THE COMPLEX EARLY TO MIDDLE PLEISTOCENE HISTORY OF LAKE MONONGAHELA: SIMPLE CONCEPTUAL MODELS TO GUIDE FUTURE RESEARCH

Late 19^th Century observations of high-terrace deposits in the upper Ohio River basin led GSA founder I.C. White to identify major reorganization of preglacial drainage, including impoundment of Lake Monongahela. White’s lake concept involved a glacial dam at its northern limit with high-stand outflows through >335 m (1100 ft) elevation cols along the southwestern edge of the Monongahela River drainage divide. Frank Leverett and others interpreted a less-extensive lake, impounded by outwash deposits derived from glaciated areas in northwestern Pennsylvania and eastern Ohio. Consensus holds that widespread major impoundment was Illinoian or older, although local fine-grained Wisconsin-aged deposits unconformably overlie old lake sequences. Nonetheless, Lake Monongahela map reconstructions drape White’s 335 m lake elevation upon modern topography, impounded by an ice margin near the Wisconsin glacial maximum at the Ohio-Pennsylvania border.

Incision and erosion of the terrace deposits, plus multiple cycles of pedogenesis, colluviation, and cryoturbation guarantee resolving Lake Monongahela’s history will remain an ambitious undertaking; yet conceptual models informed by 20^th and 21^st Century advances can help target high-yield areas for future research. High-terrace stratigraphic sequences (Carmichaels Formation) include 12 or more facies, and many sites record multiple inundations, so it is unlikely Lake Monongahela was a one-time phenomenon. Different valley segments in different parts of the lake had different development histories. Hence, research should progress expecting terraces in the unglaciated Monongahela River basin to have longer records than those in the more recently integrated, outwash-corridor Allegheny River valley or the flow-reversed, outwash-corridor Steubenville paleo-drainage system. For example, an apparent lack of paleomagnetically reversed sediments along the Alleghany River does not negate the veracity of reversed sediments at ~300 m (980 ft) elevation upper Monongahela sites at Mannington and Morgantown. Inclusion of hypothetical effects of isostacy, forebulge migration, outwash accumulation, and paleo-flooding into a model of paleo-col elevations and lake levels suggests White’s and Leverett’s paleo-lake scenarios may have operated at different times, or in different lake segments at the same time.