Cleveland to Vermilion reach, with the exception of the area offshore of Lorain, 

 there is also a basic difference at about 5 kilometers offshore; from Cleveland 

 to Lorain postglacial sediment thicknesses range from to 2 meters, whereas 

 from Lorain to Vermilion postglacial sediment thicknesses range from 4 to 5 

 meters. Offshore of Lorain, the postglacial sediment thickens to 8 meters 

 about 8 kilometers offshore and then decreases to the north as it pinches out 

 against the Pelee-Lorain ridge. Thus it appears that the nature and thickness 

 of the postglacial sediment is largely controlled by the configuration of the 

 underlying rock and till surfaces as well as by the existing wave climate. 



IV. GEOLOGIC HISTORY 



1. Introduction . 



Sedimentary deposits from the Devonian and the Quaternary periods are 

 exposed in the study area. The Devonian Ohio Shale underlies most of the area 

 except for lower Devonian carbonates near Sandusky; Quaternary glacial and 

 interglacial Pleistocene deposits and postglacial Holocene deposits overlie the 

 Devonian rocks. A good general overview of the Quaternary geologic history of 

 the Lake Erie basin is given in Sly and Lewis (1972) . 



2. Shale . 



Seismic reflection profiles extending the width of the central Lake Erie 

 basin illustrate the relatively irregular rock surface on a regional scale 

 (see Fig. 23 in Lewis, 1966). The irregular rock surface, as well as the 

 buried ancestral valleys of the Cuyahoga, Black, and Huron Rivers, indicates 

 that appreciable erosion of the shale has occurred. The relative narrowness 

 and northerly trend of these valleys, in addition to their Pleistocene fill, 

 imply that much of the erosion took place before Pleistocene Glaciation, 

 probably by streams. 



3. Till . 



The relatively smooth till surface on the seismic records is judged to be 

 the product of several Wisconsinan glacial advances and retreats with glacial 

 and periglacial processes tending to smooth and fill irregularities in the 

 earlier glacial deposits. This hypothesis is consistent with the physical 

 variations in the deposits mapped from the seismic reflection records and the 

 internal reflectors, which indicate that the till is likely made up of multiple 

 till deposits. Field studies on both sides of the lake have documented multiple 

 Wisconsinan tills (Dreimanis, 1970; White and Totten, 1979). Following the 

 classifications of White and Totten (1979) and White (1979), the olive-gray 

 basal till in the cores is interpreted to be the Titusville-Millbrook Till and 

 the yellowish-gray basal till to be either the Lavery-Hayesville Till or the 

 Hiram Till. 



The topographic lows in the till mapped off Fairport Harbor (Wall, 1968) 

 and Lorain-Vermilion (Hartley, 1960) are the most prominent features in the 

 till surface. In agreement with Wall's interpretation (1968), the lows repre- 

 sent channels cut into the till surface by eastward-flowing streams probably 

 following the retreat of the Erie glacier from the eastern end of the lake. 



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