Dallas County is located in central Iowa, in the fourth tier of counties north from the Missouri state line and in the fifth tier east of the Missouri river. It is partly in the Wisconsin drift soil area and partly in the southern Iowa loess area. Hence the soils of the county are of glacial and loessial origin, over three-fourths of the area being occupied by the drift or glacial soils.
Stevenson, W. H. and Brown, P. E., "Soil Survey of Iowa, Report No. 39—Dallas County Soils" (1926). Soil Survey Reports. 38.