A widespread, nonstratified, porous, friable, usually highly calcareous, blanket deposit (generally less than 30 m thick), consisting predominantly of silt with subordinate grain sizes ranging from clay to fine sand. It covers areas extending from north-central Europe to eastern China as well as the Mississippi Valley and Pacific Northwest of the United States. Loess is generally buff to light yellow or yellowish brown; often contains shells, bones, and teeth of mammals; and is traversed by networks of small narrow vertical tubes (frequently lined with calcium carbonate concretions) left by successive generations of grass roots, which allow the loess to stand in steep or nearly vertical faces. Loess is now generally believed to be windblown dust of Pleistocene age. Compare: adobe