Loess sheets in Mississippi are draped over hills in the uplands adjacent to the Mississippi River alluvial valley. The loess is thickest on the bluffs bordering the valley and thins eastward away from the bluffs. The loess sheets were developed by eolian transport from braided-stream deposits in the Mississippi River alluvial valley during late Pleistocene and early Recent time. Stratigraphically, the loess is separable into (1) the Vicksburg loess, which is leached in a thin uppermost layer but is calcareous in the remainder of the section; (2) a Basal transition zone, which is weathered, generally noncalcareous, and contains a distinct buried soil horizon; and (3) a Pre-Vicksburg loess, which may be calcareous and unweathered to noncalcareous and greatly weathered. Where these loess sheets thin with distance from the bluffs and become weathered throughout, the entire section is designated as Mississippi brown loam.
The calcareous Vicksburg loess is bedded as a result of periods of quiescence during deposition. Calcareous loess invariably contains an internal skeleton of calcareous concretionary material which is developed by former root penetrations. These vertically oriented root penetrations have affected both the calcite and the clay bonding in the loess so that the loess is stable in vertical cuts but is susceptible to failure along vertical planes.
Infiltration of rainfall into the loess and internal drainage occur at rates which are such that the loess does not become saturated except where there is a water table. Often the loess may remain in a relatively dry state only a few feet below the surface.
Specific gravity ranges from 2.66 to 2.73, density from 79 to 104 pounds per cubic foot, and porosity is between 43 and 54 per cent of total volume. Calcareous Vicksburg loess has a plasticity index of 2 to 12; the plasticity index of other Mississippi loesses ranges from 5 to 20. Other strength properties are such that loess makes a satisfactory foundation and construction material provided proper design measures are followed.
In general, Mississippi loess resembles loess in other areas of the world; however, it tends to contain less sand, may have a more highly variable water content, and is somewhat denser than many other loesses.