profiles, taken in 1977, and 58 vibracores ranging from 0.7 to 6.1 meters long, 

 taken in 1978. About 23 percent of Ohio's part of Lake Erie was covered by the 

 survey . 



Shale underlies most of the survey area. The shale is overlain by glacial 

 tills and by postglacial deposits. The surfaces of all the units slope lake- 

 ward. In general, the shale is exposed nearest the shore and succeeded offshore 

 by till and postglacial deposits. The shale surface is irregular in comparison 

 with the till and postglacial deposit surfaces and its slopes range from 5 to 

 20 meters per kilometer. Basal till and flow till were identified in many of 

 the cores. An olive-gray basal till was found over much of the area and a 

 yellowish-gray basal till was found in the Lorain-Vermilion area. Some of the 

 tills contain interlaminated silts and clays (varves?), and three cores contain 

 both basal till and flow till. The tills are made up largely of silt- and 

 clay-size particles composed of quartz and illite. The till surface is much 

 flatter and more uniform than the underlying shale surface and its slopes range 

 from 1 to 4 meters per kilometer. Overall, the till thickens lakeward at about 

 5 meters per kilometer. 



On the basis of cores and echo character on the seismic records, four 

 principal postglacial deposits were defined: sand, muddy sand, sandy mud, and 

 mud. At about 9 kilometers offshore, the stretch from Conneaut to Fairport 

 Harbor is characterized by postglacial sediment thicknesses ranging from 10 to 

 16 meters; the stretch from Fairport Harbor to Cleveland is characterized by a 

 lack of postglacial sediment. At about 5 kilometers offshore, the area from 

 Cleveland to Lorain is characterized by postglacial sediment thicknesses ranging 

 from to 2 meters; from Lorain to Vermilion the thicknesses range from 4 to 5 

 meters. In general, the finest sediment is found closer to shore and in 

 shallower water west of Cleveland. 



The irregular shale surface in concert with the characteristics of the 

 buried river valleys implies an irregular surface before Pleistocene Glaciation. 

 The relatively smooth till surface is likely the product of several Wisconsinan 

 glacial advances and retreats. The physical variations in the till in the cores, 

 as well as the internal seismic reflectors in the till, indicate that multiple 

 tills were emplaced. The topographic lows on the till surface off Fairport 

 Harbor and Lorain-Vermilion imply stream erosion, and the scarcity of glacio- 

 lacustrine clay implies stream and wave erosion following retreat of the last 

 Wisconsinan glacier. The postglacial deposits accumulated during a rising lake 

 level from fluvial, deltaic, and lacustrine processes. The sedimentary deposits 

 on the lake floor reflect Pleistocene and Holocene processes as well as the 

 configuration and nature of the underlying rock and till. Present-day waves 

 and currents tend to erode and rework the shale, tills, and postglacial 

 deposits in high-energy environments. Subsequent deposition of these sediments 

 in low energy environments is obscuring evidence of previous low water stages. 



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