Destructive Dynamism: Imperiled sites on Cumberland Island National Seashore

Cumberland Island formed 18,000 years ago at the end of the last Ice Age. Measuring approximately 17 miles (27.4 km) from north to south and 3 miles (4.8 km) across at its widest point, it is the largest and southernmost barrier island in Georgia and the only one that has remained largely undeveloped. As such, it is a haven for a broad range of flora and fauna and a time capsule of equally diverse and unique historic and archeological resources. The 100-mile-long (160.9 km) Georgia coast is bordered by more than a dozen other Sea Islands separated from each other by tidal inlets. The Georgia Sea Islands are, in turn, part of chain of barrier islands that extends from New England down the east coast around the Gulf of Mexico and south to Mexico. Serving as buffers by absorbing storm energy, the Sea Islands are the mainland’s first line of deference against the wind and waves of the Atlantic Ocean.

Cumberland_Island_National_Seashore_map_2007.08

Cumberland Island was designated as a National Seashore in 1972 to conserve the landscape, and natural and cultural resources, for the enjoyment and education of present and future generations. The park boundaries encompass 56.9 mi2 (147.4 km2) of beach, salt marsh, and maritime forest, including more than 9800 acres of congressionally designated wilderness, the most in a national seashore on the east coast. The island is only accessible by boat and visitation is limited to 300 persons per day. Still, every year tens of thousands of visitors travel to Cumberland Island by ferry to explore the island’s varied ecotonal landscapes, remarkable biodiversity, and unique historic sites. There is easily more than a day’s worth of experiences to be had at Cumberland Island and visitors are invited to spend the night on the island. Accommodations include primitive campsites and for those less rustically-inclined, the privately managed Greyfield Inn built in 1900 by Thomas and Lucy Carnegie offers charming suites and their own guided tours of the island.

Dungeness_Family_Layer_Mask_2
A superimposed historic image of members of the Carnegie family on the steps of Dungeness mansion. The ruined building that burned in 1959, is one of 87 structures on the island listed on the National Register of  Historic Places.

There are four major historic districts and 87 structures listed on the National Register of Historic Places on the island. Cultural resources in the park represent thousands of years of Native American prehistory, Spanish missionization and British colonization through the sixteenth, seventeenth, and eighteenth centuries,GUGE-logo the War of 1812, the era of antebellum plantations, the Civil War, Reconstruction, and the gilded age of the Carnegie and Rockefeller families during the late-nineteenth and twentieth centuries. Cumberland Island is also part of the Gullah Geechee Cultural Heritage Corridor.

While modern development on the island has been limited, invasive plant and animal species were propagated over the years for aesthetic, recreational, and practical reasons. There is a large population of feral pigs that threaten native plant and animal communities, impact buried cultural resources, and pose a danger to visitors and park employees. Non-native plants like bamboo, oleander, and Chinese tallow are a challenge to resource managers because, in some cases, they are part of the island’s historic cultural landscape but still negatively impact the indigenous ecology. Likewise, the herds of horses in the park are non-native and despite being a draw for visitors, spur erosion by eating dune vegetation and trampling paths to the beach. Efforts to manage the adverse effects of these invasive species are on-going but the most dramatic impacts to the natural and cultural resources of the park have been brought on by increased storm activity during the past several years concomitant with warmer sea water and higher sea levels.

A perusal of just a handful of the imperiled archeological sites at Cumberland Island National Seashore…

Barrier islands are dynamic landforms constantly being reshaped by wind, waves, and currents. While the shifting and natural redeposition of sediment on barrier islands by waves and currents can in effect restore soil to areas scoured during storms, the process does little to protect buried archeological sites exposed and washed away by storm surges, storm tides, and other climate-related erosional forces. With an eye for the preservation of archeological sites, parts of the back-barrier, or western shoreline of the island look as though they have experienced a natural disaster.

Prehistoric Shell Middens

Sea camp to dungeness shell midden erosion example
Shell midden exposed along the eroding back-barrier bluff on Cumberland Island.

Last July and this February, SEAC archeologists recently conducted a series of surveys to delineate the extent of prehistoric archeological sites identified in the 1970s. These sites were recorded by former SEAC director John Ehrenhard in 1975 by shell midden deposits visible on the ground surface and along the eroding bluffs on the island’s west side.

Learn more about SEAC’s recent survey here!

 

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Hurricane storm surge flooding, Burnswick, Georgia, 1898.

Cumberland Island’s natural dynamism becomes fragility when compounded by dramatic climatic events like hurricanes. The last major hurricane (Category 3 or stronger) to hit Cumberland Island made landfall in 1898 when an almost 5 m storm surge was recorded at Brunswick, Georgia, 10 miles up the coast from Cumberland. Since 1911, no hurricane stronger than Category 2 has made landfall in Georgia. As tropical cyclones travel up the east coast near the state, their centers are usually directed away from the mainland. This pattern is partly explained by the route of the Gulf Stream, the strong warm water current that affects the paths of Atlantic hurricanes which runs 50 miles offshore from Savannah. But, the eye of a hurricane does not need to make landfall for tropical cyclones to cause major damage to resources on islands and coasts.

