CRETACEOUS UPLIFT OF BITTER CREEK. 229 



lithological character. The beds dip here about 13° to the westward, with a 

 strike a Httle north of west. The upper section, A B, at the bottom of the map, 

 which passes through this point, gives an exaggerated thickness to the beds 

 of the Laramie series, from the fact that it does not cross the strike a,t right 

 angles throughout, since at Rock Springs the Laramie beds strike nearly 

 30° east of north. To the south, these sandstone ridges disappear under 

 the horizontal Tertiary beds which form the bench-like spurs to the west 

 and south of Quaking Asp Mountain. 



This mountain is a sharp narrow ridge, made up of thinly-bedded 

 brown sandstones, which strike to the northeast and dip to the southeast. 

 They are, in general, much harder and more compact than the sandstones 

 of the Fox Hill group, which is probably due to some local metamorphism. 

 They may be considered to represent the opposite side of the fold from 

 the rocks of the ridge just mentioned above, but their direct connection 

 with these beds was masked by the Tertiary benches. In the deep ravines 

 of South Bitter Creek, however, almost continuous exposures of the loose 

 white sand-rocks of the Laramie series can be traced, which, with a dip of 

 14° to the westward and a strike to the east of north, in the lower part 

 of the valley near the railroad, gradually curve in strike to the westward 

 as one goes south, and steeper in dip, till in the narrow ravines near the 

 head of the creek, to the south of Quaking Asp Mountain, they are seen 

 to dip 35° to the south with a strike to the northwest. It may therefore be 

 supposed that the beds which form Quaking Asp Mountain bend round in 

 the same way under the Tertiaries. 



On the eastern side of the valley, the lower sand-rocks, exposed in the 

 parallel lines of bluff which enclose it, have been referred to the Fox Hill 

 group, though the dividing line between this and the Laramie series is not 

 easy to determine. Corresponding beds to those containing fragments of 

 Ammonites, on the west of the valley, were found in the bluffs, about 6 

 miles northeast of Salt Wells Station. In lithological character, the dis- 

 tinction between the Fox Hill and Laramie groups is not very marked. In 

 general, the sandstones of the former are more compact, frequently thinly 

 bedded, and showing a tendency to split up into flags, or thin slabs. They 

 are characterized by the presence of marine fossils. Ammonites, Baculites, and 



