Printed on recycled paper U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2018–3011

March 2018

Figure 2B. Map of Fort Pulaski National Monument, Chatham County, 

Georgia. The red star marks the location of the drill site and the 

Cockspur Island core described in figure 3. North of Cockspur Island, 

the middle of the Savannah River forms the boundary between the 

States of Georgia and South Carolina. 

Figure 2A. Map showing the location of Fort Pulaski and the 

Floridan aquifer system (shaded in green) in the southeastern 

United States. Figure modified from Williams and Kuniansky (2016). 

Fort Pulaski was constructed from 1829 to 1847 and was designated a national monument by proclamation of President 

Calvin Coolidge in 1924. The fort is located on Cockspur Island in Chatham County, Georgia, within the Atlantic Coastal 

Plain province of the southeastern United States (figs. 1 and 2). The island lies near the mouth of the Savannah River, 

and consists of small mounds (hummocks), salt marshes, and sediment dredged from the river.

Figure 1. Photograph of Fort Pulaski 

National Monument, Chatham County, 

Georgia.

A Brief Geological History of 

Cockspur Island at

Fort Pulaski National Monument, 

Chatham County, Georgia

A Step Back in Time—

80°55’00” 80°52’30”

32°02’00”

32°01’00”

Historic 

North Pier

Historic 

Dike

FORT

PULASKI

Tybee Island

Jones Island

Cockspur Island

McQueens

Island

ALABAMA

GEORGIA

GEORGIA

FLORIDA

SOUTH

 CAROLINA

SOUTH

 CAROLINA

FORT PULASKI

GULF OF

MEXICO

ATLANTIC

OCEAN

Savannah

South Channel

Savannah River

Savannah River

DRILL SITE F l o r i d a n a q u i f e r s y s t e m

0 100 KILOMETERS 50

0 50 100 MILES

0 0.2 KILOMETERS

0 0.2 MILES

80
Figure 3. Description of the Cockspur Island 

core recovered by the U.S. Geological Survey 

(USGS) in 2010 at Fort Pulaski National 

Monument in Chatham County, Georgia. The 

drill site location (lat. 32o01’53“, long. 80 ̊54’07”) 

is shown in figure 2B. The approximate age 

estimates for the different rock and sediment 

units encountered with depth within the core 

are given on the basis of biozone identifications 

by Lucy E. Edwards (USGS) and Jean M. Self-Trail 

(USGS), and uncalibrated radiocarbon (C-14) 

ages by John P. McGeehin (USGS). Core 

description modified from Alemán-González 

and others (2016).

Several studies have been published on the geology of Cockspur Island. Weems and Edwards (2001) published a description 

of a 116.5-foot (ft) (35.5-meter [m]) core drilled on the north side of the island. In 2010, the U.S. Geological Survey drilled a 1,017-ft 

(310-m) core to determine the stratigraphy of Cockspur Island west of Fort Pulaski (fig. 3). The core from top to bottom consists 

of the following five sedimentary units: (1) a 53-ft (16-m)-thick unit of sand and mud, ranging in age from present to 4,300 years old; 

(2) a 35-ft (11-m)-thick unit of sand and mud with phosphate, ranging in age from 3.5 to 12 million years old; (3) a 94-ft (29-m)-thick 

unit of sand and mud, ranging in age from 12 to 32 million years old; (4) a 783-ft (239-m)-thick unit of limestone, ranging in age from 32 

to 41 million years old (includes ~25 ft [7.6 m] of sandy limestone); and (5) a 53-ft (16-m)-thick unit of glauconitic sand, ranging in 

age from 41 to 43 million years old. The limestone and water within the limestone are referred to collectively as the Floridan aquifer system 

(fig. 2A), which is the primary source of drinking water for the City of Savannah and surrounding communities. In addition to discussing 

the subsurface geology, this fact sheet identifies a variety of geologic materials used in the construction of Fort Pulaski. These materials 

include granite, bricks, sandstone, and lime mud with oyster shells (figs. 4A–D).

