Casey Key is a distinctly wave-dominated barrier that is very narrow throughout most of its extent. This long barrier has a straight shoreline, and has been experiencing erosion problems along most of its length. The barrier is separated from the mainland by only a few hundred meters of shallow estuary.

Three of the cores penetrated Miocene limestone that is shallow (-1.5 m) at the mainland shoreline and dips Gulfward to -6 m a few hundred meters offshore from the island (Yale, 1997). The organic-rich, muddy sand representing a vegetated paralic environment immediately overlies the limestone; no Pleistocene sediments were penetrated. This is typical of most of the coastal system in the Sarasota Bay and Little Sarasota Bay areas (Davis et al., 1989). Muddy, shelly sand is above this facies at most places, and probably represents old washover deposits that have been thoroughly bioturbated. The facies representing the nearshore, beach, and dune environments comprise the barrier itself. The most Gulfward core penetrated the clean, shelly sand of the nearshore, which is in sharp contact with an oyster-rich muddy, sandy gravel. These oysters were radiocarbon dated at 4,670 YBP, and are much older than any of the present barrier islands. This indicates that either there was an older barrier Gulfward of these oyster-rich deposits, or that the oysters were living in a low-energy, open environment where salinities were probably below normal marine levels. The latter situation is quite similar to the extant environment in the Big Bend area of the Florida Gulf Coast located immediately to the north of the study area.