the florida state university college of arts and sciences evolution of ...

Vincent Island has changed little over the long history of the island. The sand diameter has
remained in the fine sand range and the standard deviation values have fallen in the “well-sorted”
category, using the terminology of Friedman (1962). There is only one exception to this rule, a
sample in set C, which falls in the “moderately well-sorted” range. Other than that single
sample, the only real trend revealed by the scatterplot is that the sand comprising the younger
sets is generally slightly better sorted than that found in the older sets. Like the ridge set height
results, this finding also implies an increase in wave energy in recent years. This will be further
discussed in Chapter 5.
Individual sample kurtosis values range from 2.6 to 8.5. The highest values are found in
samples from the north side of the island in the older ridge sets, while the lowest values are seen
in samples from the south side. Figure 4.30 is a scatterplot of suite mean vs. suite kurtosis. The
southern beach ridge sets (G, H, I, J, K, and L) have the lowest suite mean kurtosis values. The
kurtosis values for these sets are statistically indistinguishable, with values that fall between 3.5
and 3.9. The northern (older) beach ridge sets (A, B, C, D, E and F) show a wider spread in
kurtosis values (4.2 to 5.6) than the southern sets. Kurtosis values exceeding 5.0 are seen in the
ridge sets north of Big Bayou (shown in Fig. 4.23). These relationships can be seen in Figure
4.31 Some (e.g., Tanner, 1990) believe that kurtosis is a function of the hydrodynamic regime of
an area, with higher kurtosis values resulting from lower wave energy, and vice-versa. If this is
the case, the trends seen on St. Vincent Island could suggest that the older ridges on the island
were formed during a time of lower wave energy than the younger sets. The data shown in
Figure 4.30 do not show any correlation between mean grain size and kurtosis.
Skewness values range from –0.7 (strongly coarse skewed) to 1.3 (strongly fine skewed).
Sets B, C, E and H are positively (coarse) skewed while the remaining sets are negatively (fine)
skewed. Figure 4.32 is a scatterplot of suite mean versus suite skewness. There does not appear
to be a relationship between skewness and mean grain size.
Sorting and kurtosis values were compared to the mean ridge set heights shown in Figure
4.26. Figure 4.31 shows the relationship between age and suite kurtosis while Figure 4.33 is a
plot of relative age versus suite standard deviation. These figures demonstrate that kurtosis and
sorting increase with age. It appears that ridge sets on the southern or Gulf (younger) side of the
island (Sets G through L) have lower sorting and kurtosis values than ridges on the north or bay
side of the island (Sets A through F). This implies that the sand making up the ridges on the
southern portion of the island is better sorted than sand making up the ridges on the northern
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