394 BITTER ROOT FOREST RESERVE 



The portion of the Reserve west of the summit of the Bitter Root 

 mountains (that portion in Idaho) is drained by the Clearwater 

 and Salmon rivers, about 90 per cent of the territory being tribu- 

 tary to the former. Both of these streams are affluents of the 

 Snake river, the Clearwater forming its junction at Lewiston and 

 the Salmon about 50 miles above, to the southward. The Salmon 

 has no important tributaries within the limits of the Reserve. 

 The Clearwater has four principal branches — the North Clear- 

 water, the drainage area of which is largely north of the Reserve ; 

 the Laksha, or Middle Clearwater, which has its source about the 

 base of St Mary's and St Joseph's peaks; the Main Clearwater, 

 which drains the crest line from Lost Horse pass to the Nez Perces 

 pass, and the South Clearwater, or American river, the smallest 

 of the four, whose drainage basin is in the southwestern portion 

 of the Reserve and extends within a few miles of the canyon of 

 the Salmon river. It may be mentioned that the location of the 

 Salmon river in this locality, as shown on the best existing maps, 

 was found to be in error by from 10 to 15 miles. 



The streams constituting the Clearwater system flow generally 

 in a western direction, and while the various affluents come from 

 almost every direction, the general result is a series of secondary 

 east and west ranges which have no well defined connection with 

 the main range. The summits of the ridges are from 3,000 to 

 5,000 feet above their enclosing canyons, and each ridge rises to 

 the same general elevation, so that were a surface laid through 

 all the crest lines it would be of an undulating and moderately 

 irregular character. We may therefore assume with some degree 

 of certainty that the surface represents an old topographic form— 

 an old plain or peneplain of denudation to which the country 

 was reduced after a long period of erosion. 



The rocks of the Bitter Root mountains are granites and slates, 

 the granite formation being the northward continuation of the 

 enormous granite mass of southern Idaho, one of the largest in 

 the United States. The slates, which are confined to the north- 

 ern portion of the Reserve, constitute a part of the Belt forma- 

 tion, these rocks being the oldest stratified beds of the Rocky 

 Mountain region. At some period since the Carboniferous the 

 great body of granite out of which this immense tract was carved 

 was injected as a molten fluid mass from below upward into the 

 slates. This molten rock cooled slowly, as is shown by its 

 coarseness of grain, and it must have cooled beneath a cover of 

 slates; but this cover has been almost entirely removed and the 



