20 FIELD OPERATIONS OF THE BUREAU OF SOILS, 1912. 



SOILS. 



The most important rock formation in this part of the Northwest 

 is known as the Columbia lava, a vast sheet covering neariy 250,000 

 square miles and varying in thickness from 300 or 400 to over 4,000 

 feet. This mass of rock was not formed by a single flow or volcanic 

 eruption, but by a series of disturbances, often with a considerable 

 period of time intervening, and the line of separation between the 

 flows is commonly marked either by a change in the structure of 

 the rock or by intervening strata of sands, clays, or gravel. The 

 larger part of these lavas cooled slowly, and wherever vertical sec- 

 tions of the rock are ^dsible a columnar structure nearly always 

 exists. Thi'oughout the gorge of the Columbia River, in this area 

 and to the westward, and along the courses of the Hood and Wliite 

 Salmon Rivers the vertical walls are often 500 feet or more in height. 

 These bold columnar cliffs form a prominent feature of much of the 

 wUd, rugged scenery along these streams. (PL IV.) 



The broad features of the Hood River Valley are structural. It 

 occupies a shallow downward fold or syncline of the basalt beds. 

 Since its formation, however, it has been modified in detail by 

 erosion, both by rumiing water and by glaciers. The work of the 

 latter, however, has been mainly deposition, while the former has 

 done very little beyond the cutting of the narrow valley in which 

 the river flows. Later a considerable part of the glacial deposits 

 were apparently removed, either by streams from the retreating glacier 

 or by others, the forerunnei-s of the present rivers. In some instances 

 the lava was swejjt clean of its stony mantle and veneered with a 

 deposit of finer sedimentary material, but commonly sections along 

 the gorges of the streams show remnants of the glacial deposits 

 lying between the rock and the later surface soil. Some of the 

 present soil material was probably deposited as glacial outwash, 

 being made up of fine glacial material transported to its present 

 location by streams from the melting ice. 



In the White Salmon River Valley it seems doubtful whether glacial 

 ice occupied any considerable part of the depression. So far as has 

 been determined, tlic present surface of the valley may be solely 

 the result of erosion and deposition by streams, some of which may 

 have been of glacial origin. 



In addition to the effects of glacial and alluvial agencies, the 

 lavas have been subject to the slow but certain action of weathering 

 and the decomposition of this rock, where not obscured by deposits 

 resulting from other agencies, has resulted in a mantle of residual 

 soils which cover the crests and slopes of the hills and mountains 

 in the area. 



The latest development in the formation of the soils has been 

 the deposition of recent alluvial soil in irregular, narrow areas along 



