russell] TOPOGRAPHY. 17 
more correctly, so far as the southeastern portion of Idaho is con- 
cerned, the east and west borders, are defined by rugged mountain 
ranges, several of which extend into the lava-covered country for a 
score or more miles in much the same manner that headlands and 
capes on a ragged coast project into the ocean. In several instances 
the lava poured out in the great central plain entered tributary 
stream-cut valleys and flowed up them from 20 to 30 miles, or until 
the rising gradient checked the advance of the tide of molten rock. 
The upstream ends of these lateral tongues from the main lava sheets 
are now usually covered with alluvium brought down by streams from 
higher portions of their courses. Alluvial fans and talus slopes are 
conspicuous features about the bases of the mountains, and chrono- 
logically both preceded and followed the widely extended lava sheets. 
This is an instructive fact, as the buried alluvial fans favor the pas- 
sage of water from the mountains to the strata beneath the lava sheets 
flooring the plains. 
The mountains to be seen from the Snake River Plains are bold and 
rugged, and as a whole present exceedingly sharp and serrate sky 
lines. The main range in south-central Idaho, known as the Sawtooth 
Mountains, expresses both in its name and its salient features the pre- 
vailing characteristic of the mountain crests throughout a large por- 
tion of the State. These pinnacled mountain tops are to a great extent 
composed of quartzite, but granite spires and prominent limestone 
ridges are in numerous instances nearly as sharply defined. The 
granite, even in valleys and on the immediate borders of the central 
lava plains, as, for example, between Boise and Mountain Home, pre- 
sents a vast number of monumental and spire-like forms, which rise 
from widely expanded talus slopes. 
The leading topographic features of southern Idaho may be sum- 
marized as deeply sculptured mountains surrounding a vast, nearly 
level, plain. The mountains rise boldly to a height varying from a 
few hundred to over 6,000 feet above the plain, and to elevations 
ranging very commonly from 7,000 to 10,000 or more feet above the 
sea. In its western part the plain has a general elevation of from 
2,900 to 3,200 feet, and in its broadest and most characteristic portion 
about Big Butte or between Blackfoot and the "Lost River country" 
of in general from 4,500 to 6,000 feet above the sea. 
CLIMATE. 
The climate of the Snake River Plains has for its leading character- 
istics aridity, prevailing high temperatures in summer, and severe cold 
in winter. One of the most marked features in the atmospheric condi- 
tions at nearly all seasons is the great range in temperature between 
day and night. 
Bull, 199—02 2 
