12 GEOLOGY OF SW. IDAHO AND SE. OKEGON. [bull. 217. 
sent. On Stein Mountain a few pines, firs, junipers, and cottonwoocls 
grow in the deep canyons on its western slope, and at a few localities 
along the border of the small streams which descend its precipitous 
eastern escarpment. In each of these instances the elevation is well 
below that of the cold timber line, and water for natural irrigation is 
supplied by streams. 
The snow line on mountains, or the lower limit of perennial snow, 
like the upper and lower limits of arboreal vegetation, is determined 
by climatic conditions, mainly temperature. The snow line marks 
approximate^ the elevation at which the mean annual temperature 
is at the freezing point, namely, 32° F. The snow line, like the cold 
timber line, is high above the sea in equatorial regions, and declines 
toward the poles. In the Andes, near the equator, the snow line has 
an elevation of about 17,000 feet; in the high sierra of California, it 
is at an altitude of about 14,000 feet; on the southern slope of Mount 
St. Elias it has an elevation of 2,500 feet; and in the Arctic Archi- 
pelago it reaches sea level. The snow line is higher than the cold 
timber line, and between the two lies the belt of alpine flowers. 
In southwestern Idaho and adjacent portions of Oregon none of the 
mountains are sufficiently lofty to reach the snow line, although Stein 
Mountain, the highest and finest range of the region, makes a near 
approach to the necessary height, and, as already stated, snow banks 
are found on its higher portion throughout the summer, but for the 
most part only in the noontide shadow of northward-facing precipices. 
TOPOGRAPHY. 
The portions of Idaho and Oregon under consideration have broad 
and, in general, flat-bottomed valleys, separated by abrupt and deeply 
sculptured hills and mountains. The valleys so situated that the 
streams formed in or traversing them can discharge into lower valleys 
or join Snake River are usually trenched by stream channels and 
canyons, but the valleys from which there is no escape of the surface 
water are level floored and either contain lakes or the desiccated 
beds of transient Avater bodies. The valleys for the most part owe 
their existence to the upraising of the bordering hills and mountains. 
Their generally flat bottoms are due to the breadth of the folds or 
irregular dome-like elevations and depressions resulting from move- 
ments in the rocks and to the partial filling of the depressions by 
lacustral and stream-deposited debris. 
The greatest valley of the region is the one through which Snake 
River flows. This, as was described in a previous report, a extends 
from east to west across the entire width of southern Idaho, and 
includes an area of about 1,200 square miles in Malheur County, Oreg. 
This valley, known also as the Snake River Plains, is underlain by 
lacustral and stream deposits and by extensive sheets of basaltic 
a Bull. U. S. G-eol. Survey No. 199. 
