Dry Tortugas National Park

Geology

Limestone and dolomite rocks more than 15,000 feet thick make up the platform under Dry Tortugas National Park

Viewed against the horizon, Fort Jefferson appears to rise up out of the open sea, but the fort is actually built upon a small, sandy island named Garden Key. Sometimes spelled cay or caye, a key is a small, low-elevation island formed on the surface of an ancient coral reef.

An aerial view reveals Garden Key and the hexagonal shape of the fort, as well as a mosaic of corals, sand, and vegetation beneath the surface of the surrounding waters. Made of carbonate sands and muds that consist of the remains of small marine plants and animals, the seven islands that make up Dry Tortugas National Park are considered the southwesternmost extent of the Florida Keys. These seven keys are in a state of constant change, subject to the influences of marine, atmospheric, and terrestrial environmental processes.

In addition to Garden Key, the islands that make up Dry Tortugas National Park include Loggerhead, Bush, Long, Hospital, Middle, and East keys. The total area of the islands is less than one fifth of a square mile (less than half of a square kilometer), though the actual area fluctuates annually as a result of changes in sea level, hurricane and storm impacts, and sediment erosion and accretion. Most of the 101 square miles (262 square kilometers) that make up the park are below the surface of the sea and therefore more difficult for humans to access. Pristine coral reefs, sand banks, and seagrass beds make up the majority of Dry Tortugas National Park.

Geologic Setting

Dry Tortugas National Park is near the southwestern edge of the Florida Platform. This broad, flat, carbonate platform is only partially exposed above sea level as the Florida peninsula. The submerged portion of the platform extends to water depths of about 300 feet (90 meters). Beyond this point, the sea floor drops abruptly to more than 10,000 feet (3,000 meters). The Florida Platform has been accumulating sediments since the Atlantic Ocean basin began to form almost 200 million years ago. Carbonate rocks such as limestone and dolomite more than 15,000 feet (5,000 meters) thick make up the platform and underlie Dry Tortugas National Park. These rocks continue to form as the calcareous shells and skeletons of dead sea life are deposited on submerged areas of the platform.

During the Pleistocene Epoch (2.6 million–11,700 years ago), sea level rose and fell repeatedly in response to the advance and retreat of continental ice sheets. During an advance, a portion of Earth’s water is stored as glacial ice and sea level falls. During a retreat, the water is released, and sea level rises. During glacial periods, much of the Florida Platform was exposed above sea level and its carbonate bedrock was eroded. During interglacial periods, the Florida Platform was largely submerged and coral reefs established themselves on the outside of the tiny Florida peninsula. As sea level dropped to its current location, the tops of these reefs were exposed and now make up the Florida Keys.

Bedrock

Limestone underlies all of the Florida Keys, including Dry Tortugas National Park. The Key Largo Limestone is an exquisitely preserved fossilized coral reef that forms the bedrock directly beneath the more recently deposited sand banks, islands, and reefs of the park. It consists of primarily intact coral heads embedded in a matrix of sand-sized carbonate grains. The coral-head framework functioned as a trap for smaller fragments of coral, coralline algae, and the shells of mollusks and other sea life. The framework corals are mainly star and brain corals. Nearly all of the coral species that are contained in the Key Largo Limestone still exist and currently grow in the modern reefs that are common around south Florida today.

The Key Largo Limestone formed in the Pleistocene Epoch during a warm interglacial period between ice ages when sea level was as much as 100 feet (30 meters) higher than it is today. Although the Key Largo Limestone is not exposed at the surface in Dry Tortugas National Park or anywhere near the Lower Keys, it crops out in the Upper and Middle keys. Within the park, the Key Largo Limestone underlies the live reefs and recent sediments. Sediment cores obtained from drilling have revealed its presence at various depths. The upper surface of the Key Largo Limestone is not flat; rather, it shows great topographic variations similar to those of a live reef. These variations are largely responsible for the location of the park’s reefs, sand banks, and islands.

