Southeastern Washington and Adjacent Idaho. 9 
not unlike the billowy surface of the ocean. The fine silt loam 
soil, from the disintegrated basalt, has been moulded by the ac- 
tion of wind and water into a dune-like topography (Fig. 38). 
Because of the constant shifting of the soils from the exposed 
south and southwest to the leeward slopes, the latter are much 
steeper. These hills reach a height of 100 to 360 feet. Measure- 
. ments of a large number of slopes near the base station gave an 
average gradient of 12° on the exposed sides, while the sheltered 
sides were from 5° to 8° steeper. Indeed, in parts of the region, 
many of the hillsides are so steep that they can not be brought 
under cultivation. The intervening valleys are usually narrow, 
but never form undrained basins. The general slope of the 
region is westward. While Thatuna Hills, a low, outlying spur 
of the Bitterroot Mountains, culminate in Cedar Mountain, 4,950 
feet high, the eastern edge of the lava sheet reaches an altitude 
of only 2,750 feet (Fig. 1). It descends at the rate of about 30 
feet per mile, to 1,170 feet on the western border of the region 
under consideration. 
Two main rivers with their tributaries drain the region except 
the northeastern part which is drained by tributaries of Spokane 
River. The Palouse River has its headwater tributaries in the 
mountains of Idaho, and after passing into Washington the north 
and south forks unite at Colfax. In the western part of the 
region it turns southwestward and finally enters the Snake River. 
At Colfax the river has cut a canyon into the basalt to a depth of 
300 feet. 
In general, the river canyons, whether large or small, are char- 
acterized by much steeper south and southwest than north and 
northeast banks. This is due in part to greater exposure of the 
north and northeast sides to temperature changes, and to wind 
action which erodes and carries away the soil materials, and in 
part to the constant covering of the sheltered sides by wind- 
blown material. These processes have a striking effect upon the 
vegetational cover. Locally the exposed canyon walls are known 
as rim-rock. 
Snake River is joined by the Clearwater as it enters Washing- 
ton. Tributaries of the latter drain the extreme eastern part of 
9 
