LOESS, in geology, a variety of loam. Typical loess is a soft, porous rock, pale yellowish or buff in colour ; one characteristic property is its capacity to retain vertical or even over-hanging walls in the banks of streams. These vertical walls have been well described by von Richthofen (Fiihrer fur Forschungsreisende Berlin, 1886) as they exist in China, where they stand in some places 5ooft. high and contain innumerable cave dwellings ; ancient roads, too, have worn their way vertically downwards deep into the deposit, forming trench-like ways. This character in the loess of the Mississippi region gave rise to the name "bluff formation." A coarse columnar structure is often exhibited on the vertical weathered faces of the rock. Another characteristic is the presence throughout the rock of small capillary tubules, which appear to have been occupied by rootlets ; these are often lined with calcite. Typical loess is usually calcareous; some geologists regard this as an essential property, and when the rock has become decalcified, as it frequently is on the surface by weathering, they call it "loess loam" (losslehm). In the lower portions of a loess deposit the cal cium carbonate tends to form concretions, which on account of their mimetic forms have received such names as losskindchen, ldsspuppen, poupees du loess, "loess dolls." Bedding is absent from
typical loess. The mineral composition of loess varies somewhat in different regions, but the particles are always small ; they con sist of angular grains of quartz, fine particles of hydrated silicates of alumina, mica scales and undecomposed fragments of felspar, hornblende and other rock-forming silicates.
In Europe and America loess deposits are associated with the margins of the great ice sheets of the glacial period; thus in Europe they stretch irregularly through the centre eastwards from the north-west of France, and are not found north of the 57th parallel. Most geologists are agreed that the true loess is an aeolian or wind borne rock, formed most probably during periods of tundra or steppe conditions. But it seems clear that certain deposits classed as loess in western Europe do not really belong to this category, being of alluvial origin and related to the brick-earths of south eastern England.