10 A Study of the Vegetation of 
the region. Snake River flows westward through the region in a 
canyon which is simply an immense gorge a mile and a half to 
two miles wide hewn in the basalt. Although the canyon is 
over 1,600 feet deep, the river has not yet, except in a very few 
places, cut to the bottom of the basalt, for its present bed is still 
like its bluffs. The soil of these bluffs, as is characteristic of 
similarly exposed situations in the region, presents various stages 
of decomposition from the coarse talus at the foot of the cliff to 
well formed soil. The canyon at its bottom is scarcely wider 
than the river, except at the bends, where bars of moderate area 
have been formed. The soil of the bars is partially river-sand 
of granitic origin and partially wash from the bluffs. 
Except for the Clearwater, no large streams enter Snake River 
in this region. Numerous small laterals have worked their way 
back into the basalt, and it is along these that the main canyon 
may be entered, especially from the south. 
In general, the topography is young. The streams have not 
advanced far in their task of cutting away the rocks from the 
plateau and of reducing the land to base level. Especially south 
of Snake River, in Garfield County, where the surface is less roll- 
ing, one is impressed with the view of a land with a broad plateau 
surface and an infantile drainage. Here the spaces between the 
streams are flat-topped. 
In the northwestern part of the area occur numerous small 
lakes, of which Rock Lake, about 0.5 mile wide and 7 miles long, 
is the largest. It is really a great crack in the basalt, with rocky 
canyon walls and basalt bottom. It connects northward with 
Little Rock Lake and is drained into the Palouse River by Rock 
Creek. 
About these lakes, as well as along the canyons of the streams, 
the weathering of the edge of the basaltic sheets has caused them 
to recede unequally and to form steps or terraces in the faces of 
the precipices. Where the walls are steepest, they present a 
series of narrow steps covered with talus and soils in all stages of 
formation, separating vertical masses of bare and frequently 
columnar rock 30 or more feet in height (Fig. 45). 
In the drier western part of the region, where the disintegra- 
1ge) 