Fort Saint Andrew

Fort Saint Andrew was one of two earth and wood forts built on Cumberland Island in 1736 by the British colonial military under the command of James Oglethorpe, the founder of Georgia. It was abandoned by the British in 1742 and subsequently occupied by the Spanish for two days before it was set ablaze. During the hyperactive 2005 hurricane season, Tropical Storm Tammy dumped more than 20 inches of rain on Cumberland Island and surrounding areas. The storm contributed to increased erosion, particularly along the shoreline in the northwest portion of the island, where the site of Fort Saint Andrew is located.

Fort St Andrews.JPG
The location of Fort St. Andrew as depicted an 1802 map by surveyor John McKinnon. Note the difference between the shape of the land on the overlain map and the recent satellite imagery beneath.
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The wide band of dark soil across the base of this unit is a feature interpreted as the moat of Fort St. Andrew.

The exact location of the fort was not known archeologically until a Phase I study of the 
effects of the storm damage led by Carolyn Rock (2006) succeeded in locating its remains. During the summers of 2007, 2009, 2011, and 2014, crews from SEAC conducted excavations at the fort to determine its integrity and to recover as much information as possible before any more of this important site is lost to erosion. Intact archeological deposits were encountered over 2 m below the ground surface. In some of excavation units, eighteenth century artifacts like ceramics, gunflints, kaolin pipestems, and wrought iron nails were recovered from what is interpreted as the ditch or moat that surrounded the fort. Weathered cedar planking and iron hoops, possibly for barrels, were also recovered. The British used shell mined from prehistoric Native American shell middens to help construct the rampart and parapet adjacent to the ditch or moat. Archeological excavations have also demonstrated that a shell midden lies beneath what remains of the fort and extends beyond it. Unfortunately, these investigations also suggest that the majority of the fort has already been lost to erosion of the steep back-barrier bluff.

Fort Prince William

The second eighteenth century British fort, Fort Prince William, was constructed on the south end of the island. By the time surveyor John McKinnon made his map of the island in 1802, Fort Prince William was already being overtaken by the sea.

Fort Prince William
Location of Fort Prince William as shown on McKinnon’s 1802 map. Note the dramatic difference between the shape of the island on the overlain map and the recent satellite imagery beneath.

 

Brickhill Bluff

Like Fort Saint Andrew, the Brickhill Bluff site is located on the northwest back-barrier and was damaged during Tropical Storm Tammy’s 2005 deluge.  In 2012, the rate of shoreline attrition was estimated at 0.59 m per year based on data from three other sites with analogous geography and archeological components along the back-barrier (Wise 2012). That year, SEAC archeologists demonstrated with diagnostic artifacts recovered from buried contexts on the bluff and the ground surface along the beach exposed by erosion, that the site had been in essence continuously occupied from approximately 800 B.C., the Woodland Period, until 1870.

Brickhill bluff on 1802 mckinnon.jpg
Location of “Brick Kiln” on the 1802 McKinnon map. The site became the location of the postbellum African-American Brickhill Bluff settlement. Difference between the overlain historic map and the light colored shoreline depicted on the recent satellite image beneath.

Just north of the area explored in 2012, historic maps depict a community of formerly enslaved African Americans that developed as early as 1863 and persisted at least into the 1880s (Bullard 2003).

Brickhill bluff map with eroding areas highlighted SEAC-02141
Red lines mark parts of the Cumberland Island back-barrier shoreline actively eroding in 2012.

The Brickhill Bluff site was originally recorded by SEAC archeologists during a cultural resource inventory survey in 1975 as part of the Cumberland Island National Seashore Master Plan (Ehrenhardt 1976). To evaluate the apparently severe erosion they observed, archeologist established monitoring stations at the north and south ends of the site as delineated by Ehrenhardt. Between 1987 and 1988, about four feet of the bank was lost to erosion. An effort to stabilize the sound-side boarder of the site was conceived based on observations at parts of the island where stable tidal marsh zones were developing through the natural deposition shell rakes. These rakes are essentially tightly compacted walls of shell that form basins that trap sediment while allowing water to filter out. “Once sufficient silt is deposited in these small basins, colonies of grasses being to develop… As the grass colonies expand, additional silts are trapped reducing the force of incoming waves,” (Ehrenhardt and Throne 1991:14).

jackson 2006 erosion scarps
The extent of land loss at Brickhill Bluff is captured by Jackson’s (2006) image of erosion scarps at the site.

