636 KEPOET u:nited states geological suevey. 



erates, whicli contain pebbles of limestone, and which were evidently de- 

 rived from the adj acent mountains. When these beds were deposited it is 

 evident that a large part of the land had been eroded, for the Mesozoic 

 beds must have been largely removed from the mountains.* The red 

 character of the sediments, however, is due to the wearing down of the 

 red Mesozoic rocks, as the conglomerates of the Wahsatch are cemented 

 by siliceous material, the predominating color of which is red. The red 

 color, however, is more marked farther from the mountains, where the 

 sediments are finer. 



South of Thompson's Plateau we find the Wahsatch beds resting on 

 the Jurassic rocks of Meridian ridge, beyond which they do not extend'to 

 the westward. In this region we have only a narrow line of outcrop of 

 the group along the east side of the ridge, as the Grten Eiver Group 

 forms a large portion of the surface to the eastward. As the ridge de- 

 clines in elevation to the southward, the variegated Wahsatch beds reach 

 farther to the westward, resting on the Cretaceous strata which were not 

 removed from the fold. 



North of Thompson Plateau the group rests on Juarassic, Cretaceous, 

 and Laramie beds, and isolated patches rest on the latter, close to the 

 mountains. Nowhere along the western edge do we get the entire thick- 

 ness of the group. The thickness exiDosed is only about 500 to 800 feet. 

 This is due to the fact that the region of the eastern outlying fold was 

 above the level of the lake, through the earlier portion of its existence, 

 and a portion of its sediments were derived from the erosion of the 

 ridge formed by this fold. 



An arm of the lake in the later stage of its existence reached north- 

 ward, around the southern end of Absaroka Eidge in Ham's Fork Basin. 

 A second arm extended northward in the region of Bear Lake and the 

 Bear Lake Plateau. This arm was separated from the first by a long 

 narrow peninsula, which now forms the range between Bear Biver Val- 

 ley and Ham's Fork Basin. During the early stages of the lake this 

 was probably a Mesozoic Eange, but the -rocks of that age were largely 

 removed by the time the subsidence had carried it down almost to the 

 level of the lake. In the Green Eiver epoch it was probably under 

 water. The Wahsatch Group, resting on its eastern fianks, is on the 

 Carboniferous, and is not very thick. On the Bear Lake Plateau the 

 thickness is greater, especially toward the west, and on the eastern 

 flanks of the Bear Eiver Eange it is still greater ; it increases also to 

 the southward until it is sefVeral thousand feet in thickness. In the 

 Bear Eiver Plateau it rests on Palseozoic and Mesozoic rocks, which are 

 folded and eroded, and along the Bear Eiver Eange the underlj'ing 

 rocks are Silurian quartzites. At Twin Creek, near our south line east 

 of Bear Eiver Valley, it rests on Laramie sandstones; the latter dip 

 40O or 45° to the westward and the Wahsatch 5° to the eastward. The 

 unconformability is about the same along the west side of the Green 

 Eiver Basin. All the exjjosures of Wahsatch in our district are, there- 

 fore, near shore-lines of the lake. 



The line that I have drawn separating the Wahsatch Grouj) from the 

 Green Eiver is lithological. All the variegated beds that lie below the 

 laminated light-colored sandstones have been referred to the Wahsatch 

 Group and all above to the Green Eiver. 



The softness of the variegated saiuls and marls has caused them to 

 weather obscurely, and wlierever they rise on the slopes of a ridge it is 



*I liave already alluded to the probability of a portion of this erosion having been 

 during the latter part of the Laramie. 



