164 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



bedded glaueonitic limestones whose lithological character and sequence 

 place them in the Flathead formation of the Cambrian. 



Similar rocks are exposed in the bed and walls of Falls River below 

 the Rainbow Falls, where they are covered by a light-colored andesitic tuff 

 or breccia, generally fine grained and much decomposed. This breccia 

 forms low rounded hummocks at the base of the hills, and is not yet cut 

 through by the river, whose bed it forms for a mile above the meadows. 

 The exposure in the river bank shows rude bedding, with northwest dip. 



South of Falls River the hills are continued in an irregularly accidented 

 area. The rhyolite plateau terminates in a wall several hundred feet high, 

 a deep but narrow depression separating the bluff from the sedimentary 

 ridge to the west. 



These hills south of the river present exposures of the Carboniferous 

 and Triassic beds, forming parallel ridges with benched slope and trending 

 N. 70° E., the rocks dipping 30° N. The red, fissile Teton sandstones are 

 well exposed in the lower slopes, weathering into a reddish soil not easily 

 distinguished from that of the red patches of the Quadrant formation. 



These red sandstones are here underlain by the cherty horizon of the 

 Teton, resting upon the Quadrant quartzites. The chert occurs in charac- 

 teristic rolls, rods, and nodular masses, having a chalky surface, and grading 

 at times into the inclosing arenaceous rock. The Quadrant quartzites cor- 

 respond closely in character to the formation as developed in the Gallatin, 

 consisting of white granular quartzite and interbedded limestones that are 

 often good marbles. 



West of the sedimentary area just noted there is an exposure of 

 hornblende-andesite-porphyry. The rock is clearly intrusive and cuts 

 through the Carboniferous limestones. 



The Birch Hills, by reason of the compact character of their rocks, 

 present excellent evidences of former glaciation of the region. The rocks 

 occur in rounded ice-worn hummocks, covered with glacial scorings and 

 groovings with general east-west trend. In general the eastern and 

 northern slopes are gentle, while steep cliffs bound the hills to the west 

 and south. 



