. Coastal changes, eastern Lake Michigan, 1970-1974. Coast changes. Sample composition was strongly related to bluff type. Almost pure sand was found on both the backshore and foreshore at profile lines 4, 6, 8, 9, 11, and 14, all of which are backed by sand bluffs. Similarly, only profile lines 10, 13, 15, and 17, which have till bluffs, had high silt concentrations on the backshore (greater than 50 percent). While gravel was found in varying amounts at all profile lines, high and low concentrations were characteristic of specific lines. High percentages of gravel (geater than 40 percent) wer
. Coastal changes, eastern Lake Michigan, 1970-1974. Coast changes. Sample composition was strongly related to bluff type. Almost pure sand was found on both the backshore and foreshore at profile lines 4, 6, 8, 9, 11, and 14, all of which are backed by sand bluffs. Similarly, only profile lines 10, 13, 15, and 17, which have till bluffs, had high silt concentrations on the backshore (greater than 50 percent). While gravel was found in varying amounts at all profile lines, high and low concentrations were characteristic of specific lines. High percentages of gravel (geater than 40 percent) were found on the foreshore at profile lines 1, 3, 5, and 13. Gravel concentration was not necessarily related to bluff type. For example, the beach at profile line 1 is covered with pebbles even though it fronts a sandy dune area (Davis, 1976). A not unexpected finding is the linear relationship between (profile average) sample gravel content and median sand size shown in Figure 17 for both the foreshore and backshore data from Table 5. r-. 50 60 30 40 Gravel Content (pet) Figure 17. Linear relationship between median sand size and percent gravel by weight. When the data are averaged' monthly, as shown in Figure 18, the backshore median sand size is fairly uniform throughout the year. However, the fore- shore shows an increase during the fall months, peaking at millimeter. This trend is similar though out of phase with the lake level variations. An examination of the variation for individual profile lines indicated that this seasonal increase occurred to some extent only at profile lines 2, 5, 8, 10, 11, and 12. Seasonal variations in sample content are also shown in Figure 18. 70 From the visual estimates of heavy mineral content, it was determined that heavy minerals, predominantly magnetite, were most commonly found on the back- shore. Only 6 percent of the foreshore samples had significant amounts of magnetite compared with 13 percent of the backshore samples. The av
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