Live Coral Reefs

The live coral reefs in Dry Tortugas National Park draw tourists and researchers alike. These pristine, remote reefs have been growing for the past 11,700 years and in some places are more than 50 feet (15 meters) thick. They are home to more than 50 coral species, including the threatened elkhorn (Acropora palmata) and staghorn (Acropora cervicornis) species.

Although the reefs in the park may appear stunning in comparison with the more easily accessible reefs along the Florida Keys, their health has been declining since at least the 1980s. Coral disease outbreaks and bleaching are on the rise. Coral reef rehabilitation is a primary concern in the park, and more research is needed to determine the cause of decline in stony coral cover and how to encourage new growth on reefs. In 2007, a no-take, no-anchor Research Natural Area was established in the park to protect marine life and habitats. Baseline data have been established and ongoing monitoring has determined that the Research Natural Area is improving the condition of resources within its boundaries.

Seagrass Beds

Seagrass beds play an important role in the geologic process of sedimentation by trapping sands and other fine particles that would otherwise be carried away by wave activity or ocean currents. Seagrass roots and rhizomes also stabilize the seabed. Once established, seagrass beds provide protection to reef communities and shorelines against waves and coastal erosion, which is especially important during the large tropical storms and hurricanes that often threaten Florida. High sedimentation rates in seagrass beds increase water clarity, which is beneficial for coral growth. Many varieties of marine life depend on seagrass. Small organisms find protection from predators and fast-moving currents in seagrass beds and the associated sediment accumulations into which they burrow. Seagrass beds serve as nurseries for juvenile reef fish species and provide a source of food for foraging marine animals. Several seagrass species grow in the park. Turtle grass (Thalassia testudinum) dominates the shallow flats, the deeper grass beds are a mixture of turtle and manatee grass (Syringodium filiforme), and the deepest areas are occupied by tape grass (Halophila spp.).

Unconsolidated Sediments

More than half the seafloor in the park is covered by recent, unconsolidated sediments. These sediments make up the shallow sand banks. The lack of stabilizing vegetation and coral allows unconsolidated sediments to be easily transported by flowing water. Sand ripples formed by flowing water are common features. The grains that make up the unconsolidated sediments are derived from the same species as those typically found in the underlying and older Key Largo Limestone: primarily particles of calcified algae of the genus Halimeda, along with varying quantities of fragmented coral, mollusk, and foraminifera. Sediment size ranges from gravel to mud. Gravel and sand are common in storm-degraded shoals, sand and silt are common in quieter protected areas, and fine-grained muds are common in the deepest areas.

Pavement

Also known as hardbottom, pavement is a type of flat, low-relief, mostly solid rock substrate that may be overlain by a veneer of unconsolidated sediment. Pavement makes up a very small portion of the sea floor, some of which is occupied by communities of soft corals, such as octocorals. Outside of the park, exposed areas of Key Largo Limestone form pavement, but as previously stated, the Key Largo Limestone does not crop out anywhere in the park. Rather, pavement in the park occurs where sediments have been lithified, or transformed into stone. This hardbottom, non-bedrock seafloor forms where algae or inorganically precipitated calcium carbonate partially cements granular marine sediments together as they accumulate.

Paleontology

Paleontological resources are any remains of past life preserved in a geologic context. Fossils are the primary constituent of the bedrock limestone, coral reefs, sand banks, and islands of the Dry Tortugas. Fossils help to date—in other words, put a geologic age on—the rocks in which they appear. Moreover, marine fossils that are now exposed on land surfaces provide lessons on paleoecology by showing how the environment has changed over time. Fossils that are accessible to visitors on the islands and by snorkeling in the shallow marine areas of the park include corals, mollusks, invertebrates, microfauna, and calcareous algal remains.

