Polk county is located in south central Iowa, in the fourth tier of counties north of the Missouri state line, the sixth county east of the Missouri river and the seventh county west of the Mississippi river. It is partly in the southern Iowa loess and partly in the Wisconsin drift soil area and hence its upland soils are made up of loess and drift, the drift types occurring to a very much larger extent than do the loess soils.
Stevenson, W. H. and Brown, P. E., "Soil Survey of Iowa, Report No. 24—Polk County Soils" (1922). Soil Survey Reports. 28.