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

profile and deposits a ridge. Each ridge represents a paleo-sea-level position. The best estimate
of paleo-sea-level position can be assumed to be somewhere between the ridge crest and the toe
of the shoreface. The boundaries between beach ridge sets may indicate a change in sea-level
position. Where adjacent sets have a measurable elevation difference there has been a change in
sea level (Tanner et al., 1989). Figure 1.7 illustrates Tanner’s theory of beach ridge formation.
As an alternative to this theory, Curray (1996) re-proposed that offshore bars are built to sea
level when sufficient sand is supplied to the coastline. If enough sand is available, the bar
becomes emergent during periods of low surf. With the subsequent rise and fall of tides, and if
surf conditions remain low, the bar will grow in height. Taylor and Stone (1996) proposed that
during storm activity, sand from existing ridges is eroded and stored in an offshore bar. During
calm weather conditions, the sand is transported onshore and a ridge is rebuilt.
Each beach ridge plain displays the same relative time sequence. The ridge closest to the
mainland is oldest and the ridge immediately adjacent to the beach is youngest. All others fit in
a sequence between the two extremes (Tanner, 1988). Since ridges form on the beach face, they
are indicators of the orientation of the coastline and approximate the vertical position of sea level
at the time of their deposition (Donoghue and Tanner, 1992; Fernald and Purdum, 1992; Tanner,
1988). A beach ridge plain can be divided into sets. The different orientations of the sets
indicate changes in oceanographic conditions (sea level rise, changes in river mouth position) or
a change in the rate of sediment supply (Curray et al., 1969; Dominguez et al., 1992).
Many of Florida’s barrier islands are composed of beach ridges. The organization and
the physical character of the beach ridges that make up the Holocene beach ridge plains of the
northwest Florida barrier islands can be employed to study the depositional history of the
barriers.
Sea-Level Change
Sea level is never static, but rather has fluctuated throughout geologic time. Several
factors can cause these fluctuations including changes in the volume of seawater (i.e. thermal
expansion and contraction, or advance and retreat of ice sheets), rise and fall of the land or ocean
floor (i.e. tectonics, isostatic rebound) and variations in the earth’s gravitational field (the
gravitational forces between the sun and moon will change the distribution of the earth’s water
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