The Sangamonian Stage, originally the Sangamon interglacial stage, is defined on the basis of the Sangamon Soil, a paleosol, which is developed in contemporaneous colluvium and older glacial tills and loesses and overlain by Wisconsinan loesses or tills. Although originally described from water wells in northwestern Sangamon County, Illinois, the current type sections for the Sangamon Stage are the Rochester section in eastern Sangamon County and the Chapman section in Morgan County, Illinois. In the Rochester Section, the Sangamon Soil is developed in Sangamonian colluvial sediments, called "accretion gley", that accumulated contemporaneously with the development of the Sangamon Soil. In the Rochester section, the Sangamon Soil is developed directly in Illinoian glacial till and overlain by Roxana Silt, the oldest of the two regional Wisconsinan loesses.
In its typical and broadest usage, the Sangamonian Stage is equivalent to all of Marine isotope stage 5 between 75,000 and 125,000 BP. Although it includes the same time span, the Sangamonian Stage (sensu lato) is not temporally equivalent to the Eemian in Europe. In its much less common usage, the Sangamonian Stage (sensu stricto) is equivalent to Marine Isotope Substage 5e and the Eemian. In case of this usage, Marine Isotope Substages 5a, 5b, 5c, and 5d are collectively referred to as the Eowisconsinan Stage. In its broadest sense (sensu lato), the Sangamonian Stage precedes the Wisconsinan (Wisconsin) Stage and follows the Illinoian Stage in North America.
Research concerning the age and degree of development of the Sangamon Soil demonstrates that it actively developed, at the least, over all of Marine Isotope Stage 5, which is a period of time from 125,000 to 75,000 BP. Unlike Europe, the development of ice sheets in Canada was limited during Marine Isotope Substages 5b, 5c, and 5b and either completely disappeared or were greatly reduced in size during Marine Isotope Substage 5a.  Because of the continuous development of the Sangamonian Soil in the Midwest and the limited development of ice sheets in North America during this marine isotope stage, the Sangamonian Stage, unlike the Eemian in Europe, is regarded as encompassing all of Marine Isotope Stage 5.
The start of the Sangamonian Stage is constrained by optically stimulated luminescence (OSL) dates obtained from fluvial deposits of the Pearl Formation and Illinoian glacial tills of the Glasford Formation, which fill an ancient and buried Mississippi River valley in north-central Illinois. The age of fluvial sediments overlying the youngest glacial till (Radner Member) of the Glasford Formation yield optically stimulated luminescence (OSL) dates that averaged 131,000 BP. These OSL dates demonstrate that the Illinoian Stage ended and the Sangamonian Stage started about 125,000 BP. These dates refute older dates, i.e. between 220,000 and 450,000 BP, given by older publications for the start of the Sangamonian Stage.
^ abcdHansel, A.K. and E.D. McKay, in press, Quaternary Period, in D.R. Kolata, ed., The Geology of Illinois. Illinois State Geological Survey, Urbana, Illinois.
^ abcdWillman, H.B., and J.C. Frye, 1970, Pleistocene Stratigraphy of Illinois. Bulletin no. 94, Illinois State Geological Survey, Champaign, Illinois.
^ abRichmond, G.M. and D.S. Fullerton, 1986, Summation of Quaternary glaciations in the United States of America, Quaternary Science Reviews. vol. 5, pp. 183-196.
^ abcdMcKay III, E.D., R.C. Berg, A.K. Hansel, T.J. Kemmis, and A.J. Stumpf, 2008, Quaternary Deposits and History of the Ancient Mississippi Valley. North-Central Illinois, Guidebook for the 51st Midwest Friends of the Pleistocene Field Trip, Streator, Illinois, May 13-15, 2005: Illinois State Geological Survey, Guidebook 35, 98 p.
^Fulton, R.J., P.F. Karrow, P. LaSalle, and D.R. Grant, 1984, Summary of Quaternary stratigraphy and history, Eastern Canada, in R. J Fulton, ed., p. 193-210, Quaternary Stratigraphy of Canada -- A Canadian Contribution to IGCP Project 24, Geological Survey of Canada Paper, no. 84-10.
^Curry, B.B., and M.J. Pavich, 1996, Absence of Glaciation in Illinois during Marine Isotope Stages 3 through 5. Quaternary Research. v. 46, no. 1, p. 19-26.
^Grimley, D.A. , L.R. Follmer, R.E. Hughes, and P.A. Solheid. 2003, Modern, Sangamon and Yarmouth soil development in loess of unglaciated southwestern Illinois. Quaternary Science Reviews. 22 no. 2-4, p. 225-244.
^Clague, J.J., D.J. Easterbrook, O.L. Hughes, and J.V. Mathews, 1992, The Sangamonian and Early Wisconsinan Stages in western Canada and Northwestern United States. in Clark, P. U., and Lea, P. D., p. 253-268, , The Last Interglacial-Glacial Transition in North America. Special Paper no. 270, p. 171-184. Geological Society of America, Boulder, Colorado. ISBN978-0-8137-2270-2
^Lamothe, M., M. Parent, and W.W. Shilts, 1992, Sangamonian and early Wisconsinan events in the St. Lawrence lowland and Appalachians of southern Québec, Canada. in P.U. Clark and P.D. Lea, eds., p. 171-184, The Last Interglacial-Glacial Transition in North America. Special Paper no. 270, p. 171-184. Geological Society of America, Boulder, Colorado. ISBN978-0-8137-2270-2
^Hintze, L.F., 1973, Geologic History of Utah. Brigham Young University Research Studies, Geology. v. 20, Part 3, no. 8.
^Ericson D.B., and G. Wollin, 1968, Pleistocene Climates and Chronology in Deep-Sea Sediments. Science. v. 162, no. 3859, p. 1227-1234.
^Wornardt. W.W., and P.R. Vail , 1991, Revision of the Plio-Pleistocene Cycles and their Application to Sequence Stratigraphy and Shelf and Slope Sediments in the Gulf of Mexico. Gulf Coast Association of Geological Societies Transactions. v. 41, p. 719-744.
Ehlers, J., and P.L. Gibbard, 2004a, Quaternary Glaciations: Extent and Chronology 2: Part II North America, Elsevier, Amsterdam. ISBN0-444-51462-7
Gillespie, A.R., S.C. Porter, and B.F. Atwater, 2004, The Quaternary Period in the United States. Developments in Quaternary Science no. 1. Elsevier, Amsterdam. ISBN978-0-444-51471-4
Sibrava, V., Bowen, D.Q, and Richmond, G.M., eds., 1986, Quaternary Glaciations in the Northern Hemisphere, Quaternary Science Reviews, vol. 5, pp. 1-514.