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

K and L have crest elevations of approximately 2.5 m (Stapor and Tanner, 1977). The ridge sets
have been assigned relative ages inferred from the erosional truncations and based on field
mapping and aerial photography. Archaeological evidence and a few scattered radiocarbon dates
add corroborative evidence that the ridge sets developed in the order proposed by Stapor (1973)
and shown in Figure 1.2. Few radiometric dates have been obtained prior to the current
investigation. Those that have been obtained are based on shell deposits. To date, the geologic
history of the island is based on relative dating and inference. The goal of this project was to
directly date ridge depositional ages and use this information to reconstruct the island’s complex
history.
Stapor (1973) calculated coastal sand budgets for the past 100 to 150 years based on
historic bathymetric chart comparisons and maintenance dredging data for coastal stretches
between Pensacola and Dog Island. The results indicated that the northwest coast of Florida is
characterized by many longshore drift cells and some of these cells experience little net exchange
of sand with adjacent cells or offshore regions. The bulk of the coast is eroding, with eroded
material being deposited at barrier island tips rather than being lost offshore. The stretch of coast
between Dog Island and the St. Joseph Peninsula contains at least 7 distinct cells, which
experience little net exchange of sand (Figure 2.2). Dog Island is influenced by two longshore
drift cells. One transports sand to the northeast and one to the southwest. The ends of the island
are growing, while the Gulf shoreline is eroding. St. George Island also has two longshore cells,
one transports material to the northeast end of the island and the other to the southwest towards
Cape St. George. The shoreline at the middle of the island has experienced extensive erosion as
evidenced by the presence of exposed stumps on the island’s gulf front and the presence of
washover deposits extending into Apalachicola Bay. The lagoonal side of the island has
experienced accretion. The two cells are separated by a stretch of coast that has experienced no
shoreline change between 1860 and 1943. A comparison of net erosion and deposition between
the east and south shores of St. Vincent Island, the lunate bar across West Pass and the Sand
Island region indicate that erosion and deposition rates are nearly equal. The same is true of the
St. Joseph Peninsula. The coast between Cape San Blas and St. Joseph Point contains a
southward and a northward transporting drift cell. Cape San Blas is eroding, while St. Joseph
Point is prograding. Figure 2.3 shows the shoreline changes for the barrier island chain of which
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