Are there any rock outcrops in Florida?

Dave,
I often vacation in Florida during the winter. I don't recall ever seeing a rock outcrop! Why is that? Where can I go to see rocks in Florida?
Joan Osborne
Cleveland, Ohio

Dear Joan,

The short story: there are a few areas of Florida with exposed bedrock. Virtually all of it is limestone. Your best bet is in the northeast part of the state. You won't find any bed rock in Miami, for instance. Much of the state is covered in unconsolidated clay or sand. The clays were eroded from the Appalachian highlands and deposited on top of the limestone by rivers entering the shallow sea. The sand is beach deposits. For roadside locations, see Roadside Geology of Florida by Brian, Scott and Means, and  Florida's Geological Treasures by Iris T. Comfort. I confess I haven't read any of these books, but they seem like a good place to start.

I found this map at http://www.floridacaving.com/pages/misc/karst1.htm

Dark brown areas are where limestone is at or near the surface.

This is a fossil coral from Florida. It is preserved

on a pillar in the Westlake Mall in downtown

Seattle (yep, no kiddin'! I write about it in my up-

coming book, Geology Underfoot in

Western Washington.)

The long story: It wasn't that long ago that all of Florida was below sea level, beneath the shallow warm waters of the Caribbean. Limestone is the fossilized remains of animals, many of them microscopic, that have shells made of calcium carbonate. Clams, oysters and crabs are large animals with calcium carbonate shells. When these animals die, their shells sink to the seafloor and accumulate hundreds of feet thick. With sufficient thickness, water is driven out of the spaces between the shell and they become cemented together (calcium carbonate makes great cement). If the seafloor is lifted above sea level, the limestone is exposed. The highest point in Florida is only 345 feet above sea level, so little erosion has cut into the bedrock to expose it. Your best bet to see rock in Florida is in a few road cuts, or in caves.

Florida is proud of its high

point, and has erected a

roadside sign so you don't

miss it as you drive by. It's near

Lakewood in Walton County.

The limestone in northern Florida is very young, around 50-60 million years old. It is exposed in the northern portion of the state only because the rocks have been slightly folded along a north-south axis which raised some of the rocks high enough to allow the clay and sand to largely erode away, exposing the rock beneath. The Florida Keys have some limestone, too, though it is much younger, only around 130-150,000 years old. Some was quarried to build the buildings of the Keys.

Limestone is dissolved in fresh water that is slightly acidic. The result is a swiss-cheese type of topography called 'karst'. The famous limestone caves of Florida result from this chemical weathering. Some caves must be explored by diving, but there are dry caves in the northern part of the state.