GLACIAL ISOSTASY AND THE MODERN OHIO RIVER DRAINAGE

The modern Ohio River was formed by glacial diversion of north-flowing pre-glacial streams. It is composed of segments of these ancient streams, connected wherever extensive proglacial lakes overtopped drainage divides south of the ice margin. The river's modern course, therefore, should cross the lowest pre-glacial drainage divides. However, this is not always the case. The river crosses a divide near Manchester, Ohio, that is significantly higher than a drainage divide further to the south. Furthermore, erratic boulders are found south of the accepted limit of glaciation, and higher than the southern drainage divide. The emplacement of these boulders and the overtopping of the Manchester divide have remained enigmatic since the pioneering work of Leverett and Jillson in the early 20th century. Here, we use a simple elastic plate model for lithospheric flexure, loaded by an ice sheet with a semi-empirical square-root profile, to reconstruct paleo-topography near the former ice margin. We show that glacial loading of the crust was probably sufficient to lower drainage divides near the ice margin, causing proglacial lakes to spill over previously elevated divides. Similarly, the high ground on which the erratics rest was lowered, allowing the boulders to be deposited by icebergs. Post-glacial rebound subsequently lifted both the old drainage divide and the ice-rafted boulders to their present positions. The modern Ohio River thus follows a course near the ice margin, rather than one that crosses lower drainage divides to the south.