Marion County is situated in south-central Iowa in the third tier of counties north of the Iowa-Missouri line. Knoxville, the county seat and largest town, is located 38 miles southeast of Des Moines, the state capital, and 51 miles northwest of Ottumwa. With the exception of 55 square miles in the northeastern corner, all of the county lies within the drainage basin of the Des Moines River. The region is a part of a loess-mantled drift plain, once apparently level over extensive areas but now generally rolling because of the dissection of the older land surface by streams. Occasional flat upland divides still mark the level of the former plain.
Simonson, Roy W. and Benton, T. H., "Soil Survey of Iowa, Report No. 81—Marion County Soils" (1941). Soil Survey Reports. 71.