STREAM INCISION RELATED TO RAPID DRAINAGE OF PLEISTOCENE LAKE TIGHT: LEO PETROGLYPH GORGE, JACKSON COUNTY, OHIO

The stream networks of southern Ohio were complexly modified during the Pleistocene as a result first of the impoundment, and later the drainage of Lake Tight; however, the detailed drainage history of Lake Tight is poorly understood. Lake Tight, with an average depth of some 250 ft and a surface area of 9,800 mi², was impounded in pre-existing stream-incised valleys near the western edge of the Allegheny Plateau, resulting in a complex lake basin characterized by many islands and peninsulas. As a result of the drainage of Lake Tight, new channels were incised into Paleozoic strata in the form of narrow valleys and steep-walled gorges. One of these, Leo Petroglyph Gorge in Jackson County, exhibits geomorphic features which are observed in similar valleys throughout the Lake Tight basin. Leo Petroglyph Gorge is developed in the Pennsylvanian Sharon Conglomerate (Pottsville Group), a variably cemented, quartz-pebble unit with an average clast size of 5 mm and a maximum clast size of 40 mm. These characteristics of the Sharon Conglomerate are central to the development of the features observed at Leo Petroglyph Gorge. Erosion of the conglomerate, and transportation of the quartz pebbles as bed load under intense energy, led to incision, perpendicular and lateral cavitation, and possible kolking in the gorge. The rapid incision of this, and similar, gorges helps to elucidate the drainage history of Lake Tight. Geomorphic evidence of long-lasting shorelines at the 900 ft and 825 ft contours suggests an episodic drainage model, punctuated by divide breaches related to the establishment of the present-day course of the Ohio River. The absence of shoreline features below 825 ft, and the occurrence of the heads of narrow valleys and gorges consistently at or below 800 ft, indicates a rapid (potentially catastrophic) outflow model for Lake Tight.