GEOLOGICAL SURVEY OF THE TEREITORIES. 205 



it had been very fluid. That these mounds were not all formed by an 

 undermining and sinking of the surrounding mass, to which some of 

 them have very properly been referred, is proved by the tapering shape 

 of the closely-fitting blocks which form the arch. But there is still 

 room for study on all these points. 



As the canon opens toward the plain, white and drab friable sand- 

 stones, probably of Pliocene age, make tlieir appearance on the flanks 

 of the bordering hills. As nothing of the kind was apparent within the 

 caiion, it would seem either that the whole width of the canon has been 

 thoroughly scoured out since the deposition of the Pliocene beds, or 

 else that the stream carried so much and so rapidly-flowing water dur- 

 ing the Pliocene epoch as to prevent the deposition of such beds. 

 These sandstones dip about 30^ IsT. 11° W., showing that the dis- 

 turbances, of which we have such abundant evidence, continued to a 

 very late period. The terraces themselves show no evidence of any 

 local disturbances ; their levels were not determined, however, with 

 sufficient accuracy to enable us to judge whether or not any general 

 upheaval, such as has already been suggested, might have affected this 

 region. 



Leaving the Snake Eiver jilain at Eoss's Fork, we followed up the 

 valley of that stream some six or eight miles, and then crossed a divid- 

 ing ridge to Lincoln Valley and Fort Hall, which post we reached about 

 noon on July 3. Thus far we had followed well-traveled roads, and had 

 transported o^r baggage and provisions in wagons ; but now we were 

 soon to leave roads entirely, and accordingly turned in our wagons and 

 fitted out a train of pack-mules. During the week thus occupied, we were 

 enabled to examine a considerable territory in the neighborhood of the 

 post. 



At Eoss's Fork we had struck the line of the old emigrant-road across 

 the mountains to Oregon. This road crossed from Green Eiver to the 

 head of Bear Eiver, down that stream to Soda Springs, thence across 

 the head of Port jSTeuf, and through a low gap to the head of Eoss's Fork. 

 Xear the mouth of Eoss's Fork it crossed Snake Eiver, and thence ran 

 in almost a direct line past the westernmost of the Three Buttes to the 

 foot of the Salmon Eiver Mountains. 



Lincoln Yalley, as a whole, is broad and flat, with mostly gentle 

 slopes rising to the crests of the bounding ridges. On the wet 

 bottoms, large quantities of a coarse grass are cut for the stables at the 

 post. The slopes are covered with various grasses, sedges, and other 

 forage-plants, which make fine grazing for nearly the whole year. In 

 this excessively dry atmosphere, these plants, which are ripe and have 

 stopped growing by midsummer, lose their moisture so rapidly as to 

 escape the decay common in the damp air of the Eastern States, and so 

 become natural hay while still standing, and mostly retain their nutri- 

 tious qualities until the melting of the snow and the setting in of the 

 spring-rains, which start the new growth while occasioning the decay 

 of the old. Here in the foot-hills, the snows sometimes lie deep enough 

 to prevent the stock from reaching the standing grass, and so a supply 

 of hay is essential to their wintering. Only a few miles westward, how- 

 ever, the snows are generally so light and melt so rapidly that, in an 

 ordinary winter, no hay is needed ; and stock range through the whole 

 winter, never even coming to the ranches of their own accord. This 

 neighborhood was formerly one of the favorite winter-rendezvous of the 

 trai>pers and fur-traders ; and the plains and foot-hills then supported 

 vast herds of buffalo, as well as many antelope, deer, elk, and bears. 

 The region is now so much frequented that none of these animals are 



