Research Article:
The Story of Water Movement and Land Formation

In Investigation One, "Southern Florida: A Study Site for Coastal Communities," you examined the bathymetry and topography of the Florida Plateau. You also looked at aquifers and the flow of water from Lake Okeechobee into the Everglades, to Florida Bay, to the reefs, and beyond. In this investigation, you will take a closer look at the geological processes such as erosion, rising sea levels, and sedimentation that produced Florida's geomorphologic forms. You will also learn about ground-testing techniques and other technologies that scientists use to determine the origin of particular geomorphologic forms.

The geomorphologic forms in Florida result mainly from erosion and sedimentation. These are complementary processes in which materials such as rock, coral, soil, sand, and bottom sediment are worn away, removed from one area of the Earth's surface, and deposited in another area. Materials are often carried long distances by ocean or other aquatic currents before being deposited.

Glaciation and Rising Sea Levels: How Natural Disturbances Affect Erosion and Sedimentation

Natural disturbances, such as changes in sea level, can greatly affect erosion and sedimentation, which in turn influence land formation. The changes in southern Florida during and after the Pleistocene era ("Ice Age") provide dramatic examples of these effects. About 100,000 years ago, during the Pleistocene era, the tectonic plates beneath Florida began to move. The southern part of Florida began to sink as it broke away from the Florida Plateau, eventually forming Pourtales Terrace and shelf areas.

Also around 100,000 years ago, several parallel lines of patch reef formed along the shoreline of southern Florida. At that time, the sea level was about 8 m (about 25 ft) higher than it is today. As glaciers formed during the Ice Age, much of the ocean's water was withdrawn from the ocean basins. As the sea level fell, water drained off the patch reefs and barrier reef, exposing the coral to erosion. Erosion removed the top 17 meters (about 50 ft) of the barrier reef. The patch reef closest to the Everglades shoreline remained and became what is now known as the upper Florida Keys.

When the Earth's temperature rose and the glaciers began to melt, sea levels rose. When the water reached the level of the old reefs, a new barrier reef began to grow at the seaward edge, and patch reefs grew back between the barrier reef and the shoreline. Limestone sediment that had eroded from the reefs began to move to the south, covering the coral growing there and forming mounds of hard Miami limestone, now known as the Lower Keys.

The formation and melting of glaciers also shaped Florida Bay. Before the Pleistocene era, Florida Bay was a lagoon between the living coral reef and the mainland. Limestone that had eroded from the reef and was suspended in the water eventually covered the floor of the lagoon. When glaciation occurred, the sea level fell 33 meters (about 100 ft). The bay became eroded and the limestone hardened, forming Miami limestone. When the glaciers melted, the lagoon filled up with freshwater and looked similar to the Everglades. As the climate continued to warm up, sea levels rose. The sea reached its maximum level 4,000 years ago, flooding the lagoon and producing Florida Bay.

The Florida East Coast Railway: How Human Activities Influence Erosion and Sedimentation

Human disturbances can also affect the rates of erosion and sedimentation. In 1904, the first major human disturbance in the Florida Keys began with the building of the Florida East Coast Railway between Miami and Key West. Entrepreneur Henry Flagler undertook this project to facilitate travel between Cuba and New York City. The railway had to cross 37 miles of open water between Key Largo and Key West. The original proposal for the railway involved building a solid causeway over the entire route. A solid causeway would have blocked the exchange of water and nutrients between Florida Bay and the Everglades from the Atlantic Ocean. This would have disrupted the reefs, which need nutrient-rich sediments from the Everglades, as well as Florida Bay, which depends on variations in salinity produced by tidal flows.

To prevent these disruptions, the United States government proposed that at least 17 miles of bridges be included in the railway to allow for tidal flow between the ocean and Florida Bay. Although these bridges were part of the final design, the railway still harmed the natural environment of the Florida Keys. The causeways buried thousands of acres of grass beds found in the shallow waters between the Keys. Hundreds of acres of mangrove forests were filled to make a firm roadbed. Sedimentation was widespread, because the bottom was churned and disturbed by filling, dredging, and bridge construction. Because the tidal flow was blocked in some areas, the flow doubled in open areas. The rapid currents that resulted changed the life on the bottom. Channels that were once deep became shallow through sedimentation. Seagrass beds and calcareous (calcium-containing) algae grew and changed the habitats of organisms living in the channels.

In 1935, a hurricane destroyed most of the railway. Between 1936 and 1944, an overseas highway was built to replace the railway. The highway used more bridges and fewer causeways than the railway did, and the highway builders used railway bridges that remained standing rather than constructing new ones. For these reasons, the impact of highway construction on the Keys was limited, some tidal flow was restored, and the Keys ecosystem recovered to some extent.

How Scientists Investigate Geomorphologic Forms

Geologists use a systematic approach to determine the origin of geomorphologic forms. For a marine system such as a coastal area, this process begins by constructing a bathymetric map using data from a depth recorder. A depth recorder measures the depth of a body of water by sending sound waves from a ship to the bottom. To obtain more detail about the marine system, geologists use magnetometers, which measure magnetism, and gravimeters, which measure gravity. Geologists can also use side-scanning sonar, which sends sound waves at low angles to scan a wide area on both sides of a ship.

Direct observation is also critical. Scientists can directly observe geomorphologic forms using a wide range of technologies:

Scientists also examine the seabed through core sampling. To take core samples, researchers lower coring tubes to the bottom. The corer falls into the sediment and the sharpened, open lower end of the corer penetrates 10 meters (30 ft) or more into the soft sediments. This creates what is called a well. The corer extracts samples of the sediment from the well, and these samples are examined by scientists to determine the age and origin of the sediments.

During JASON VII, Dr. Ballard and his team of engineer-researchers will use these tools to look at marine geomorphologic features, such as Pourtales Terrace and a shelf in the Straits of Florida maintained by the constant movement of sediment in the Gulf Stream. He will also explore ancient coral reefs 73 to 183 meters (240 to 600 ft) below the surface, located 29 kilometers (18 mi) south of the Florida Keys. One of these reefs, known as "the hump"-a pre-Pleistocene reef located on Pourtales Terrace-has never been explored before!

The Florida Bay research team headed by Dr. John Hunt also will investigate marine communities. This team will examine the geomorphologic forms of the hard-bottom shoals and lakes in Florida Bay and the soft-bottom mud banks that join at irregular intervals to create a lace-like pattern around the lakes.

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Gene Carl Feldman (gene@seawifs.gsfc.nasa.gov) (301) 286-9428
Todd Carlo Viola, JASON Foundation for Education (todd@jason.org)
Revised: 17 Oct 1995