Northern Ohio furnishes unrivaled opportunities for estimating the amount and rate of erosion since the final withdrawal of Wisconsin ice from that region. The watershed between the basin of the Great Lakes and that of the Mississippi Valley is nowhere more than 100 miles south of Lake Erie, averaging not more than 50 miles. The elevation of the cols through which the drainage passed into the Mississippi Valley, as the ice retreated northward from the watershed, ranges from about 300 feet above Lake Erie, at Warren, Ohio, and Lodi, to 200 feet at Fort Wayne, Indiana. Of these, the col at Fort Wayne is most important in regulating the level of the temporary glacial lake which was formed north of the watershed. The occupation of this col by the drainage stream was so long that a well-defined shoreline, 200 feet above Lake . . .