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Devonian shale overlain by Quaternary glacial tills and postglacial 

 deposits underlies most of the survey area. In general, the shale is exposed 

 nearest the shore and is succeeded offshore by till and postglacial deposits. 

 The shale surface is commonly more irregular than the till and postglacial 

 deposit surfaces; slopes on the lakeward dipping shale surface range from 

 about 5 to 20 meters per kilometer. 



Pleistocene tills — both basal and flow tills — also underlie most of the 

 survey area with extensive exposures between Fairport Harbor and Avon Lake 

 and off Lorain. Interlaminated silts and clays are interbedded with the 

 flow till in some cores; three cores contain both basal and flow tills. The 

 tills are made up largely of silt and clay-size particles composed of quartz 

 and illite. The till has a flatter and more uniform surface than that of the 

 underlying shale, with lakeward slopes ranging from about 1 to 4 meters per 

 kilometer. The till varies in thickness from to 26 meters and thickens 

 lakeward at about 5 meters per kilometer. 



Sand, muddy sand, sandy mud, and mud are the four principal postglacial 

 deposits. These deposits commonly lie lakeward and overlie rock and till. 

 In general, the coarser deposits lie nearest the shore. However, the two 

 principal sand deposits at Fairport Harbor and Lorain-Vermilion are well 

 offshore. Also, the finer deposits are found closer to shore and in shallower 

 water west of Cleveland. Combined postglacial deposit thicknesses range 

 from to 22 meters and like the till, the postglacial sediment thickens 

 lakeward . 



The tills were first deposited on an irregular, erosional shale surface. 

 Till deposition continued intermittently on both shale and previously 

 deposited till until eastward retreat of the last Wisconsinan glacier from 

 the Erie basin. Drainage of the lake ponded west of the glacier then exposed 

 the till to subaerial erosion which led to the formation of stream channels 

 in the till off Lorain and Fairport Harbor. Isostatic rebound of the outlet 

 then led to a rise in lake level with associated erosion and deposition along 

 the expanding lakeshore, which tended to smooth the till surface. The early 

 postglacial (Holocene) deposits, which accumulated during the rise in lake 

 level and cover the underlying till and shale, were deposited in a complex of 

 fluvial, deltaic, and lacustrine environments. Modern lacustrine muds are 

 now being deposited over these early Holocene deposits. 



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