Lower Mississippi Valley loess resembles in all essential respects that of the Rhine Valley and other parts of Europe. Its field relationships preclude the possibility of eolian, lacustrine, fluvial, or other direct sedimentary origin. Typical loess grades upslope into parent material, from which it has been differentiated by a process here called loessification. Initial parent materials are terrace deposits physically similar to backswamp clays of the Recent Mississippi River. Parent materials weather into brown loam that creeps downslope, accumulating in greatest thickness in valleys and as mantles of bluffs. During loessification, carbonates accumulate, the size of particles becomes restricted mainly to 0.01–0.05 mm., snails are incorporated, and other loessial characteristics appear. More widespread development of loess east of the Mississippi results from wider areas of Pleistocene backswamp deposits. Loess occurs along major Mississippi tributaries, on such residual eminences as Crowleys Ridge, Sicily Island, and various hills west of the river in Arkansas and southeastern Missouri. Under some exposure conditions deloessification occurs, carbonates leach, coarse granules disintegrate, loessial characteristics disappear, and a type of brown loam is the end product. If a sharp distinction be made between loess and loesslike materials, it appears that the origin of loess in other regions is similar to that in the lower Mississippi Valley.