Beachrock

Loggerhead Key is unique among the islands of the park because it is the only island with outcrops of beachrock, a sedimentary rock that has formed underneath a thin layer of unconsolidated sediments along its shorelines. Beachrock formation typically occurs in the subsurface area between high and low tide where evaporation causes seawater to become supersaturated with calcium carbonate. The highly concentrated calcium carbonate contained in such waters cements together shallowly buried beach sediments, such as mollusk shells, coralline algae, coral, and encrusting algal debris, causing them to lithify into stone. Beachrock typically occurs in fractured blocks that are inclined toward the sea. The fracturing, commonly known as jointing, is visible in the Loggerhead Key beachrock deposits. Erosion has widened the joints and caused pitting over the surface of the beachrock.

Shifting Sands

The seven small islands that currently exist in the park are constantly changing. The islands are composed of carbonate sand and smaller amounts of coral rubble, with the exception of Loggerhead Key, which contains the most extensive occurrence of beachrock in south Florida. Carbonate sands and, to a lesser extent, coral rubble are easily transported by wind and water, especially during tropical storms and hurricanes. The size, shape, and location of the islands are constantly changing, sometimes rapidly. During a 4-month period in 2005, four hurricanes impacted the park. Tropical storms and hurricanes change the shape of the islands, create and destroy channels between islands, and sometimes cause islands to disappear completely. For example, a land bridge made of sand intermittently joins Bush Key and Garden Key. In general, sediment transport is to the west and Bush Key has been slowly migrating closer to Garden Key. Over time, however, storms and hurricanes have interrupted this overall trend. These shoreline changes affect the ecosystems in the park, facilities on the islands (particularly on Garden Key), and accessibility.

see original article here: https://www.nps.gov/drto/learn/nature/geology.htm

Park History

How Dry Tortugas became a National Park

Fort Jefferson National Monument was designated by president Franklin D. Roosevelt under the Antiquities Act on January 4, 1935. (Comprising 47,125 acres (19,071 ha) The monument was expanded in 1983 and redesignated as Dry Tortugas National Park on October 26, 1992 by an act of Congress.

Dry Tortugas was established to protect the island and marine ecosystems of the Dry Tortugas, to preserve Fort Jefferson and submerged cultural resources such as shipwrecks, and to allow for public access in a regulated manner.

The rich cultural heritage of the Dry Tortugas all begins with its location 70 miles west of Key West, Florida. The seven keys (Garden, Loggerhead, Bush, Long, East, Hospital, and Middle) collectively known as the Dry Tortugas, are situated on the edge of the main shipping channel between the Gulf of Mexico, the western Caribbean, and the Atlantic Ocean. The strategic location of the Dry Tortugas brought a large number of vessels through its surrounding waters as they connect the Atlantic Ocean and the Gulf of Mexico. Early on, the shipping channel was used among Spanish explorers and merchants traveling along the Gulf Coast.

Fort Jefferson on Garden Key

Fort Jefferson, the largest all-masonry fort in the United States, was built between 1846 and 1875 to protect the nation’s gateway to the Gulf of Mexico. Supply and subsidence problems and the Civil War delayed construction. The fort was never completed because of fears that additional bricks and cannon would cause further settling and place more stress on the structure and the cistern system. Distinguishing features include decorative brickwork and 2,000 arches. Time, weather, and water continue to take their toll, necessitating ongoing stabilization and restoration projects.

Fort Jefferson and a Harbor Light

A large military fortress, Fort Jefferson, was constructed in the mid-19th century as an effort for the United States to protect the extremely lucrative shipping channel. Low and flat, these islands and reefs pose a serious navigation hazard to ships passing through the 75-mile-wide straits between the gulf and the ocean. Consequently, these high risk reefs have created a natural “ship trap” and have been the site of hundreds of shipwrecks. A lighthouse was constructed at Garden Key in 1825 to warn incoming vessels of the dangerous reefs and later, a bricktower lighthouse was constructed on Loggerhead Key in 1858 for the same purpose. Discover and explore the rich heritage of Dry Tortugas National Park on the history and culture pages.

see original article here: https://www.nps.gov/drto/learn/historyculture/index.htm

Attractions

The best things to do at Dry Tortugas National Park

Dry Tortugas National Park is rich in both significant cultural and natural resources. You can enjoy dramatic landscapes and the history of Fort Jefferson, or explore our beautiful reefs, marine life and shipwrecks. There are many areas to explore this park — both above and below the water!

