CHICKEN RIDGE. 193 



Creek, there were found in the shale a cross-bedded sandstone 3 feet in 

 thickness, carrying' a number of characteristic Colorado fossils. From this 

 collection Mr. T. W. Stanton has identified the following species : Inoceramus 

 undabundus, I. umbonatus, I. flaccidus, I. acuteplicatus, Bacidites asjier, Sca- 

 phltes ventricosus. Along Snake River the base of the Colorado formation 

 is nowhere exposed; consequently no estimate can be made of its thickness; 

 but there are at least 400 feet of fissile impure sandstones and limestones 

 above the heavy shale belt assigned to the Montana. 



The Colorado formation extends along Snake River for more than 2 

 miles, the deep trench of the river exposing a sharp anticline in the shales. 

 A mile below the mouth of Sickle Creek, on the northwest bank, occurs an 

 exposure of blue clays inclined nearly 70° SW., while half a mile upstream 

 the beds dip 30° NE. Near the point where the Colorado fauna was 

 obtained the shales dip from 10° to 15° NE. Along the south and west 

 banks of the river the Colorado beds, on the west side of the anticline, 

 rapidly pass beneath the Montana sandstones at the base of Big Game 

 Ridge. On the bottom of Heart River, the stream having cut completely 

 through the flow of rhyolite, thei'e is an exposure of dark clay shale 

 which has been referred to the Colorado and may prove to be an extension 

 northward of the Cretaceous shales exposed between Sickle and Crooked 

 creeks. 



Between Sickle and Outlet creeks Chicken Ridge is made up mainly 

 of yellowish-gray, brown, and white sandstones, which in their lithological 

 habit closely resemble those found on Mount Hancock and Barlow Peak. 

 Along the southern slope the upper beds are a very dense steel-gray rock. 

 North of Coulter Creek the beds strike obliquely across the ridge, but 

 northward trend with the ridge in a nearly north-south direction. The 

 general dip varies from 20° to 30° E. 



South of Overlook Mountain the sandstones are well exposed, and are 

 inclined 30° E., toward Grouse Creek, with evidences of local faulting, 

 accompanied by variations in clip. Underlying these beds on the west side 

 of Chicken Ridge fissile sandstones with interbedded arenaceous shales 

 prevail. Lithologically these latter beds resemble the Pierre shales of the 

 Montana, and have been correlated with them, although no evidences of 

 organic remains were secured. They form the slopes until obscured by the 

 rhyolite flows, which attain elevations of between 8,200 and 8,300 feet above 



MON XXXII, PT II 13 



