The maps you see here are created from high-resolution Laser Airborne Depth Sounding (LADS). The data is used to create an image depicting the actual bottom contour topography. Not only can you clearly see reef contours but you can also identify shipwrecks.
You´ll notice on many places of the outer reefs a collapsed feature on the reef top. Some scientists believe this feature was due to frame-work collapse and was inevitably covered with sand. This creates the ledges we now dive today.
The GeoHistory of SE Florida´s Reef Tract
The current SE Florida reef tract sits on top of ancient beach ridge complexes. As sea level began to rise after the last glacial period, lithified sand dunes became submerged and coral growth initiated. These linear reefs run parallel to the shoreline from Miami-Dade to Palm Beach County, forming one of the longest continuous reef structures in the Western Atlantic.
The gaps in the reefs were created by ancient rivers and inlets cutting through the beach ridges at different periods. Over time as sea level continued to rise each of the reefs (inner, middle and outer) met its demise for varying unknown reasons. The last reef to stop upward growth was the middle reef which ceased upward growth around 3,700 years ago. Since then, corals have reformed on top of the old structures, creating the current reef communities we see today.
Not all areas along the greater reef tract have all three formations present. Some areas may only have a middle and outer reef while others may have all examples represented. many divers confuse the nearshore ridge complex with the inner reef. For example, the Pompano Drop Off is actually part of a ridge complex composed of coquina and Touchdown Reef is on the inner reef which is built by fossil corals.