88 REPORT UNITED STATES GEOLOGICAL SURVEY. 



POST-CRETACEOUS. 



Laramie Group. 



Along Beaver Creek we find a series of beds superimposed upon the 

 Fox Hills group, which I refer to the Post-Cretaceous period. We have 

 in adjacent regions a considerable development of the Lignitic strata, 

 but here it is limited. It is not my purpose here to enter into any dis- 

 cussion as to the proper position of this group in the geological succes- 

 sion of formations. In a paper treating of the coal-beds near Evanston 

 I shall shortly have occasion so to do. Overlying the upper sandstones 

 of the Fox Hills we see a succession of shales and yellow sandstones 

 forming low, long-continued bluffs. Our work did not carry us far 

 enough to make any careful examinations of this group, but sufficient 

 information was obtained to enable us to recognize the formation. Indi- 

 cations of coal were found at a number of points, but the large amount 

 of debris covering the outcrops prevented any determination as to 

 whether they were workable or not. So far as could be seen, the thick- 

 ness of the group amounts to about 400 feet. Its horizontal distribution 

 is determined, essentially, by the amount of erosion and transportation 

 of material to which the overlying beds have been subjected. We find 

 the beds exposed along a bine trending about north 45° west, following 

 the general strike of the strata. The dips, at this distance from the cen- 

 tres of disturbance, is almost obliterated, amounting to but a few 

 degrees. A series of heavy, yellow sandstones close the Lignitic Group. 

 Neither in topographical features nor in geognostic importance does this 

 series occupy a very prominent position. From observations made else- 

 where, we find that the group develops to a much greater extent farther 

 south and southwest. It passes under the Tertiary beds and reappears 

 in the southern portion of our district, occupying conspicuous areas. 



I have placed the group under the head of " Post-Cretaceous " in con- 

 formity with the views expressed in the annual report of 1875. In this 

 report I shall continue to do so, expecting soon to present a paper which 

 defines the reasons for taking this position. Literature upon the subject 

 is accumulating, but as yet it is impossible to say which opinions may 

 ultimately be adopted. 



TERTIARY. 



During the past twenty years an almost endless number of Tertiary 

 groups and subgroups have received names and have been placed into 

 relatively different positions by various authors. We have, within the 

 region to which this chapter is devoted, but one of the Tertiary groups, 

 if we exclude the northern edge of the Sweetwater Plateau. This latter 

 will be considered in the chapter next following, and mention of its geo- 

 logical character will here be omitted. 



Wasatch Group. 



Besting upon the yellow sandstones of the upper portion of the Lara- 

 mie Group, we find a series of variegated, arenaceous marls. They are 

 nearly horizontally stratified, and are carved into typical " bad-lands " 

 by fluviatile erosion. A variety of colors presents itself in these marls. 

 Gray and reddish-brown predominate, interchanging, however, with yel- 

 low, white, greenish, and maroon. Without any apparent separation of 

 strata, these colors and shades form bands resembling well-defined bed- 

 ding. Eapidly denuded by erosion, the slopes presented by these marls 

 are generally entirely bare of vegetation. Thin bands of highly argil- 



