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Distribution and Characteristics of Loessial Soil Parent Material in Northwestern Oregon¹

ABSTRACT

Field study, mechanical analysis, and X-ray diffraction analysis (of fine silt) were used to identify as loess a silty deposit bordering the Columbia River near Portland, Oregon. The loess material is 65 to 75% silt and pale brown. It commonly overlies reddish brown clay derived from Columbia River basalt. Quartz and feldspars were found in fine silt of the loess. Only quartz was positively identified in fine silt of the underlying material from basalt. Thinning with distance from the Columbia River and also from a point north of Portland indicates that the loess source was a former Columbia River floodplain, particularly the wide area north of Portland. Vertical discontinuities in particle size distribution and mineralogy, not coinciding with genetic horizons, suggested the existence of more than one period of loess deposition.

¹ Technical Paper No. 1206, Oregon Agr. Exp. Sta., Department of Soils, Corvallis. A limited number of lithograph copies of the M.S. thesis by the senior author, on which this paper is based, are available. Presented at the Western Society of Soil Science, Logan, Utah, June, 1958.

² Research Fellow and Associate Soil Scientist, respectively.

Received for publication February 10, 1959. Accepted for publication June 10, 1959.