The Yarmouthian stage and the Yarmouth Interglacial were part of a now-obsolete geologic timescale of the early Quaternary of North America.
Later, the Yarmouth (Yarmouthian) stage in Illinois was defined on the basis of the Yarmouth Paleosol (Soil), developed in the surface of what were thought at that time to be "Kansan" glacial tills, and buried by Illionian glacial tills of the Glasford Formation in southeast Iowa and east-central Illinois.
[5][6][7] For example, the so-called "Kansan" glacial sediments in which the Yarmouth Soil developed are now known to date to different periods of glaciation depending on where it is examined within the Midwest and other parts of North America.
Elsewhere in North America, as in Illinois, the Yarmouth Soil also has developed over a variable number of multiple glacial - interglacial cycles.
Pollen samples and wood recovered from these deposits indicate that the vegetation consisted of a Picea-Larix forest and that climate was full-glacial rather than interglacial during their accumulation.