 Arkansas County is located in the Arkansas Delta (in Arkansas, usually referred to as "the Delta") a subregion of the Mississippi Alluvial Plain, which is a flat area consisting of rich, fertile sediment deposits from the Mississippi River between Louisiana and Illinois.Within the Delta, Arkansas County is almost entirely within the Grand Prairie subregion, historically a flat grassland plain underlain by an impermeable clay layer (the Stuttgart soil series).Prior to the 19th century, flatter areas with slowly to very slowly permeable soils (often containing fragipans) supported Arkansas's largest prairie, covered in prairie grasses and forbs, with oaks covering the low hills and ridges, and pockets of floodplains with bottomland hardwood forests.