As glaciers expanded, they consumed bodies of water, including the shallow tropical seas covering Florida, causing the land to emerge. Then, as they melted, the seas returned, submerging the Florida peninsula. Each time the glaciers melted and the seas returned, the water level was lowered because much of the land mass was slowly rising. The lower areas of Florida, however, still held water, creating inland lakes such as enormous Lake Okeechobee. During the interglacial periods when the land was underwater, millions of plants and animals decomposed and deposited calcium carbonate onto the sea floor.
Limestone composed of oolites is very porous and spongelike.
Today, this oolitic limestone forms the gently sloping bedrock of the Everglades. The foundation for the Florida Keys was laid 100,000 years ago by billions of tiny coral animals. As the land mass of southern Florida began its slow rise from the sea, the reefs also began to emerge. What resulted were the many islands of the Florida Keys, constructed mainly of fossilized coral rock. You will see these keys in Biscayne Bay.