Until very recently, geologists working in Iowa have been content to refer the various boulder clays represented in the state to two till sheets, a so called "upper" and "lower," separated in many places by the "forest beds" of McGee, or in other localities by gravels, often in conjunction with a vegetal horizon, the Aftonian of Chamberlin.
Proceedings of the Iowa Academy of Sciences
©1896 Iowa Academy of Science, Inc.
Beyer, S. W.
"Evidence of a Sub-Aftonian Till Sheet in Northeastern Iowa,"
Proceedings of the Iowa Academy of Science, 4(1), 58-62.
Available at: https://scholarworks.uni.edu/pias/vol4/iss1/11