This graph shows the general trend of changing ice volume on the Earth over the
past 750,000 years. The extent of ice is estimated using changes in the amount
of different isotopes of oxygen found in the calcium carbonate shells of planktonic
sea creatures (foraminifera). These data are from Imbrie and others
(1984). The portions of the curve marked with "G"s are times when glaciers
may have reached into the midwestern U.S. The timing of midwestern U.S. glacial
advances prior to 40,000 years ago are not well known. The advances indicated
on this graph are taken from Richmond
and Fullerton (1986).
