The Encyclopedia Americana (1920)/Loess

LOESS, lės or lō'es, a loamy deposit of Pleistocene Age, abundantly developed in the valleys of the Rhine, the Danube, the Rhone and many of their tributaries. It is a pulverulent yellowish-gray or brownish loam, homogeneous and non-plastic, and consists principally of clay with small angular grains of quartz, and extremely minute scales of mica, together with a larger or smaller admixture of carbonate of lime and some iron oxide. It has a tendency to cleave in vertical planes, and thus forms cliffs where streams intersect it. The organic remains of the loess consist principally of land-shells of existing species, but now and again fresh-water shells are met with. Occasionally, also, the remains of man and the Pleistocene mammals are encountered. The deposit varies from a small thickness up to nearly 300 feet, and occurs at greatly differing levels, so that more than one agency would seem to have been active in its formation. Escaping floodwaters from glaciers are believed to have made some of the deposit; some of it may have been the result of weathering and rain-washings. The European loess is undoubtedly associated with the glacial deposits of the Continent, and in North America the same relationship obtains. Many geologists of the United States Geological Survey maintain that the accumulations which cover enormous areas in the great basin traversed by the Mississippi and its affluents are essentially fluviatile. Richthofen believes the Chinese accumulation, the largest in the world, to have been of Æolian origin, and this theory of wind-blown material has also been advanced with respect to deposits in the United States. There is no unanimity of opinion as to its origin, but loess is probably in part fluviatile and in part wind-blown. Consult Sixth Annual Report of the United States Geological Survey (1888); Chamberlin and Salisbury, ‘The Driftless Area of the Upper Mississippi Valley’; McGee, ‘The Pleistocene History of Northeastern Iowa,’ in the United States Geological Survey, Eleventh Annual Report (1891); Geikie, ‘Prehistoric Europe’ (1881).