Figure 2. Aerial image (2004) of the Sebastian Inlet area and adjacent flood tide delta. Note white lines of impoundment dikes. Source: St. Johns River Water Management District: http://sjr.state.fl.us/programs/data.html

A second high stand about 2 meters higher than present sea level occurred approximately 30,000 years ago. By 20,000 years ago sea level was approximately 100 meters lower than present and what is now the IRL was dry land (Bader and Parkinson 1990). As sea level again began to rise, the present coast of Florida began to take its present shape. 

The present barrier island might have formed in one or a combination of two ways: spit growth across an embayment by longshore sand transport, or flooding of coastal areas behind relict beach ridges, as many barriers are thought to have formed (Cooper, 1994). The IRL probably began as an embayment that was gradually enclosed by a lengthening of the barrier island due to longshore transport of sands eroded farther north along Florida’s east coast. 

As sea level continues to rise, the barrier island migrates toward the mainland. Storm-driven overwash and flood tide delta formation transport beach and dune material into the lagoon (Parkinson, 1995). These deposits can be seen along the west sides of the barrier at the southern and northern ends of Brevard County (Fig. 3). As these deposits continue to fill in the lagoon, the open lagoon water gradually fills in to become marsh or mangrove swamp, as is seen in the northern end of IRL. The west side of the barrier island in central Brevard County shows a strong cuspate spit formation (Fig. 4), indicating a stable barrier with little or no inlet or washover formation (Bader and Parkinson, 1990). The reason for this is not known but it might result from offshore deposits of beach rock.

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