326 EEPOET WITED STATES QEOJ.OGICAI, &UEVEY. 



In the sontliwestern section, situate in tlie great northern or lower bend 

 of the Snake^ occur a series of low hill-ranges which in the main extend 

 into the country to the south^with perhaps a single principal ridge, the 

 Blackfoot Range, which occupies a topographically isolated position in 

 the southwestern half of this area. On the east and facing the Snake 

 Eiver Eange, occurs a widish belt of hiUs, the Caribou Eange, which 

 falls away inta the basin of Willow Creek, wl&ch intervenes between its 

 southwestern foot and the Blackfoot Eange. This basin area is ridged 

 with low parallel elevations, having the same general northwest and 

 southeast direction. In the extreme southwest the Portheuf Eange 

 terminates in Mount Putnam and a belt of low highlands in the angle of 

 the Blackfoot Eiver. 



Valleys and plains. — Encompassing the western and northwestern 

 borders of the highland region above briefly outlined, the great plain of 

 the Snake Eiver stretches miles away to the foot qf the distant mountain 

 barriers which define the western and northern bmindary of this arid 

 waste. Within our present limits this region is scarcely broken by a 

 single elevation, the Crater Buttes at the apex of the northern bend of 

 the Snake, and the Sand-Hills a few miles to the north of the latter, being 

 the only exceptions. But, approaching the highlands, the plain rises in 

 gentle grassy acclivities, whose surfaces are scored by narrow caiions 

 hemmed in by precipitous walls of dark basaltic lava, which in places 

 rise to the height of several hundred feet above the stream-beds. Through 

 such portals the majority of the streams pass on their way -fcom the high- 

 lands to the plain. 



The highlands are intersected by broad, bay-like recesses,,which open 

 out to the north into the plains country, of which, indeed, they form a 

 part i)hysically and geologically. Such are the basins situated between 

 the Caribou and Blackfoot Eanges and Pierre's Basin, between the Snake 

 Eiver and T6ton Eanges, extensive level, grassy tracts, which are par- 

 tially hemmed in by mountain ranges. Jackson's Basin, at the eastern 

 foot of the Teton Eange, is an interesting example of mountain-locked 

 valley, which comprises an area of four or five hundred square miles, 

 environed on the east by the comijaratively gentle wooded slopes which 

 form the outlying flanks of the mountain borders, and on the west by 

 the Teton Eange, whose precipitous walls and massive towers rise 5,000 

 to 7,000 feet above its surface. The Snake Eiver winds its course througli 

 low, willow-fringed flats, between beautiful gravel terraces, and over 

 pebble- and bowlder-strewn bed, receiviug on the west bank the numer- 

 ous snow-fed torrents which debouch into pretty laklets at the foot of 

 the great range, and on the east the larger affluents of iceKiold water 

 which descend from the continental divide. 



SiiMistrictSj or sections. — As above outlined, the district may be di- 

 vided into mountain and plain regions, the superficial areas of which are 

 respectively as three to four, or thereabouts. Further considered in 

 reference to the topographical and geological characteristics, of the dis- 

 trict, it may be divided into the following sections, which will be sepa- 

 rately noticed hi the following chapters devoted to detail geology: The 

 southwestern section, embracing the area reaching up into the great 

 northern bend of the Snake Eiver j the Teton, or middle section, in- 

 cluding that portion of the district embraced in the great southern bend 

 of the Snake; and, lastly, the eastern section, which comprises all the 

 territory to the east, lying between the T6ton, Eange and the main 

 water-shed crest, of which, however, only a narrow strip' along the west- 

 ern border in the vicinity 'of Jackson's Hole feU under actual examina- 

 tion during the past season. 



