Siesta Key is a pronounced drumstick barrier island with multiple generations and orientations of beach-ridge sets. Development is essentially complete on the northern part of the island and has included dredging of numerous channels to provide water access for residents. These activities have obscured the geomorphology of the island. Nevertheless, Stapor and others (1988) were able to define several sets of beach ridges and obtained numerous radiocarbon dates of shells from these ridges. They found that the oldest beach-ridge set is 3,000 YBP and is located near the center of the island, landward of Point OÕRocks (see aerial photo below), where dates of 4,300 to 1,900 YBP have been obtained from "beachrock" that extends from present mean sea level to -2.5 m (Spurgeon, 1997).

The stratigraphic cross section (lower right) extends across the northern, widest portion of Siesta Key where Stapor and others (1998) determined that two ridge sets had ages of 1,000-500 YPB on the landward portion, and less than 500 YBP on the Gulfward set. Clean sand and shell make penetration difficult, and only one of the cores on the cross section penetrated into a distinctly pre-island unit. This organic-rich, muddy sand is at about 4 to 5 m below present sea level and is interpreted to represent a vegetated paralic environment. Overlying the muddy sand is a series of interfingering facies that represent backbarrier, washover, beach and nearshore environments progressing from the landward, older side of the island to the Gulfward, younger side. Dune ridges cap the island.

The ebb-tidal delta at Big Sarasota Pass has been cored in detail (Kowalski, 1995) and shows two distinct facies; shelly sand and sandy shell gravel. Gravels represent channel-margin and floor environments and sand dominates on the shoals and platform of the tidal delta.