BITTER ROOT FOREST RESERVE 395 



granite itself deeply cut and dissected. At the time when vol- 

 canic activity was so predominant a feature in the Yellowstone 

 Park and the great lava flows of basalt dammed up the Snake 

 and Columbia rivers west of these mountains, the Bitter Root 

 valley was effected by tilting of the earth, so that the drainage 

 was in many cases reversed and the Bitter Root river was 

 dammed back, forming the Bitter Root lake, which was over 

 1,000 feet in depth. 



The overflowing waters of the lake gradually deepened the 

 outlet and drained the lake, clearing out a large part of the sed- 

 iment, a work not .yet entirely accomplished, as the valley has 

 not been cut down to its former level. The many lakes which 

 nestle in the mountain amphitheaters and dot the plateaus are 

 the result of glacial occupancy. 



In connection with the reconnaissance survey a forest map 

 was prepared, and it is published herewith. This map indicates 

 the features of the forest in the broadest way, no attempt hav- 

 ing been made to differentiate the species. Two zones of forest 

 trees are native to the Montana slopes of the Bitter Root Reserve, 

 the yellow pine and the subalpine fir, about one-fourth of the 

 growth belonging to the former, which has a range from the 

 lowest elevations to 5,800, feet, and three-fourths to the latter, 

 which has a range from 4,200 feet to the highest altitudes. In 

 the yellow-pine zone the yellow pine constitutes about 20 per 

 cent of the growth and the hemlock spruce about 60 per cent, 

 the remaining 20 per cent being distributed among the other 

 trees included in the zone, the lodge-pole pine, white fir, and 

 balsam fir. In the subalpine zone the lodge-pole pine consti- 

 tutes by far the greatest portion (about 90 per cent) of the 

 growth, the remaining 10 per cent being Lyall tamarack, sub- 

 alpine fir, white-bark pine, white fir, Engelmann fir, and yew. 

 Strictly speaking, only the yellow pine should be classed as 

 commercial timber, as it alone is used for lumbering purposes ; 

 but on the map are included under this head the tamarack, the 

 fir. and the white-bark pine, as they may be applied to local 

 purposes and have to that extent some commercial value. The 

 yellow pine may be considered as constituting the entire growth, 

 as shown on the map, between the Bitter Root valley and the 

 summit. 



The areas indicated as bare on the map are not wholly so, 

 there being no portion of the Bitter Root Reserve entirely above 

 timber line, as everywhere, even on the loftiest summits and 



