All the soils of Iowa without exception are, in respect to their origin, referable to one or the other of four easily distinguishable classes, which, are to be found in plainly marked areas. These are:
1. Geest, or soils resulting from the secular decay of indurated rocks.
2. Soils Of Fluviatile Origin, or stream made soils (alluvium).
3. Soils Of Aeolian Origin, or wind made soils (loess).
4. Soils Of Glacial Origin, or ice made soils (till).
Stevenson, W. H.; Christie, G. I.; and Willcox, O. W.
"The principal soil areas of Iowa,"
Bulletin: Vol. 7
, Article 1.
Available at: https://lib.dr.iastate.edu/bulletin/vol7/iss82/1