PF.ALE.] DESCRIPTIVE GEOLOGY — GREEN RIVER JSASIN. Odd 



OYSTER RIDGE. 



In Vol. II, Descriptive Geology, Exploration of the Fortieth Parallel, 

 Mr. S. F. Emmons describes a Cretaceous fold, forming what he has 

 named Oyster Eidge, from the abundance of fossil remains of Ostrea 

 found upon it. The culminating point of this ridge, he says, is Ham's 

 Hill. This point lies within the district examined by us, and was our 

 Station No. 33. What was a simple fold in the district examined by 

 Mr. Emmons has, in the region about Ham's Hill, become complicated 

 by a fault and a secondary fold. On the west side of this northern end 

 of the ridge, Ham's Fork flows southward in a monoclinal valley in 

 sandstones that are of Fox Hills Cretaceous age. On reaching the south 

 line of our district it turns abruptly to the eastward, and cuts a canon 

 through the ridge at right angles to the strike of the rocks composing 

 it. Just before it emerges from the eastern end of the cafion we have 

 the following section, which is represented in the accompanying diagram : 



Section JSfo. 6. 



1. Pink and red sandstones folded, dipping 8° to 10° on the eastern side and 25° on 



the western. 



2. Resting on the eastern side of the fold uncomformably are soft yellow sandstones 



dipping east about 4°. The outcrop of these sandstones is rather obscure, and 

 on the bluff, a short distance east of the outcrop, in fragments of sandstone that 

 appear to belong in these strata, I obtained the following invertebrate fossils : 



Campeloma maci'ospira, Corbula ? Pyrgulifcra ? and a fragment of a 



leaf which Professor Lesquereux says resembles au Araliopsis of the Cretaceous. 



3. Purplish, reddish, and greenish sandstones and shales. 



4. Conglomerate, dark in color, and somewhat metamorphosed, dip 20° to west. 



5. Sandstones. 



6. Same layer as No. 3, dipping west 6° to 8°. 



7. Same conglomerate as No. 4, dip 6° to 8° west. 



8. Sandstones, like No. 5. 



9. Sandstones and quartzites of Station 32. 



Above these are black shales and yellow sandstones, followed by mas- 

 sive sandstones, whose strike carry them through Ham's Hill. In the 

 upper part of these sandstones, at the bend of Ham's Fork, there is a 

 bed of coal. We have, therefore, in this cafion an anticlinal fold toward 

 the east, with its western side faulted. Besting on the eastern side are 

 beds that appear to be of Laramie or Post Cretaceous age. These beds 

 do not show as we follow the eastern side of the ridge north, but appear 

 to be covered by the red beds of the Wahsatch Group, which overlap 

 them. Before the deposition of these layers, therefore, the ridge must 

 have been subjected to a vast deal of erosion, especially on its eastern 

 face, for the western members are the most important topograpically, 

 the Cretaceous sandstones forming the mass of the ridge. The red sand- 

 stones in which the fold is so well marked in the canon probably repre- 

 sent the Triassic Bed beds and the conglomerate I have provisionally 

 considered to be the No. 1 Cretaceous. Looking north from Station 33, 

 on Ham's Hill, the sandstones of which it is composed are seen forming 

 the Fontenelle hogbacks, which are much lower, while the continuation 

 of the fold is distinctly noted in the Meridian Bidge, Whether the fault 

 is continuous or not, I cannot say. In the Meridian Bidge the greatest 

 amount of erosion has taken place on the western side of the fold, and 

 it has somewhat obscured the relations of the rocks. As to the age of 

 the elevation of the ridge, it is evident that it was Post Cretaceous, and 

 that the irplift was followed by a period of erosion before the beds of the 

 Wahsatch were deposited upon the upturned and eroded edges of the 

 Cretaceous strata. The strike of the Fox Hills beds, which in Mr. Em- 

 mons's district is a little east of north, changes to north, and can be traced 