Key West

Florida Keys Eco-Discovery Center
Key West Bight Museum
Dry Tortugas National Park

Garden Key
Loggerhead Key
Bush Key
Swimming Beaches
Diving and Snorkeling Sites

Vegetation

A subtropical climate, extreme weather events, and harsh maritime environment control this ecosystem

The variety of vegetation that flourishes at Dry Tortugas National Park is influenced by the area’s unique combination of subtropical climate, extreme weather events, harsh maritime environment (sun, sand, and salt), and history. Both native and nonnative plant species compete for the same basic resources: space, sunlight, water, and nutrients. Tides, currents, and migrating wildlife are sources of seeds from distant lands, and humans have cultivated an assortment of plants throughout history. Of the 125 plant species reported in Dry Tortugas National Park, 81 species (65% of the total) are of exotic origin; the result of accidental and deliberate introduction by man during the past century. The smaller islands consist mainly of native species, while the larger islands (Garden and Loggerhead keys), with their long history of human habitation and disturbance, have a substantially larger abundance of exotic species.

The rich cultural heritage of the park is reflected in the wide variety of cultivated plant species that have fallen in and out of favor over time by past residents. Vegetation in national parks is generally managed to allow native plant species to occur naturally. Although invasions of nonnative plants are controlled by park managers within time and budgetary constraints, certain nonnative, but noninvasive, species with historical value are permitted to remain.

A tour of Fort Jefferson immerses visitors within the vegetation of Garden Key, on which the fort is built. But Garden Key’s story is just one small part of the park’s vegetation history. Visitors to Loggerhead Key are treated a close-up view of the successful results of a recent vegetation management program to remove invasive exotic vegetation that had been interfering with the ecology of the island. And don’t be fooled into thinking that vegetation grows only on the islands, above the surface of the water. The vast majority of the park’s 101 square miles (262 square kilometers) are submerged under the surface of the sea. Accessible by snorkeling and diving, marine plants such as seagrasses make up the majority of the vegetation in Dry Tortugas National Park.

see original article here: https://www.nps.gov/drto/learn/nature/plants.htm

Animal Life

There is a very diverse wildlife at Dry Tortugas National Park

Dry Tortugas National Park is home to many historical and natural wonders above and below the water’s surface. Teeming with life, this area has long been an inspiration to visitors, researchers, and adventurers.

The park’s coral reef and sea grass communities are among the most vibrant in the Florida Keys, providing habitat for myriad species of marine wildlife. The Sooty Tern finds its only regular nesting site in the entire United States on Bush Key, adjacent to Fort Jefferson. Large sea turtles lumber onto the park’s protected beaches to bury their clutches of eggs. Patient visitors who are willing to get wet and go snorkeling will glimpse many species of reef fishes and other marine life beneath the surface of the water.

Visit the links below to learn about some of the wildlife that inhabits the keys and waters of Dry Tortugas National Park.

see original article here: https://www.nps.gov/drto/learn/nature/animals.htm

Hiking Trails

The best hiking trails at Dry Tortugas National Park

Almost 70 miles west of Key West lies the remote Dry Tortugas National Park. The 100-square mile park is mostly open water with seven small islands. The park is known the world over as the home of magnificent Fort Jefferson, picturesque blue waters, superlative coral reefs and marine life, and the vast assortment of bird life that frequent the area.

To learn more about things to do in the park, follow the links below:

Fort Jefferson Tours

Ranger Guided Programs

Junior Ranger Program

Swimming

Snorkeling and Diving

Camping

Boating

Geocaching

Paddlesports

Recreational Fishing

Wildlife Viewing

Guided Tours

see original article: https://www.nps.gov/drto/planyourvisit/things2do.htm