310 KEMP AND KXIGHT — LEUCITE HILLS OF AVYOMIXG 



strata that is approximately 40 miles long and 30 miles wide. Bitter 

 creek in its westward course crosses the greater axis nearly at right 

 angles and has carved a broad valley which cuts this elevated area into 

 two nearly equal parts. Along this valley the Union Pacific railroad has 

 constructed its line. This dome has been greatly reduced by erosion, 

 for from its crest there has been removed not less than 5,000 feet of 

 Laramie, and practically an equal thickness of Cretaceous rocks below 

 the Laramie. The pre-Laramie formations have not been satisfactoril}^ 

 determined, and occupy a tongue-like mass 12 or 14 miles in width along 

 the Union Pacific railroad and extend from Salt Wells a distance of about 

 8 miles to the northward. The Laramie formation north of the Union 

 Pacific railroad occupies a semicircular area and is made up of a series 

 of concentric hogbacks. On the western side of the dome this formation 

 has a width of 6 or 7 miles, and Rock Springs is nearl}^ in the center 

 froni east to west. From this point it extends to the northward and 

 gradually widens, and where it swings to the eastward it is not less than 

 14 miles from north to south, the northern limit being at the base of 

 steamboat mesa. It extends considerably farther to the eastward, but 

 eventually swings to the southward and crosses the Union Pacific rail- 

 road at Black buttes, where the formation is about 8 miles in width. 

 About the exterior border of the Laramie there is a slight development 

 of Wasatch Tertiary. This is well exposed 3 miles west of Rock Springs 

 and 4 miles east of Black buttes. To the northward these beds have 

 been covered with sand and soil and are only occasionally seen. Above 

 the Wasatch there are extensive developments of Green River shales, 

 which along the western flank of the dome rise in precipitous blufi's, 

 varying from 400 to 900 feet high. The Green River formation has been 

 traced several miles east of Steamboat ; but southward from the bluffs 

 east of Steamboat it has been removed by erosion and is not seen again 

 until one approaches the railroad. To the northward of the Leucite 

 hills there are isolated areas of Bridger resting on the shales. 



Owing to the fact that the Green River shales along the western flank 

 of this dome dip to the westward at an angle of from 4 to 6 degrees, it is 

 evident that the elevation of the dome must have been post-Green River 

 and possibly later. Since its uplift, sufficient time has elapsed to remove 

 all of the Tertiary rocks from the greater portion of the U])lift, and to 

 have cut deeply into the Cretaceous below. 



Resting on the northern half of this dome or on the Tertiary rocks 

 covering it are the Leucite hills, all of which, with the exception of Pilot 

 mesa, are supported by the Laramie or are very near the edge of its 

 eroded surface. 



The elevation of the region varies from 6,300 to nearly 8,000 feet. 



