spaced internal reflectors which either intersect the main reflector at a low 

 angle or parallel it in a regular pattern (Fig. 11). On the other hand, the 

 till reflector generally is continuous and not as irregular nor as well defined 

 as the shale reflector. In addition, irregular but fairly continuous internal 

 reflectors are fairly common in the till (Fig. 12). The shale and till reflec- 

 tors were mapped at two confidence levels. The first confidence level was used 

 where the reflector was well defined on the seismic record. The second confi- 

 dence level was used where the reflector was not as well defined but its 

 position could be reasonably well inferred from the overall geologic setting 

 and character of the reflector. In addition, the echo character type of the 

 surface sediment, in conjunction with reference sediment samples, was used to 

 map the surface sediment. Seven surface echo character types were defined: 

 rock, till, sand, muddy sand, sandy mud, mud, and rock waste (Table 1) . 



Following the mapping of the reflectors, acoustic traveltimes were meas- 

 ured to determine sediment thicknesses (App. F) . The velocities of sound 

 through the till and postglacial sediment are used to convert these measure- 

 ments into sediment thickness. The velocity of sound in water was calculated 

 to be about 1.54 kilometers per second from measured depths at several core 

 locations (App. G) . The average velocity in the postglacial sediment was calcu- 

 lated to be about 1.3 kilometers per second in a similar way by using the thick- 

 ness of the postglacial sediment in the vibracores (Apps. H and I). A velocity 

 for the till was not calculated because none of the cores penetrated till to 

 rock; however, laboratory work by Morgan (1964) suggests that 1.8 kilometers 

 per second is a realistic velocity for Lake Erie till so this value was used 

 in the calculations. 



5. Previous Studies. 



The first comprehensive bottom sediment study of the Ohio part of the 

 central Lake Erie basin was done by Hartley (1961) . Bottom grab samples were 

 taken on 1.6- or 3.2-kilometer grids and some subbottom sampling was done by 

 coring or jetting. Hartley's work provided important data that were used 

 extensively in the planning stages of this study. Shore and nearshore deposits 

 within 610 meters of the shoreline were mapped in the 1970' s as part of DGS's 

 county shore erosion studies (Benson, 1978; Carter and Guy, 1980); this infor- 

 mation was used in the current study to map the sediment adjacent to the shore. 

 Also, several hundred kilometers of seismic reflection profiles and 32 borings 

 and vibracores were collected in a rectangular nearshore area off Cleveland 

 (Dames and Moore, 1974) . 



Three shallow seismic reflection surveys of central Lake Erie have been 

 reported (Morgan, 1964; Lewis, 1966; Wall, 1968). The works by Morgan (1964) 

 and Wall (1968) are too general to be of use, whereas the work by Lewis (1966) 

 is sufficiently detailed, particularly on the geologic history of the lake, 

 and of use. 



II. PHYSICAL SETTING 



1. Introduction . 



The Lake Erie basin is underlain by middle Paleozoic sedimentary rocks that 

 are overlain by Pleistocene- and Holocene-age deposits. The Pleistocene deposits 



