russell.] TOPOGRAPHY. 13 
lava. The fine, light-colored sediments, mostly of Tertiary age, are 
best developed in southwestern Idaho and the adjacent portion of 
Oregon, while the associated lava sheets are more continuous and 
apparently more numerous in southern Idaho east of Ada County. 
This difference in the rocks is accompanied by marked contrasts in 
the topography. The vast smooth plain through which the Snake 
River flows has been deeply trenched bj^ that river from American 
Falls westward, and where the walls of its canyon are of basalt they 
stand in bold cliffs, and if sedimentary beds are present beneath a 
surface sheet of basalt, there occurs along the borders of the plateaus 
a characteristic feature common in many areas in Oregon. The sheets 
of resistant material, with weak strata beneath, give origin to rim 
rocks, as they are locally termed ; that is, precipitous and frequently 
vertical escarpments, surmounting more gentle slopes which are 
sheathed with debris derived from the crumbling margin of the pro- 
tecting layer (PL XVI). The details in the valley sides present great 
variety, dependent on the number of alternating strong and weak beds, 
and their variations in thickness. Where the strong, resistant rocks 
greatly predominate, the valley sides due to erosion are bold, and if 
canyons have been excavated they are characteristically narrow, with 
nearly vertical walls. Where weak, easily eroded rocks predominate, 
other conditions being the same, the valleys are wide and have flaring 
sides, perhaps with a few vertical steps, marking the positions of 
resistant beds, and narrow canyons with vertical walls are absent. 
Where all the rocks cut through in the excavation of a canyon or 
valley are soft or weak, as for example where lava sheets are absent, 
and only lacustral beds, stream deposits, or beds of volcanic dust and 
lapilli are present, the valleys have widely flaring sides which are 
sculptured into a great variety of details, owing to the action of rain, 
rills, and usually ephemeral streams. The results just indicated, 
which depend mainly on rock texture and composition, or perhaps 
more strictly on the varying degree of resistance rocks offer to 
mechanical and chemical erosion, are modified, as is well known, by 
differences in the way the earth's surface is being sculptured, 
dependent on climatic conditions, elevation above sea level or the 
base-level of erosion. In the portions of Idaho and Oregon under 
consideration the climatic conditions are essentially the same through- 
out, but the elevation and the position of the rocks are markedly 
different from locality to locality. 
MOUNTAINS. 
The entire region here considered may be designated as mountain- 
ous, and although at present not enough is known of the underlying 
structure to permit a detailed description of the manner in which even 
the more prominent elevations were produced, it is apparent that two 
types of mountains are present. 
