Yarmouthian (stage) - Reassessment

Reassessment

Because of the flaws with the early conceptual climatic and chronological framework for Midwestern glacial - interglaciations, the Yarmouthian (Yarmouth) Interglacial (Stage) completely lacks any meaning or usefulness in North American glacial - interglacial nomenclature. First, the incorrect presumption that there was only one major ash bed within the Midwestern United States lead to the misclassification of Middle to Early Pleistocene interglacial deposits containing the 602,000 year-old Lava Creek B volcanic ash; the 1,293,000 year-old Mesa Falls volcanic ash, and the 2,003,000 year-old Huckleberry volcanic ash as being the same age. Because it was thought that there was only one significant ash bed, not three of them, any nonglacial deposit containing an ash bed, regardless of its true age, were thought to be the same age. As a result, the nonglacial desposits attributed in the scientific literature as dating to the Yarmouthian (Yarmouth) Interglacial (Stage) consist of an assemblage of sediments deposited during a number of differing interglacial periods, including some dating to 0.60, 1.22, and 2.02 million years ago. An additional problem is that recent research demonstrates that the glacial tills, the Glasford Formation, of the Illinoian Stage are limited in age to Marine Isotope Stage 6. Thus, within Illinois and adjacent parts of Iowa, the Yarmouth Soil (paleosol), which defines the Yarmouthian (Yarmouth) Interglacial (Stage) in Illinois, spans a period of geologic time equivalent to Marine Isotope stages 7, 8, 9, 10, and 11. Elsewhere in North America, as in Illinois, the Yarmouth Soil also has developed over a variable number of multiple glacial - interglacial cycles. Thus, the presumption that the Yarmouth Soil, by which the Yarmouthian (Yarmouth) Interglacial was later defined, represents a single interglacial stage or period has been completely discredited.

Also, the "interglacial" deposits used by Leverett to originally define the Yarmouthian (Yarmouth) Interglacial in 1898 actually consist of interbedded glacial tills, diamictons, peats, sands, and silts, that are part of the Kellerville Till Member of the Glasford Formation. 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. Thus, the criteria, i.e. (1.) its sediments being all of the same age, (2.) it consisting of sediments deposited during a single interglacial period, and (3,) as originally defined by Leverett in 1898 consist of interglacial sediments, by which the Yarmouthian (Yarmouth) Interglacial (Stage) was defined and recognized have all been found to be false.