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. i). 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 



