Heavy mineral concentrations were evident at many of the beaches , 

 particularly after erosion periods. These lag concentrates were typical 

 of sites 8, 10, and 14, but were never found at sites 6, 12, 13, and 17. 



IX. SUMMARY AND CONCLUSIONS 



An analysis of the profile changes on the shore of eastern Lake 

 Michigan suggests the following (see also Davis, Fingleton, and Pritchett, 

 1975, pp. 54-58) : 



1. There is evidently a critical mean annual lake level beyond which 

 erosion is universal. This level is apparently about 580.0 feet in east- 

 ern Lake Michigan. Although beach erosion and adjacent bluff erosion are 

 definitely linked to high lake levels, the lake level plays a passive role, 

 not an active one. Primary evidence of this is the fact that erosion was 

 intermittent during the first 2 years, but erosion was universal in the 

 third year when mean lake level was the highest. 



2. Local factors predominate in controlling erosion and accretion 

 during most conditions. Of these factors, the nature of longshore sand- 

 bars, composition of the bluff and beach, and coastal configuration appear 

 to be most significant. Local embayments or protuberances in the shore- 

 line are significant factors. Clayey till is relatively resistant to wave 

 attack. 



3. Available aerial photos suggest that long-term changes are tied 

 closely to lake level conditions. Photos indicate that sand terraces be- 

 hind the active beach have remarkable capabilities of regeneration during 

 periods of low lake level. Large terraces can apparently form in a decade 

 or two. 



4. Storms are the dominant force, with the bulk of the coastal erosion 

 occurring in only a small percentage of the total elapsed time. As few as 

 two or three major storms per year can account for the majority of the ero- 

 sion. However, there is extremely rapid recovery of beaches between storms. 

 Only a week or two is necessary in most instances. 



5. There are obvious seasonal variations in the coastal processes. 

 Late fall and early spring are the periods of highest erosion due to the 

 increased frequency of storms. Winter ice cover affords excellent protec- 

 tion for the coast against these storms. 



6. Beach sediments are closely related to adjacent sources and are 

 generally well-sorted, medium sand. No correlation was found between the 

 nature of the sediments and coastal erosion. 



7. Based on this report, it appears that longshore bars do have some 

 effect on coastal erosion, but the cause and effect relation is not clear. 



