feet) . The accuracy of the position fixes using the dead-reckoning and 

 one-range method is estimated to be ±1.8 kilometers (1 nautical mile) in 

 the worst case. 



III. RESULTS 



1. Geologic Character of the Study Area . 



a. Bottom Topography . Depths in the study area generally increase 

 lakeward and at the lakeward limit of survey vary from about 18 to more 

 than 61 meters (60 to 200 feet). The most prominent lake floor relief 

 features were submarine ridges or bars which vary from subtle, low re- 

 lief undulations to distinct ridges with relief up to 6.1 meters. These 

 shore-parallel features occur throughout the study area but are most 

 common in the northern half. Internal reflectors in some of the larger 

 ridges indicate that their internal bedding consists of foreset series 

 with accretion progressing in a lakeward direction (e.g., profile 9 in 

 App. A). Cores indicate that most, if not all, of these ridges are com- 

 posed of unconsolidated sand. Near Pentwater and in the Holland area 

 (Fig. 1) the lake floor is roughened by sets of sand waves up to 1.5 

 meters (5 feet) high. These may be ephemeral features which reshape or 

 shift position in response to prevailing hydrodynamic conditions; how- 

 ever, study data provide no evidence on this supposition. Other lake 

 floor features consist mostly of aggregates of submarine hills and ridges 

 which are highly irregular in distribution, shape, and relief. This 

 topography suggests a relict glacial surface. Areas of smooth almost 

 featureless lake floor occur interspersed with the topographically 

 irregular areas throughout the study area. 



b. Subbottom Structure . The seismic reflection profiles for this 

 study reveal some elements of the shallow geological framework of south- 

 eastern Lake Michigan. Deeper features associated with pre-Pleistocene 

 bedrock are generally below the depth of penetration of the seismic 

 systems; however, it is the shallow reflectors that are most important 

 in sand resources investigations and shallow reflector continuity is 

 reasonably good throughout the study area. In general, the maximum depth 

 of penetration and reflector continuity is greatest in the southern half 

 of the area where acoustically transparent silt often occurs. Sand is 

 dominant in the northern half except in offshore areas where thick seg- 

 ments of lakp muds often underlie the bottom. 



The absence of subbottom reflectors on the records or very shallow 

 penetration may have been caused by the impenetrability of surficial or 

 near-surf icial deposits. Alternately, penetration may have been quite 

 deep but the section penetrated was acoustically homogeneous and lacked 

 suitable reflecting surfaces. Probably both factors are involved at dif- 

 ferent places. More powerful sound sources would reveal subbottom reflec- 

 tors in the former case, but not in the latter case. 



The interface between the first or surface sediment layer and under- 

 lying sediments is of particular interest in sand resource studies because 



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