KNDUCH.] EAST OF WIND EIVER EANGE EROSION. 89 



laceous sandstones, occurring sporadically witbiu the series, sometimes 

 give rise to the formation of low, regular bluffs. About 450 to 500 feet 

 may be regarded as the thickness of these marls southeast of Beaver 

 Creek, where we find them. The outlines of horizontal distribution 

 shoAvn by them are irregidar, owing to the varied forms produced by 

 erosion. In a general way, the western edge runs m a direction north 

 and south, while the eastern passes beyond the limits of our district. 

 From analogy of geological position and lithological character (no fossils 

 having been found), 1 regard this series as parallel to Puerco marls of 

 New Mexico and Colorado. They resemble them in every essential fea- 

 ture. 



Above the marls we observe a succession of yellow sandstones and 

 shales, which also belong to the Wasatch. They are cut short in their 

 vertical development by the overlying beds of the Sweetwater Group. 

 From certain featiu-es, subsequently to be discussed, it appears that a 

 considerable amount of erosion must have taken place before the Sweet- 

 water strata were deposited. This accounts for the imperfect vertical 

 development of the Wasatch grouj) at the point from which it has been 

 described. 



With the exception of a small portion of late Tertiary beds lying 

 between the subsidiary range and the western base of the Wind Eiver 

 Mountains, we have no additional sedimentary beds to sijeak of in this 

 chapter. The occurrence just mentioned will be re\'iewed, together with 

 other Tertiary groups, in the fourth chapter. 



POST-TEKTIARY EROSION. 



After the deposition of Tertiary sediment a period of great activity of 

 a peculiar character existed Avdthin the area east of the Wind Eiver 

 Kange. At that time, essentially, was the surface moulded into its 

 present shape. Omitting the existence of most recent Tertiary groups, 

 we can picture to ourselves the ajjpearance of the country after the 

 upheaval of the mountain range. The two main elevations were the 

 range itself and the rise determined by the course of the anticlinal axis 

 east of it. Erosion had, even before the advent of the latest tertiary 

 waters, modified the surface very considerably. Upon the arrival of the 

 latter, they found basins ready to receive the sediment they carried with 

 them or derived from neighboring regions. In this manner an equaliza- 

 tion of elevations and depressions took place, which greatly affected 

 certain portions of the region. Subsequent to the deposition of large 

 masses of Tertiary sediment a period of maximum erosion followed, which 

 left the surface of the country api)roximately in the condition we now 

 find it. Within the area to which this chapter is devoted we have occa- 

 sion to observe the enormous effects produced by erosion. Final eleva- 

 tions along certain lines, or depressions on others, may, to-day, tend to 

 magnify the results, but enough can be seen to obtain a general idea of 

 its magnitude. Parallel to the strike of the sedimentary strata they 

 have suffered severely from this primary erosion. Along the lines of 

 disturbed portions, where the continuity of strata was probably broken 

 and the rocks more or less shattered, the erosive forces have exerted a 

 powerful influence. Nearly all of the material belonging to formations 

 younger than the Carboniferous are composed of mechanically deposited 

 sediment. This necessarily coidd offer but comparatively slight resist- 

 ance to flowing water, and enormous masses of it were carried away to 

 build up most recent groups at some other locality. Along the junction 

 line of Carboniferous and Triassic beds this feature of erosion, denuda- 