An experimental erosion-barrier was constructed comprised of dead trees, an artificial shell rake made up of dead shell in burlap bags and GEOWEB to prevent trampling on the rake and predation of vegetation by horses and feral hogs. Unfortunately, by 1994 the only trace of the erosion-control barrier was a thin scatter of oyster shell.

 

Artificial shell rake drawing
Schematic of the artificial shell rake built bu NPS at Brickhill Bluff.

 

The Future?

Climate is a complex system of interrelated variables like sea level, atmospheric and oceanic temperatures, storm frequency and intensity, and seasonality, among others. Research into these dynamics can support the development of predictive models, and inspire new avenues for human adaptation to our changing environment.

The cultural resources of Cumberland Island National Seashore exist in a dynamic environment. The same forces that allow the island to adapt to cyclical changes in the force and direction of wind, waves, and currents threaten the preservation of stationary archeological sites. Punctuated and gradual climate-related events and processes have already impacted and in some cases destroyed archeological sites at the park.

NOAA vulnerable CUIS

The powerful punctuated impacts of severe storms cannot be overstated, but the more subtle, ongoing impact of rising sea levels has been understated particularly in regard to erosion along sandy beaches. Interactive predictive models developed by NOAA show that much of the Cumberland Island back-barrier is vulnerable to inundation by rising sea levels. The most vulnerable areas are the low salt marshes and sandy beaches that, while ecologically important, are not generally where intact archeological sites are located. However, as research by the International Hurricane Center at Florida International University has shown “Sea level rise is an enabler of erosion,” (Leatherman, Zhang, and Douglas 2000).  “[S]ea level rise does not actually cause erosion; rather, increased sea level enables high-energy, short-period storm waves to attack further up the beach and transport sand offshore.” (Leatherman, Zhang, and Douglas 2000). Shocking figures suggest that “long-term shoreline retreat rate average[s] about 150 times that of sea level rise. Thus, a sustained rise of 10 cm (3.9 inches) in sea level could result in 15 m (49.2 feet) of shoreline erosion” (Leatherman, Zhang, and Douglas 2000).

Data suggests that ocean waters are becoming warmer and sea-levels are rising. Whether these changes are human-induced or part of natural atmospheric cycles, they foretell increasingly powerful tropical cyclones and more intense storm surges and tides. “At most of the East Coast tide gages, relative mean sea level has maintained a steady rise over the past century,” (Kraus, Gorman, and Pope 1994:23). Though a number of factors are contributing to this process, increasing atmospheric and oceanic temperatures are considerably influential and intensified by the accumulation of greenhouse gasses including water vapor, carbon dioxide, methane, and nitrous oxide. Some studies show that since 1975, human activity has been contributing increased amounts of carbon dioxide to the atmosphere while at the same time decreasing the extent and quality of ecosystems like forests and wetlands that remove it. Inundation, caused by rising sea levels, of coastal wetland ecosystems like marshes along the Cumberland Island back-barrier may eliminate an ally in our struggle to balance our impact on climate.

Have you been to Cumberland Island National Seashore?

We would love to hear about your experience!

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References:

Bullard, Mary R. 2003. Cumberland Island: A History. The University of Georgia Press. Athens, Georgia.

Ehrenhard, John. 1976. Cumberland Island National Seashore: Assessment of Archeological and Historical Resources. Southeast Archeological Center. Tallahassee, Florida.

Ehrenhard, John E. and Robert M. Throne. An Experiment in Archeological Site Stabilization: Cumberland Island National Seashore. CRM. 14:2, Pp. 13-16. National Park Service.

Jackson Jr., Chester W. 2006. Historic Back-barrier Shoreline Changes Along Cumberland Island, Georgia1857 to 2002. Department of Geology, University of Georgia. Athens, Georgia.

Kraus, Nicholas C., Laurel T. Gorman, and Joan Pope. 1994. Kings Bay coast and Estuarine Physical Monitoring and Evaluation Program: Coast Studies. Volume 1, Main text and appendix A. U.S. Army Corps of Engineers. Vicksburg, Mississippi.

Leatherman, Stephen P, Keqi Zhang, and Bruce C. Douglas. 2000. Sea Level Rise Shown to Drive Coastal Erosion. Eos, Transactions American Geophysical Union 81:6 pp. 55-57. Online ISSN:2324-9250. Accessed March 9, 2015. John Wiley & Sons.

Rock, Carolyn. 2006. Archaeological Investigations at the Dungeness Wharf Site and the Fort St. Andrews Site, Cumberland Island  National Seashore, Camden County, Georgia. Manuscript on file at Southeast Archeological Center, Tallahassee, Florida.

Wise, Stephen Andrew. 2012. Archeological Investigations at Brickhill Bluff. National Park Service. Manuscript on file at Southeast Archeological Center, Tallahassee, Florida.

 

 

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