EED CREEK SANDSTONE. 175 



numerous drainage channels, tributaries of the western affluent of Coulter 

 Creek, afford good exposures of the higher Cretaceous formations. 



REGION BETWEEN EED AND BASIN CREEKS. 



East of Red Creek the gorge of the river narrows, and the clear 

 shallow water presents a striking appearance, flowing for more than half 

 a mile over a smooth, polished floor of Teton red beds, with the highly 

 colored sandstone forming the brilliant banks on both sides of the stream. 

 On the north and east side of Snake River, between Red and Basin creeks, 

 there is a mountain area consisting mainly of Mesozoic rocks. It extends 

 back from the river about 5 miles, and across its broadest expansion meas- 

 ures somewhat less than 4 miles. Rhyolite surrounds it on all sides, except 

 along the river gorge, effectually isolating it from adjoining areas. Geo- 

 logically this region is closely related to the Mesozoic rocks on the south 

 side of the river, every formation being represented, from the Teton to the 

 Montana, inclusive. 



Red Creek, which is appropriately named from the red rocks found on 

 both sides of the stream, marks the eastern boundary of the rhyolite flow, 

 stretching southward from the Sheridan volcano. Red Creek enters Snake 

 River through a narrow chasm cut in the sedimentary rocks. At the 

 mouth of the creek there is a bluff of red arenaceous limestone, similar to 

 the Carboniferous limestone found elsewhere underlying the red sandstone 

 of the Teton formation. It has been designated on the map as the Madi- 

 son limestone, but it may belong to the Quadrant formation, for the upper 

 beds are nearly pure sandstone. 



Passing up Red Creek the limestone soon disappears and is overlain 

 by the red sandstone. The stream for nearly its entire length has carved 

 its channel in these beds, which are here exceptionally well exposed, show- 

 ing the varying character of the sandstones and the intercalated red clays 

 and marls. The formation here has an estimated thickness of 400 feet, the 

 beds dipping north and east. Between Red Creek and Basin Creek the beds 

 are pressed into a syncline, followed by a sharp anticline. Overlying the 

 Teton beds of Red Creek come the Ellis and Dakota formations, followed 

 by the black clays and arenaceous shales of the Colorado, which lie in the 

 syncline, the latter formation being characteristically shown in a depression 

 between two ridges of less easily eroded beds. 



