84 KEPOET UNITED STATES GEOLOGICAL SUKVEY. 



By supplementing Section I with Section II a view of the entire series 

 of conformable sedimentary strata may be obtained. 



CKETACEOUS. 



In discussing the Cretaceous groups of this region, I shall make use 

 of the same nomenclature that has generally been applied to the same 

 series in Colorado. The original five numbers into which this forma- 

 tion was separated are reduced to three. No. 1 remains intact and is 

 designated as the Dakota Group ; Nos. 2 and 3 are comprised under the 

 name of Colorado Group, and Nos. 4 and 5 form the Fox Hills Group. 

 This division is one which can readfly be carried out in the field, and it 

 expresses the most natural classification of the entire group at many 

 localities. Cretaceous strata occupy a prominent area on the Wind Elver 

 drainage. Thoroughly regular in their arrangement, except where they 

 take part in the disturbances which have so seriously affected older beds, 

 they reach far eastward into the lower country. Showing a smaller vertical 

 development than farther to the south, the individuality of the groups 

 is nevertheless preserved. Not only is it apparent in the successive ar- 

 rangement of strata, but many of the detail features, such as produced 

 by weathering, are strikingly similar. A deplorable want of good fossils 

 is felt throughout the entire formation. Fortunately this interferes but 

 little with the identification of the groups, as they are sufficiently charac- 

 teristic to be easdy recognized. 



Daltota Group. 



Eesting directly upon the upper light shales and marls of the Jurassic 

 series we find a succession of yellow and brown shales, interstratified 

 with sandstones of the same color. In these shales, above some of the thin 

 beds of sandstone, we find slight indications of coal. The seams are but 

 halt an inch in thickness, and the coal is of that variety called jet coal. 

 Higher up the sandstones predominate, separated by thin layers of ho- 

 mogeneous, dark shales. Near the top of the group we once more find 

 a heavy bed of shale, which is covered by massive white, yellow, and 

 brown sandstones. A small thickness of arenaceous shales closes the 

 group. This is the general section of the Dakota as exposed west of 

 the anticlinal axis. In some of the upper sandstones indistinct remains 

 of plants were observed, and in the higher shales occurs a Gryphcea. For 

 the total series we can accept a thickness of about 400 feet. 



In Colorado the Dakota Group usually occupies a prominent position, 

 but here this is not the case. Forming a low, long-continued ridge, the 

 members of this series trend about north and south on the plateau-like 

 elevation between Sheep Mountain and the base of the third chain. To 

 the southward the group pinches out horizontally, owing to the change 

 in the direction of the anticlinal axis. Following it north, we see it in its 

 normal position accompanying the Jurassic beds. Within the areas of 

 vertical distur bance the Dakota, overlaid by Colorado shales, takes part 

 in all the flexures and plications. In consequence of this fact, the horizon- 

 tal projection of the series assumes a curious shape. Between the an- 

 ticlinal axis and the outlying lulls we find two diverging lines of expo- 

 sure, trending northward. After crossing the little Popo-Agie, the one 

 runs upward toward the hills ; the other takes part in the westerly dip of 

 strata near the mouth of Twin Creek and subsequently joins the first. 

 Another line of the outcrop occurs parallel to the anticlinal axis, and 

 east of it. From there the Dakota beds dip under the younger groups 

 and are lost, untfl once more appearing at the terminus of the anticlinal 

 fold near Sweetwater Plateau. A glance at the two sections above 



