338 EEPOET UNITED STATES GEOLOGICAL SURVEY. 



The broader upland ridg^e next west is covered mth a fine soil, with 

 some water-worn debris of red sandstone, quartzite, and gray lime- 

 stone. On the western border, in the ravines which cnt the abrui)t 

 slope on this side, heavy ledges of conglomerate are met with, dip- 

 ping 10° to 20° in a northeasterly to southeasterly direction, and 

 reaching a thickness of at least 300 feet, probably much more. These 

 deposits are quite variable in their lithological aspect, sometimes appear- 

 ing as coarse sandstones easily crumbling, again showing a conglom- 

 erate structure made up of water-worn pebbles and small bowlders of 

 quartzite, limestone, trachyte, and basalt, arranged in more or less dis- 

 tinct layers and cemented with a fine light-drab x)aste. The deposit is 

 Ijrobably identical with similar accumulations occurring elsewhere in 

 this region, which have been provisionally referred to volcanic origin. 

 To the north the ridge rises into a high point, which is capped by 

 westerly -inclined volcanic ledges, and which once formed a continuous 

 sheet with the wide belt of flows filling the Blackfoot Valley. 



BLACKFOOT BANGE. 



Parallel with, and lying a few miles to the northeast of, the caFioned 

 'Course of Blackfoot Elver, the country rises into a low range of mount- 

 >ains, which forms the divide between the latter stream and a wide 

 basin which drains northward through Willow Creek into the Snake 

 Plain. This range, known as the Blackfoot Mountains, has a north- 

 northwest and south-southeast extent of some 20 to 25 miles, and with 

 the lower parallel ridges on the west a breadth of 4 to 7 miles. To the 

 south the main ridge gradually sinks into a low plateau and level basin 

 expanse, through Avhich lies the upper course of Blackfoot Eiver and 

 the sources of Willow Creek, while to the north it divides, sending oif 

 a low rocky spur, across which Wolverine Creek has cut a picturesque 

 cailOH, the eastern and more rugged branch culminating in Blackfoot 

 P^ak, a high dominating point near the northern end of the range, 

 which attains an altitude of 7,400 feet, or about 2,600 feet above the 

 Snake Plain. The southwestern face of the range presents a compara- 

 tively bare, rocky escarpment, the opposite slope falling away in succes- 

 sively lower branches, and is more or less densely clothed with pine and 

 ^ruee forests and thickets of undergrowth. 



Our route of ajjproach followed the Blackfoot, which, just before it 

 emerges into the plain at the northern bend, has cut a deep narrow 

 gorge through the volcanic rocks which incline at a moderate angle in 

 the direction of the plains, and which form the coping of a line of bluffs 

 along the w^st side of the Blackfoot above the can on, south. In the 

 lower walls of this canon. Dr. Hayden, who visited the locality in 1871, 

 found heavy ledges of rusty weathered quartzite, overlaid by Carbonifer- 

 ous limestone, all dii^ping steeply to the northeast. In the north side of 

 this canon the way passes high up over the basaltic ledges, which bury 

 the sedimentaries beneath an accamulation of several hundred feet 

 thickness of volcanics. 



Wolverine Creek joins the Blackfoot at this point from the east, its 

 lower course lying in a narrow valley with sloi)ing debris benches on 

 either hand, on the north covered with cedars and merging into the vol- 

 canic foreland, which sweeps down to the level of the plain in that direc- 

 tion, the south side acclivities rising in smooth, grassy curves up into 

 isolated trachytic domes which mark the remnant of a once extensive 

 flow dating back to the earlier period of volcanic activity. 



The first limestone ridge, or the western spur of the Blackfort Eange, 



