Clarke County is located in southern central Iowa in the second tier of counties north of Missouri, in the fifth tier east of the Missouri River and in the seventh tier west of the Mississippi River. It is entirely in the Southern Iowa loess soil area and almost one-half of the soils of the county are of loessial origin, the remainder being mainly derived from the underlying drift, exposed thru the action of erosion.
Stevenson, W. H. and Brown, P. E., "Soil Survey of Iowa, Report No. 51—Clarke County Soils" (1928). Soil Survey Reports. 69.