No fossils were found in No. 2, but it is evidently a part of a continuous 

 deposit with those beneath. 



No. 3 is remarkably prolific in fossils, especially the genus Gorbicirta, 

 of which there are no less than six or seven distinct species. In this 

 member of the section I also found the majority of the examples of Me- 

 lania wyomingensis Meek, although all its associates are regarded as 

 brackish- water forms. This member of the section is variable, being in 

 some places soft, sanely, and argillaceous, while in others it is mainly 

 composed of harder sandy or argillaceous rock. 



No. 4 constitutes one of the most if not the most persistent fossil-hori- 

 zons in this neighborhood. It is especially characterized by an abund- 

 ance of the Ostrea and Anomia which has been before mentioned as 

 characterizing a definite limited horizon in the neighborhood of May- 

 nard's ranch and elsewhere. 



No. 5 is a local development of irregular and somewhat concretionary 

 layers, the rock being siliceous, and also somewhat argillaceous and 

 calcareous. The masses in this layer, that are referred to as concretion- 

 ary, are comparatively large, and are abundantly charged with fossils, 

 while the intervening portions of the layer are less fossiliferous. This 

 member of the section was recognized at only one point, and that at the 

 southern end of the series of exposures in the valley of Crow Creek, now 

 under discussion. Unlike the other fossiliferous layers wmich compose 

 this section, all of which contain brackish-water forms, No. 5 contains 

 only those which are properly regarded as of purely fresh- water origin, 



