Steven Dutch, Professor Emeritus, Natural and Applied Sciences, University of Wisconsin - Green Bay
|Valderan and GreatLakean Red Tills (11,000 B.P.)
Valderan and GreatLakean Red Tills (11,000 B.P.)
It seems to be a common feature of ice retreats that late surges occur. The ice returns briefly in a thin sheet, perhaps spreading rapidly because of basal melt water. The ice advance lasts only a century or two, and leaves a thin blanket of till.
The Valderan and Greatlakean advances followed this pattern. The ice retreated beyond the Upper Peninsula, allowing Lake Superior flood waters to spill over through the Whitefish-Au Train outlet, bringing with it distinctive red sediment that was spread southward by the Valderan and Greatlakean ice. The ice advance had little effect other than spreading a few meters of till across the landscape; it created no distinct moraines nor does it seem to have had much erosive effect.
Between the retreat of the main ice sheet and the brief Valderan-Greatlakean readvance, a boreal forest grew up, consisting mostly of black spruce. As the ice re-advanced, it dammed up short-lived proglacial lakes that drowned the forest, then the ice bulldozed the trees and buried them under till. This so-called Buried Forest deposit is found almost everywhere in northeastern Wisconsin under the red till. Radiocarbon dates place the buried forest at about 11,000 years B.P. Trees 60 cm in diameter and meters in length have been found; without exaggeration, one could build a log cabin from Buried Forest wood. The forest deposit contains very few animal fossils - you have to be pretty slow to be overtaken by a glacier. The animal fossils consist mostly of beetles and - fittingly - snails.
Created 4 October 2000, Last Update 11 January 2020