PLEISTOCENE GLACIATION OF OHIO, U.S.A.: CURRENT STUDIES ON THE STRATIGRAPHY OF GLACIAL DEPOSITS AND RATES OF RETREAT OF WISCONSINAN ICE

A relatively new map of the Quaternary geology of Ohio and older paleodrainage maps serve as a base maps for re-evaluating the late Pleistocene of Ohio. A pre-Illinoian ice advance reversed the drainage of Ohio that was dominated by a large northwesterly to westerly flowing river system referred to as the Teays River. Pre-Illinoian deposits underlie Illinoian deposits in southwestern Ohio and in deep buried valleys. Illinoian glacial deposits crop out beyond the Wisconsinan glacial limit in central and southwestern Ohio. Ohio was ice free from the Sangamonian Interglaciation to the Late Wisconsinan advances. The first Late Wisconsinan advances deposited sandy tills throughout the state, whereas younger Wisconsinan tills are clay rich.

Recent research in Ohio has sought to differentiate among Illinoian tills and to obtain subsurface data on the extent of these units. Current research into the origin and age of the interlobate area in northeastern Ohio has begun. Dating the timing of glacial retreat from the Wisconsinan limit through the use of bog bottom dates is a continuing process. Because the glacial lobes in Ohio reached closer to the equator than any ice sheet during the last glaciations, the retreat pattern provides insights to minor environmental changes. The ice sheet remained in a nearly fully extended position for approximately 5,000 years with readvances reaching similar limits until 17,500 14C yr BP.

Recent dating of multiple small depressions (n ~ 25) along a former flow line approximately 100 km from the outer boundary and extending up ice shows that ice retreat had reached the location of a major moraine boundary by 16,000 14C yr BP. This significant retreat averaged ~70 m/yr, and in the Scioto sublobe deposited several small moraines. All geomorphic landforms (frost-wedge polygons on drift) and sediment indicators (extensive loess deposition), as well as regional climate indicators (e.g. Greenland ice cores), indicate that this retreat occurred during conditions similar to those of the full glacial. Although the details remain sketchy, the next major event in deglaciation occurred as the ice margin retreated north of a continental divide; a series of glacial lakes developed in the Lake Erie basin starting at ~14,000 14C yr BP as the ice margin retreated along the axis of the basin.