Geologically, the Florida Keys are characterized by the limestone that forms the islands. For example, the Key Largo Limestone is the surface rock of the Upper Keys, while the Miami Oölitic Limestone covers the Lower Keys. Oölite is compacted small egg shaped deposits of calcium carbonate, meaning this whole area was once a live, thriving community of corals and other marine organisms. As the glaciers reformed taking water from the ocean, sea level dropped and the coral forests died, which then ultimately collapsed into the now known islands. Big Pine Key, as shown in the photograph, signals the beginning of the Oölite Keys. A contact found here shows that the oölitic limestone overlaps the Key Largo, and core borings show that the Key Largo Limestone underlies the oölitic cover for the entire area of the Lower Keys.

Several rare plants, including yellow satinwood (Zanthoxylum flavum), Florida silver palm (Coccothrinax argentata), Key thatch palm (Leucothrinax morrisii), and the endangered small-flowered lily-thorn (Catesbaea parviflora) are found in this area. Marine life is also quite plentiful with many species of small reef fish, rays, barracuda, and sharks. Inland, researchers have discovered the only known natural colony of the now rare Miami blue butterfly (Cyclargus thomasi bethunebakeri). (Photograph taken February 2016 by Tara M. Delaney, Allen Press, Lawrence, Kansas, U.S.A.)