Off Lorain-Vermilion the surface slopes lakeward toward the Pelee-Lorain 

 ridge at about 2 meters per kilometer to a depth of 23 meters before rising at 

 the same slope. In general, the till surface is fairly flat in contrast to the 

 underlying shale; however, topographic lows on the till surface were mapped off 

 Lorain-Vermilion (Hartley, 1960) and off Fairport Harbor (Wall, 1968). 



The till thickens offshore, forming a wedge on the underlying rock; till 

 thicknesses determined from the seismic records range from to 26 meters. 

 However, data indicate a till thickness of more than 28 meters off Cleveland 

 (Dames and Moore, 1974) . Between Conneaut and Moss Point the till thickens 

 offshore at about 4 meters per kilometer. The till at west Cleveland thickens 

 offshore at about 31 meters per kilometer and then decreases uniformly west- 

 ward to about 6 meters per kilometer at Lorain (Fig. 20). 



4. Postglacial _Sedlment . 



Postglacial sediments ranging from sand to mud commonly lie lakeward of and 

 overlie rock and till (Fig. 21). The principal exceptions are off Cleveland, 

 where postglacial sediment is succeeded offshore by till, and west of Huron, 

 where postglacial sediment lies adjacent to the shore. The sediments can be 

 classified into four echo character types: sand, muddy sand, sandy mud, and 

 mud (Table 1; Figs. 22 to 25). In addition, rock waste from harbor dredging 

 was mapped off Ashtabula. In general, the sand is less than 0.5 millimeter in 

 diameter and is composed largely of subangular to subrounded grains of quartz, 

 feldspar, and rock fragments. The clay-size particles are similar in mineral- 

 ogy to the clays in the till — mostly quartz, illite, and chlorite — although 

 several samples contain montmorillonite. Macrofossils (clams and snails) were 

 common in some cores, primarily in the coarser sediment. Specific data on the 

 texture, composition, engineering characteristics, and fossils of these deposits 

 are given in Appendixes B to E. 



Between Conneaut and Fairport Harbor, muddy sand directly overlying shale 

 is generally found close to shore. This sediment is succeeded offshore by sandy 

 mud, which is commonly succeeded by mud. Between Fairport Harbor and Cleveland, 

 with the exception of the complex Fairport Harbor area, sandy mud was the only 

 postglacial sediment mapped at the lakeward ends of the traverses. Between 

 Cleveland and Vermilion mud generally lies lakeward of till or rock, although 

 coarser grained deposits lie offshore of the mud in the Lorain -Vermilion area. 

 West of Vermilion muddy sand is commonly succeeded lakeward by sandy mud. Sand 

 was mapped in three areas: Conneaut, Fairport Harbor, and Lorain-Vermilion; 

 see Williams, et al. (1980) for a discussion of the sand deposits at Fairport 

 Harbor and Lorain-Vermilion. Also, the finer sediments (mud and sandy mud) are 

 generally found closer to shore and in shallower water west of Cleveland. 



The postglacial sediment increases in thickness offshore, except off Lorain, 

 reaching 18 meters between Conneaut and Fairport Harbor, nearly zero between 

 Fairport Harbor and Lorain, and up to 8 meters thick between Lorain and 

 Vermilion (Fig. 26). The thicknesses, however, are difficult to compare 

 because of the short (<5 kilometers) seismic traverses west of Cleveland. 

 Along the Conneaut to Cleveland reach, there is a principal difference at 

 about 9 kilometers offshore; from Conneaut to Fairport Harbor postglacial sedi- 

 ment thicknesses range from 10 to 16 meters, whereas from Fairport Harbor to 

 Cleveland there is an apparent lack of postglacial sediment. Along the 



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