Reevaluating Evidence for the Pre-lllinoian Entrenchment of Northeastern Iowa River Valleys

Previous workers have proposed a pre-Illinoian age for the deepest bedrock entrenchment of northeastern Iowa valleys. Lines of evidence used to support a pre-Illinoian age of entrenchment are: (1) deep bedrock-entrenchment interpreted to take long periods of time, (2) valleys filled with red-weathered drift , which was interpreted as old , (3) differences in cross-valley profiles supposedly related to glacial advances, (4) thick beds of gravel could only be deposited by outwash streams, and (5) glacially buried (drift-filled) bedrock valleys exist outside of the Paleozoic Plateau to the west and south. These inferences are questionable because: ( 1) erosion (entrenchment) rates are variable, (2a) valleys were interpreted as anything below the controversial peneplains or highest uplands, (2b) red oxidation colors can occur quickly in coarse-grained sediments, (2c) drift is an ambiguous term meaning till and/or outwash (stratified sediments), (3) crossvalley profiles are related to the erosional resistance of the bedrock, (4) gravels can be deposited by non-glacially derived waters, and (5) rivers of the Paleozic Plateau cut across (and are therefore younger than) the glacially buried valleys to the west. More recent evidence indicate that the river valleys of Iowa\u27s Paleozoic Plateau may have been at least partially entrenched after the last local pre-Illinoian glaciers receded