LEAFLET 14 1
GEOLOGY OF THE STATE PARKS IN THE FLORIDA KEYS
by
Ed Lane
The chain of sun-drenched islands of the Florida Keys and associated coral reefs are unique in the continental United States. There are five state parks in the Keys, each with its own special features that will enhance a visit to this subtropical realm. The state parks are: John Pennekamp Coral Reef, Lignumvitae Key, Indian Key, Long Key, and Bahia Honda (Figure 1). The geological history of the Florida Keys is an interesting story that will add to one's enjoyment of these parks.
The Florida Keys lie along an arc from Miami to Key West, a distance of about 135 miles. The islands have been divided into Upper and Lower Keys, based on their orientations and on the differences between the two types of limestone that compose them. The Upper Keys, composed of the Key Largo Limestone, extend from Biscayne Bay southwest to Big Pine Key. The Lower Keys, made of the Miami Limestone, encompass Big Pine Key to Key West (Figure 1). Figure 1 also shows the distinctive orientations which characterize the Upper and Lower Keys. The Upper Keys are oriented in a linear northeast-southwest direction, while the Lower Keys are oriented perpendicular to them, in a northwest-southeast direction. The reasons for their orientations are discussed below.
GEOLOGICAL HISTORY
The Florida peninsula is the emergent portion of a wide, relatively flat geologic feature called the Floridan Plateau, which forms a rampart between the deep waters of the Gulf of Mexico and the Atlantic Ocean (Figure 2). The Florida peninsula is located on the eastern side of the plateau. The edge of the plateau lies over 100 miles west of Tampa, while on the east it lies only three or four miles off the coast from Miami to Palm Beach.
Near the southern rim of the plateau's escarpment lies a fringeline of living and dead coral reefs. The dead coral reefs form the islands of the Florida Keys. The edge of the Floridan Plateau, marked by the 300-feet depth contour line, lies four to eight miles south of the Keys. Today, living coral reefs grow in the shallow waters seaward of the Keys. This environ-