Union County is located in southwestern Iowa in the second tier of counties north of Missouri, and in the fourth tier east of the Missouri River. It is entirely in the southern Iowa loess area, and almost half of the soils of the county are of loessial origin, the remainder being of drift origin derived largely from the underlying glacial material which has become exposed through the action of erosion.
Brown, P. E.; Elwell, J. A.; Meldrum, H. R.; and Englehorn, A. J., "Soil Survey of Iowa, Report No. 68—Union County Soils" (1932). Soil Survey Reports. 52.