III. BOTTOM DEPOSITS 



1. Introduction . 



Shale, till, and postglacial sediment ranging from sand to mud make up most 

 of the bottom deposits. Shale underlies most of the Ohio part of Lake Erie. 

 The shale is overlain by glacial tills that are overlain by postglacial deposits. 

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

 tills and postglacial deposits. 



2. Shale . 



The contact between the Devonian limestones and dolomites to the west and 

 the Devonian shales to the east lies between Huron and Sandusky (Bownocker, 

 1920). The northeast trend of this contact, a gentle southeast dip, and shore 

 and nearshore exposures indicate that the Devonian Ohio Shale underlies most of 

 the survey area. In northern Ohio, the shale is made up of two carbonaceous, 

 blue-black shale members separated by a largely noncarbonaceous, blue-gray, 

 silty shale member (Hoover, 1960) . 



Shale is exposed close to the shore in a continuous 2-kilometer-wide band 

 between Conneaut and Fairport Harbor, in a broken 1- to 2-kilometer-wide band 

 between Cleveland and Lorain, and in a continuous 2- to 3-kilometer-wide band 

 off Vermilion. The echo character of the shale surface on the seismic records 

 ranges from smooth to irregular (Fig. 11). In the subsurface, the shale reflec- 

 tor could be mapped along most of the tracklines; however, the reflector was 

 not apparent in places off Ashtabula, Fairport Harbor, Cleveland, and west of 

 Vermilion (Fig. 13). 



The shale surface slopes lakeward between Conneaut and Ashtabula at about 

 10 meters per kilometer. West of Ashtabula, the slope is about 5 meters per 

 kilometer, decreasing to about 2 to 3 meters per kilometer at Fairport Harbor 

 and then increasing to about 10 meters per kilometer at Moss Point. The seismic 

 records do not show a shale reflector between Moss Point and west Cleveland. 

 From west Cleveland to Avon Point the slope is 15 to 20 meters per kilometer; 

 from Avon Point to west Vermilion the slope is gentler, about 4 to 6 meters per 

 kilometer. The irregular nearshore shale surface characterized by these 

 variable slopes is further exemplified by a major buried river valley (about 

 90 meters deep) entering Lake Erie at Cleveland (Crowell, 1979). In addition, 

 there are major valleys eroded into the shale at Huron (Stein, 1962) and 

 Lorain (Free, 1962) . 



3. Till . 



Till overlies the Devonian Ohio Shale over most of the region covered by 

 the tracklines (Fig. lA) . It can generally be characterized as a hard, stony, 

 unstratified deposit interpreted as basal till (Fig. 15; gravelly clay in 

 App. A) or a soft, clay-rich, commonly stratified deposit interpreted as flow 

 till (Fig. 16). Some of the till sections (e.g., cores 95, 98, and 100) con- 

 tain interlaminated silts and clays resembling varves (Fig. 17; clay in App. A); 

 cores 92, 98, and 99 contain both basal and flow till. The tills are made up 

 largely of silt- and clay-size particles composed of quartz and illite. 

 Detailed information on the texture, composition, and engineering properties 

 of the till is in Appendixes B and C; Table 2 is a summary of the information. 



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