Sand dunes

But, we soon learned that the Bahama Banks are a collection of ancient sand dunes formed a million years ago. As the North American glaciers melted, the water level rose, leaving only the tops of these vast sand dunes exposed.  Rain water dissolved the surface layers of the calcium carbonate rich sand and trickled down between the sand to the sea, creating a fresh water layer, called the lens, which provides limited fresh water to the vegetation and animals living on these barren surfaces.   The sun causes chemical changes to occur in the sand and turns the loose sand into black porous rock. The forces of wind and rain continue to sculpt these ever changing dunes.