Grundy County is located in central Iowa in the fourth tier of counties south of the Minnesota state line and in the fifth tier west of the Mississippi River. It lies partly in the Iowan drift, soil area and partly in the Southern Iowa loess area. The soils of the county are, therefore, of glacial and loessial origin, the major portion of the area being covered by the loessial soils.
Stevenson, W. H. and Brown, P. E., "Soil Survey of Iowa, Report No. 47—Grundy County Soils" (1927). Soil Survey Reports. 50.