UINTA UPLIFT. 201 



thinly-bedded sandstones, with some clay beds, having at its base the per- 

 sistent conglomerate carrying small pebbles of black chert. The Colorado 

 group, of about 2,000 feet in thickness, is made up mostly of clays and yellow 

 marls, with some sandstones at the base, which enclose one prominent coal 

 seam; the outcrops of this group are generally occupied by valleys. The 

 Fox Hill group consists of about 8,000 feet of heavily-bedded white sand- 

 stones, with a few coal seams and comparatively little clay. The Laramie 

 group, whose actual thickness cannot be definitely estimated, since it is 

 directly overlaid by an unconformable series of beds, consists also of gray 

 and white sandstones, often iron-stained, containing a greater development of 

 clay beds, and very rich in coal seams. In the fauna of this formation, brack- 

 ish and, locally, even fresh-water forms, are found associated with marine 

 types. 



While throughout the greater portion of the range the folding, which 

 has raised its crest, has been mainly produced by forces resulting from the 

 contraction of the earth's crust acting on meridional lines, or at right angles 

 to its longer axis, there has also been some, though considerably less, lon- 

 gitudinal compression, or contraction of the beds involved in the folding, in 

 an east and west direction, producing minor, transverse corrugations, whose 

 position and outline are indicated by the present curves and reentering 

 angles in the ridges, formed by the outcrops of these flanking formations. 



In the eastern portion of the range, the longitudinal compression has 

 been proportionately greater, and this, combined with the presence of an 

 underlying ridge of unconformable Archaean beds, whose highest summit 

 has been exposed by denudation around Red Creek, has produced a highly- 

 complicated structure. The crest of the main fold is much widened, and 

 bent somewhat to the southward. Along its northern angle, lines of dis- 

 placement were developed, whose movement has continued even into Ter- 

 tiary times. There was probably also some fracturing in the crest of the 

 fold, which determined the position of the great central valley of Brown's 

 Park. To the south of the main fold, the region of Yampa Plateau was 

 uplifted bodily, developing' a double anticlinal at its western extremity, 

 which merges into a single fold at the east end, while the sharp, rectangular 

 folds on either flank were accompanied by a certain amount of dislocation 



