On the southwest coast of Florida a shoreline reentrant t Cape Roma D forms Greater Gullivan Bay. Evidence is available which suggests that the reentrant may be structurally controlled. The bay lies between the Cape Romano shoals to the north and the irregular, low energy, mangrove ridge shorelinz of the Ten Thousand Islands to the southeast.
Two main currents affect the area. One, the longshore current from the north, enters the bay through the shoals, whereas the other, a tidal current, enters from the south, proceeds through the mangrove island archipelago and sweeps down the bay and back out to the open Gulf.
Regionally the sediment becomes coarser seaward, a reflection of the fine material of the mangrove island shoreline and of the coarse shell material on the shelf beyond the shoals. In specific parts of the area such as the bay, shoals, islands and shelf, size of the detritus is relatively constant. The mean grain size of the carbonate, however, is variable. Sorting of the bottom sediment is best over the shoals and in the bay indica- ting the location of the most effective currents.
Per cent carbonate is lower in the bay sediments than in any other part of the region. A similar situation exists for the organic matter. The average per cent of organic matter in the sediments is about the same as that 011 the shelf.
In the lee of the shoals, at the head of the bay and in the bays of the lagoonal chain current activity is low, and a mixed sediment of quartz sand, not whole shells, silt and clay appears to be accumulating. The lower bay, because of the turbulent ebbing of the tidal current, is, in general, receiving only quartz sand. Furthermore, the shoals appear to be moving shoreward.