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Coastal & Marine Geology Program > Center for Coastal & Watershed Studies > Professional Paper 1751

Systematic Mapping of Bedrock and Habitats along the Florida Reef Tract—Central Key Largo to Halfmoon Shoal (Gulf of Mexico)

USGS Professional Paper 1751

by Barbara H. Lidz, Christopher D. Reich, and Eugene A. Shinn

Table of Contents
Project Overview
Project Objective
Geologic Setting
Primary Datasets
Primary Products - Overview Maps & Evolution Overview:
Bedrock Surface map.
Depth to Pleistocene Bedrock Surface
Reef & Sediment Thickness
Benthic Ecosystems & Environments
Sedimentary Grains in 1989
Summary Illustration Index Map
Evolution Overview
Tile-by-Tile Analysis
Satellite image of the Florida Keys showing location of tiles.
Organization of Report
Tiles: 1, 2, 3, 4,
5, 6, 7/8, 9/10,

Tile 7/8

Ephemeral Islands and Lighthouses: Physical changes other than reef decline have also occurred along the outer shelf. Many shelf-edge reefs whose names contain the word 'key' were once small islets, or keys, in historical times. Sombrero Key, Looe Key, and Sand Key are examples (e.g., Fig. 113). Many such ephemeral islands must have existed in prehistoric times earlier in the Holocene.

In historical times, one of the first lighthouses in Florida was constructed in 1827 on a sand- and coral-rubble island at Sand Key. Hurricanes destroyed most of the island in 1842 and 1844, and the hurricane of 1846 washed the island away along with the brick lighthouse and light-keeper's home. A lightship replaced the lighthouse while the island reformed to near its former size. A new lighthouse, constructed on wrought-iron pilings that were screwed 3 m into the coral and sand, was officially re-lighted in 1853. The structure stands today (Fig. 113), having survived many hurricanes. The hurricane of October 1865 again removed the island and again it reformed. In October 1870, the island was struck by back-to-back hurricanes, and again it reformed. In 1903, a large weather station was built on the reformed island next to the iron lighthouse. Six years later (October 1909), the island and weather station were washed away in a storm when the sea flooded the island to a depth of 6 m. Again the island reformed and the weather station was replaced in 1910, only to be removed by two October storms. Wind speed of 125 mph was recorded, and winds above gale force (32-63 mph) lasted for more than 30 hours. Although the island reformed, today it is a small sand spit just to the north of the iron lighthouse (Fig 113). The lighthouse has been surrounded by water for the past half century.

The maritime history surrounding Florida Keys lighthouses is rich (e.g., Dean, 1992). Not all were built on ephemeral coral-rubble islands. Fifteen lighthouses were built from Cape Florida (on Key Biscayne; Fig. 97B) to the Dry Tortugas in the 1800s. Of the 12 original structures, six still stand and are functioning today: at Carysfort Reef (1852), Loggerhead Key (1858) in the Tortugas, Sombrero Key (1858), Alligator Reef (1873), Fowey Rocks (1878), and at American Shoal (1880). Use of those built at Cape Florida (1825, destroyed by fire in an 1836 Seminole Indian attack) and Key West (1826, destroyed by an 1846 hurricane) to replace the original structures at those sites has been discontinued.

Natural destruction and rebuilding of offshore islands have occurred in more recent times as well. Ball et al. (1967) documented the annihilation of a small sand- and coral-rubble island near Molasses Reef during Hurricane Donna in 1960 (Tile 1). The island, generally referred to as Little Molasses Island or Little Molasses Reef, reformed and was large enough to support, temporarily, a truck-mounted drill rig in the mid-1960s (Perkins and Enos, 1977). There is no island at Little Molasses Reef today. These small sand-and-rubble islands differed from those formed of Pleistocene bedrock in the early Holocene. Long, linear rock ridges once lined the shelf edge until the rising sea submerged them (Lidz and Shinn, 1991; Figs. 111, 114).

Coastal & Marine Geology Program > Center for Coastal & Watershed Studies > Professional Paper 1751

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