For millions of years, the area we now call Florida was covered by oceans. During this time sea creatures such as snails, clams, corals, sea urchins, sand dollars, fish, and others, lived and died. Their remains slowly built up layers of sediment thousands of feet thick. These sediments are the limestone, shell, and dolomite formations that are mined today.
Limestone is a sedimentary rock which is more than 50% calcite (calcium carbonate, CaCO3) and dolomite (calcium magnesium carbonate, CaMg(CO3)2). It can vary widely in consistency and hardness. In Florida it can be found as a soft chalky material with microfossils, a hard recrystalized mass, a grainy sand-lime mass, or a fossiliferous mass. In some areas of Florida, the limestones have been converted though a chemical process to dolomite. Dolomite is a sedimentary rock containing more than 50% of the minerals calcite and dolomite, with dolomite being the most abundant.
Shell formations vary from unconsolidated sand and shell, to loosely cemented shell. This includes the coquina formations found in the coastal areas from St. Johns to Palm Beach Counties. Some sand and clay formations may include shell material; however, there is not enough shell to consider these true limestones.
The limestone, shell, and dolomite formations are generally covered by layers of sand and clay. Where the covering is thin or absent, commercial mining of these formations is possible. This includes most areas between the Choctawhatchee River and the Florida Keys.
The mineable formations in Florida range in age from the Middle Eocene (42 million years ago) to the Pleistocene (0.5 million years ago). These formations may also be found exposed in caves, stream valleys, sinks, and in the coastal lowlands. Many of the state's best fossil hunting sites can be found in these areas.
Many of Florida's unique habitats are the result of the underlying limestone. When rain water mixes with decaying surface vegetation, it becomes mildly acidic. Where the overlaying clay layers are thin or absent, the acidic water dissolves the limestone. Caves, sinks, springs, depressions, and stream and rivers beds are the result of this process.
Limestone was first used by Native Americans for the creation of tools and art. Limestone caves and overhangs were used for shelters.
Later, the Spanish settlers used the coquina of St. Augustine (1672 - 1696), and the limestone of St. Marks (1759) to build fortifications. When newly exposed the coquina and limerock can be cut with saws and shaped. Exposure to the air allow these materials to case harden over time into long lasting barriers. These structures and the abandoned Spanish mines may be still be seen today.
When limestone or dolomite are heated, they lose carbon dioxide and become lime (CaO). Lime from burned oyster shells can be mixed with whole shells and sand to form a cement like material called "tabby." Buildings with walls and floors of tabby may be seen in St. Augustine's history district.
Today limestone, shell, and dolomite have wide variety of uses. The following is only a partial list of general uses:
Mining and Minerals Regulation Home
Last updated: September 21, 2011
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