Limestone, shell and dolomite are types of marine deposits that have accumulated in Florida over millions of years during times when the state was covered over with seawater or shallow marshes. During this time, limestones formed by chemical deposition and the accumulation of shells from sea creatures. Many invertebrate animals (animals without backbones) take calcite from sea water to construct their shells. When they die, the shells fall to the sea bed. Over the millennia, 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.
The mineable formations range in age from the Middle Eocene (42 million years ago) to the Pleistocene (0.5 million years ago). These formations may also be exposed in caves, stream valleys, sinks and in the coastal lowlands.
Limestone is a sedimentary rock that is composed primarily of calcite (calcium carbonate, CaCO3) and dolomite (calcium magnesium carbonate, CaMg(CO3)2). It can vary in purity, consistency and hardness. The limestone can be a soft chalky material with microfossils, a hard recrystallized mass, a grainy sand-lime mass, or a mass of fossil corals, urchins and shells. In some areas a chemical process converted the limestone to dolomite,
which is characterized by a greater proportion of magnesium in the mineral composition. Shell formations vary from unconsolidated sand and shell to loosely cemented shell. This includes the coquina (Spanish for “small shells”) formations found in coastal areas from St. Johns to Monroe counties.
Many of Florida’s unique habitats are the result of the effects of weathering on the underlying limestone. Rain water becomes mildly acidic when it mixes with decaying surface vegetation. Where the overlaying clay layers are thin or absent, the acidic water dissolves the limestone.
Caves, sinks, springs, depressions and stream beds are the result of this process. The type and quality of the mined resources determines how it may be used. Generally, these materials are used as general fill, crushed stone, aggregate for concrete and roadway asphalt, rip rap (large stones used to control erosion),
lime, Portland cement, plaster, fertilizer (aglime), and as an acid neutralizer for power plant smoke emissions. Limestone mining began in Florida during the First Spanish Period.
In 1672, construction began on the Castillo de San Marcos in St. Augustine. Locally mined coquina was used. This is the oldest masonry fort in the continental United States. In 1753, construction began on a limestone fort in St. Marks to replace older wooden structures. The limestone, shell and dolomite formations are generally covered by layers of sand and clay.
Commercial mining of these formations is possible where the covering is thin. This includes most areas between the Choctawhatchee River and the Florida Keys.