210 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



terraced, the floor of each bench being the bedding plane of the stratum 

 below. These limestones are capped by a thick bed that is the mottled 

 limestone forming the base of the Gallatin formation. This bed is about 

 200 feet thick, and from the meadow lands of the valley it can be seen 

 as a persistent cliff that is a readily recognizable horizon, and forms an 

 almost impassable barrier in the ascent of the slopes. It is overlain by less 

 massive limestones, capped by basic volcanic breccias. The irregularity 

 of the surface upon which these breccias were deposited is beautifully 

 illustrated by the exposures seen in this escarpment. The face of the cliff 

 shows the massive bed of mottled limestone, with a deep indentation that is 

 clearly a cross section of an old drainage way filled by volcanic breccias 

 which are now firmly indurated. The top of the mottled limestone bed 

 forms a broad and distinctive bench which extends along the slopes. Above 

 this bench the thinly bedded limestones are seldom seen, and though the 

 slopes are terraced and are open and grass covered, no good exposures are 

 found, owing to the abundance of rounded andesitic drift from reasserted 

 breccia beds which cover the summit of the plateau. 



At the upper end of the Slough Creek Valley the mountains close in 

 and the stream flows through a gorge cut in Archean gneiss and massive 

 igneous rocks. At one point the stream forms a picturesque waterfall in 

 which the water, cutting along joint planes of the rock, is separated into 

 two cascades, given the name of Twin Falls. Above tins canyon is the 

 upper mountain valley of Slough Creek, whose bottom is a labyrinth of 

 beaver ponds, sluggish channels, and willow groves, making the name of 

 the stream most appropriate. West of the valley the same sedimentary 

 formations noted are seen in terraced slopes, above which the persistent cliff 

 of mottled limestone is again present, extending northward to the forking 

 of the creek, where gneisses and volcanic breccia replace the sedimentary 

 rocks. The eastern side of this upper valley of Slough Creek shows no 

 sedimentary rocks, the dark, heavily timbered slopes rising to sharp craggy 

 summits of volcanic breccias. 



SODA BUTTE CHEEK. 



Carboniferous limestones are found at the southeastern base of Druid 

 Peak, and form the valley floor about the hot-spring cone which has given 

 Soda Butte Creek its name. The rocks are horizontal or but gently tilted 



