Florida started out as part of Africa, back when Africa was part of Gondwana -- a continent that shifted and broke up into Africa and South America during the Paleozoic era (600 to 230 million years ago). Geologists have found similar sets of fossils and similar rock layers deep underneath both Florida and West Africa. They have found evidence that basement rocks in Florida and in West Africa were affected by the same major geological event about 550 million years ago.
These basement rocks are now called the Florida Block, and they are the deepest parts of what we call Florida. The Florida Block parted company from Africa's ancestor mega-continent, Pangaea, during the Triassic period (181 to 230 million years ago). Gradually, the Florida Block moved north across the Equator until it butted up against the southeastern tip of North America, where it has stayed ever since.
The top of the Florida Block is about three-quarters of a mile (1.2 km) below the eastern Panhandle's land surface and continues downward to the bottom edge of the earth's crust. Although we don't know the exact extent of the block, its northern edge is up in Georgia. Its southern, eastern, and western edges probably extend beyond the edges of the Florida Platform, a flat, U-shaped plate that sits on top of the Florida Block.
The Florida Platform supports Florida as we know it. Its boundaries are the edges of the modern continental shelf. The Florida Platform is about 300 miles wide and 450 miles long and is constructed mostly of the bodies of miniscule marine creatures. For millions and millions and more millions of years, as untold numbers of these creatures died, their remains -- primarily carbon (in the form of carbonate) -- sank to the sea floor and eventually were compressed into rock by the great weight of more zillions of their descendants as they, in turn, fell to the bottom of the sea. Imagine how many little animals lived and died and piled up and up and up to create the Florida Platform -- a 135,000-square-mile pedestal that dips down as far as 3,000 feet below sea level! You could count the stars as easily.
Right now, there's more of the Florida Platform under water than above it, which is why waters off most of Florida's shore are shallow for a long way out from land. At today's sea level, about half the platform is submerged.
The Big Picture
In the couple hundred million years since Florida merged with North America, sea levels have risen and fallen, risen and fallen, over and over, each time changing the shape and nature of Florida. When sea levels were low, the exposed sea floor became land, and some of the surface was dissolved by rain or carried off by streams. When sea levels covered the land again, new layers of sediments accumulated. And so the cycle went on for tens of millions of years.
Where ancient oceans ate away the seaward edge of the uplands, they left behind a line of cliffs, called escarpments (scarps, for short), that are now miles inland from today's shore. Wind played its part, too, sculpting sands into dunes that are now far from any beach. Ridges and uplands that are now high and dry were shaped by currents that carried away the soils at the edges of those old shallow oceans.
These old coastlines are most evident in a series of terraces, step-like surfaces of erosion representing shorelines developed by advances and retreats of the sea during the Pleistocene Epoch. One of the state's best examples of terracing is found on the eastern and southern borders of Gadsden County, along the Ochlockonee River. The elevation changes quickly (at least, for Florida), and geologists have found five steps, or terraces, each associated with a different range of elevations above sea level.
But the layers beneath our feet are not stacked like pages in a book, one on top of another, in a tidy sequence. Instead, they dip and rise, get thicker and thinner, and come and go. Geologists study the rocks in samples that well-drilling rigs bring to the surface. They identify the sequence of layers of different kinds of rock in the samples, and then do the same thing in other wells. By charting the changes in the depth to similar layers in different samples, geologists map the spatial relationships between the various layers. Geologists call these layers "formations." Different geologists may group these sequences differently, and these categories can change as more samples are collected.Florida -- 250 Million years in the making