44 EEPORT UNITED STATES GEOLOGICAL SUEVET. 



From various points along the crest of the range the regular arrange- 

 ment of these ridges can be seen. Extending northward for distances 

 varying from 3 to 12 miles, they usually present an unbroken slope. If it 

 is broken, they are terraced. At varytag distances from the river these 

 ridges terminate more or less abruptly and the vaUey proper begins. 

 This is level, and changes in width in accordance with the strata or 

 rocks adjoining either side. 



The Sweetwater Hills and their eastern extension, the Seminole Group, 

 have a trend of nearly east and west, forming the last range from the 

 Sweetwater southward to the Union Pacific EaHroad. On the first- 

 named the highest point reaches an elevation of 9,200 feet above sea- 

 level, on the latter an altitude of 9,900 feet. 



Inasmuch as two names have been given to the same range, and have 

 beeai repeatedly pubhshed on maps and in reports, they will be retained 

 in the subjoined report. It will there be seen that no structural differ- 

 ence justifies the separation, as, too, it is certainly not warranted by 

 orographic or topographical features. 



SWEETWATER PLATEAU. 



Looking from Sheep Mountain towards the south and southeast, we 

 observe a long line, almost unbroken, of steep white cliffs, resembling 

 somewhat the southern face of the Book-Cliffs of Colorado. The edge of 

 these cliffs does not run in a straight line, but shows numerous deep 

 indentations and projecting sharp points. The general trend is a little 

 north of east. Towards the north the slopes presented are very steep, 

 sometimes precipitous. Erosion has cut narrow gulches, timbered with 

 quakin gasp and spruce into the soft strata. Without many horizontal 

 breaks, these faces sweep down in a straight or slightly curved line to the 

 lower country, northward. If we ascend the sharply-defined edge, we find 

 that it maintains an almost uniform absolute elevation, averaging about 

 6,500 feet. To the southward we will see a gradual gentle slope toward 

 Sweetwater Eiver. Eetatning this character throughout its entire ex- 

 tent, I regard the designation "Sweetwater Plateau" an appropriate 

 one for this region. The distance of its edge from the river varies some- 

 what, dependent upon curves made by either. At some points along 

 the plateau-rim, we wiU find hiUs projecting above the usual level. In 

 this case the difference of relative elevation is either slight, or the hills 

 are formed by some older geological formation than that exhibited on 

 the faces of the cliffs. Descending along one of the north to south ridges, 

 we find that they are cut off sharply upon reaching the vaUey of the 

 Sweetwater. This plateaiu, as weU as the ridges leading from the Sweet- 

 water Hills to the river, owe their formation to a tertiary deposit of con- 

 siderable thickness. Speaking from the standpoint of a geologist only, 

 the two together might aptly be designated as a " basin," but the con- 

 figuration of the surface is such as to render this term inapplicable. In 

 one sense of the word a " basin" is formed by the gradual slopes towards 

 the river, which are exhibited by both sides. Their relations, however, 

 firstly to the Sweetwater Hills, and secondly to the low country north- 

 ward, forbid their classification as such. The area occupied by this 

 plateau extends from a point nearly due south of Sheep Mountain east- 

 ward to the terminus of our district. On some maps the plateau-edge 

 may be found indicated by notched lines coimectiag the region of Stam- 

 baugh, through isolated hills, with the western end of the Eattlesnake 

 Mountains. 



