Löss, or LOESS, 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 löss consist principally of landshells of existing species, but now and again freshwater shells are met with. Occasionally, also, the remains of man and the Pleistocene mammals are encountered, such as mammoth, woolly rhinoceros, reindeer, glutton, &c. In some places again are found relics of lemming, marmot, jerboa, &c., and other forms which are suggestive of steppe conditions. Geologists are still in some doubt as to the origin of the löss. The deposit is of such variable thickness (from a few feet up to 100 yards), and occurs at such very different levels, that it seems probable that more than one agency was concerned in its formation. Much of the löss was probably deposited from the flood-waters that escaped from the great ice-fields and melting snows of glacial times. Some of it again seems to have been the result of the weathering and disintegration of pre-existing accumulations, and the washing down of the disintegrated material by rain. And it seems likely enough that the superficial portions of fluvioglacial loams may have been modified to some extent by the action of wind. Richthofen, indeed, has maintained that the löss is essentially a wind-blown accumulation—a conclusion he came to from a study of the löss of China (q.v., p. 184). This theory, however, does not explain many of the phenomena, and the general opinion of geologists is in favour of the aqueous origin of löss as a whole. The European löss is undoubtedly associated with the glacial deposits of the Continent, and in North America, where löss is strongly developed, the same relationship obtains. The geologists of the United States Geological Survey maintain that the lössic accumulations which cover enormous areas in the great basin traversed by the Mississippi and its affluents are essentially of fluviatile origin.
For a general account of the European löss, see J. Geikie, Prehistoric Europe (1881), and Address to Geol. Section, Brit. Assoc., Newcastle Meeting (1889). For American löss, see Sixth Annual Report, U.S. Geol. Survey (1885). For Chinese löss, see Richthofen's great work on China; also Geol. Magazine (p. 293, 1882).