TWO OCEAN PLATEAU. 197 



distance above the lake level occurs an outcrop of compact blue quartzite, 

 for the most part obscured by soil and vegetation. It has been referred to 

 the Sheridan quartzite of the Algonkian period. Resting unconformably 

 upon the Sheridan quartzite is a series of limestones, marls, and sand- 

 stones of the Ellis formation. They extend up the slope of Flat Mountain 

 for about 300 feet, where they are finally lost beneath the rhyolite. In the 

 limestone were found Rhynchonella and Camptonectes. The characteristic 

 Ellis sandstone has furnished the following species: Rhynchonella gnath- 

 ophora, Camptonectes pertenuistriatm, Avicula wyomingensis. 



WEST BASE OF TWO OCEAN PLATEAU. 



Between Big Game and Chicken ridges and the top of the broad 

 elevated mass of Two Ocean Plateau lies a comparatively narrow strip of 

 country characterized by distinctive and peculiar features. It presents 

 an aspect entirely different from that of the country westward, and stands 

 out in strong contrast to it, both in its geology and in its topography. 

 The Cretaceous sandstones which have been so persistent a feature of the 

 country are wanting and other and older rocks come to the surface. 



snake River fault.— This remarkable fault, to which allusion has already been 

 made, sharply defines geologically the east base of Big Game Ridge and 

 Chicken Ridge from the country to the east. The line of the fault can be 

 traced for 18 miles from the head of Grouse Creek southward, beyond 

 the limits of the mapped area, until obliterated by overlying masses of 

 Tertiary breccias. For this entire distance the west side of the fault 

 exposes only Montana and Laramie sandstones. In places the sandstones 

 are obscured by glacial accumulations. Occasional outbursts of basalt are 

 met with along the line of displacement, which coincides with the line of 

 least resistance. On the east side of the fault either Madison limestone 

 of the Carboniferous or the Teton beds of the Juratrias are brought to the 

 surface, but they are by no means so persistently exposed, their continuity 

 being broken by outlying masses of basic breccia from Two Ocean Plateau. 

 Contacts between the Paleozoic and Mesozoic rocks are admirably shown 

 all the way from Grouse Creek to Crooked Creek. At the latter locality 

 there is a dense black basalt whose outlines are partially obliterated by 

 loose alluvial deposits, but the large detached bowlders and black basaltic 

 soil help to define its area along the line of the fault. Still farther south, 



