Sediments which underlie the Ten Thousand Islands have been deposited over the past 5000 years, during a more or less continuous marine transgression. Maximum sediment thickness is approximately 25 feet. Both the macrofauna and microfauna were used to help interpret the depositional environment for these sediments. The clean, slightly peaty basal sand was deposited in a lagoon with very restricted tidal circulation. When more marine waters entered the area about 3000 years ago, a chain of gastropod reefs appeared along the coastline. During the ensuing sea level rise of about six feet, the reefs have grown in size and number to form a reef barrier which has greatly influenced sedimentation throughout the Ten Thousand Islands. Sand from the adjacent shallow shelf has been transported into the reef area by the abnormally high tides and violent wave action accompanying hurricanes. A complex of bay bottom sands and silts, tidal pass sands, oyster bars, and thick mangrove peats has accumulated behind the reef barrier.
Reefs were formed by the sessile gastropod Vermetus (Thylaeodus) nigricans Dall. The wave resistant reef core consists of fused vermetid tubes in a matrix of calcareous silt. Talus deposits typically underlie portions of the reef core. Formation of reef rock was confined to the intertidal zone, a vertical distance of less than four feet. Reef cores, as much as nine feet thick, were built up during the period of sea level rise. The vermetid reefs are compared with post-Wisconsin serpulid reefs from Laguna Madre, Texas, and with rudistid reefs from the Edwards Limestone of Texas.