EXPLANATION

Sand and mud

Sand and mud with phosphate

Limestone

Sandy limestone

Glauconitic sand

Rock and sediment description Stratigraphic name(s) Approximate age

Sand and mud

Sand and mud

Glauconitic sand

Marks Head Formation, 

Tiger Leap Formation(?), and (or)

Lazaretto Creek Formation(?)

 Satilla Formation

Sand and mud with phosphate

Limestone

Limestone

Sandy limestone

Coosawhatchie Formation

(Tybee Phosphorite Member)

Suwannee Limestone, 

Santee Limestone, and (or)

Ocala Group(?) (undivided)

Warley Hill Formation

100

0

1,000

feet

200

300

400

500

600

700

800

900

50

100

150

200

250

300

meters

Depth in core

Present to 

4,300 years old 

32 to 41

million years old

12 to 32

million years old

3.5 to 12

million years old

41 to 43

million years old

Stratigraphy of Cockspur Island
Figure 4. A–D, Photographs of building stones of Fort Pulaski National Monument. Data are from Young (1936), Eldridge (1996), Weems and Olsen 

(1997), Ryan (2008), and LeTourneau (2010). A 3.5-inch (8.9-cm)-long pocketknife (in A, B, and D) and a hand (in C ) are shown for scale. Smaller 

photographs in A–D by Mercer Parker, U.S. Geological Survey.

A. Granite—Blocks of granite were used as stairs, the 

tops of brick posts, walls, and as a lining along the old wharf 

at the Historic North Pier (fig. 2B). Granite is an igneous rock 

formed by the cooling and solidification of liquid magma. Most 

of the granite was quarried in New England, transported to 

New York, and then transported by ship to Fort Pulaski. At Fort 

Pulaski, four varieties of granite were used that may range in 

age from 280 million to 1 billion years old. 

B. Bricks—The following two kinds of bricks were made for 

building stone using mud and sand:

1. “Rose-red” bricks were used in arches and interior walls. 

The mud and sand for these bricks were excavated from 

land near the fort.

2. “Rose-brown” bricks were used in the exterior walls and in 

the walls that line the moat. These bricks were made in 

Baltimore, Md., and Alexandria, Va. The mud was probably 

derived from extremely weathered metamorphic rocks.

C. Sandstone—Blocks of brownish-red sandstone were 

used to border the moat and to line thresholds of interior 

doorways and brick arches. Sandstone is a sedimentary rock 

composed of sand-size particles eroded from other rocks. 

This sandstone was quarried in Connecticut from the Portland 

Formation of the Newark Supergroup, and then transported 

to Fort Pulaski. The quarried sandstone is informally called 

“Portland brownstone.” This sandstone is approximately 190 to 

200 million years old, and accumulated in basins on land when 

the Atlantic Ocean was opening during the splitting apart of the 

supercontinent Pangaea.

D. Lime mud with oyster shells—Cement composed of 

lime mud and oyster shells was used in the walkway through the 

demilune moat of Fort Pulaski. This cement (informally called 

“tabby”) was used as mortar between bricks and as coating 

over bricks. The cement was made by crushing and burning the 

shells to make a powdered material called lime. The lime was 

then combined with water, sand, ash (from burned wood), and 

more crushed oyster shells, and was poured into wooden molds 

and left to harden. This method of making cement was used in 

the construction of many coastal forts during the 1800s. The 

oyster shells were probably obtained from Native American 

middens in the vicinity of the Savannah River. 

Granite

Granite

Sandstone

Building Stones of Fort Pulaski
ISSN 2327-6916 (print)

ISSN 2327-6932 (online)

References Cited

Alemán-González, W.B., Swezey, C.S., Self-Trail, J.M., Edwards, L.E., 

and McGeehin, J.P., 2016, Preliminary Cenozoic lithostratigraphy of the 

Cockspur Island core, Fort Pulaski National Monument, Chatham County, 

Georgia: Geological Society of America, Abstracts with Programs, 

v. 48, no. 3, accessed December 2017, at https://doi.org/10.1130/abs/

2016SE-272508. 

Eldridge, D.P., 1996, Brick versus Earth―The construction and destruction 

of seacoast Forts Pulaski and McAllister, Georgia: Jacksonville, Florida, 

University of North Florida, M.A. thesis, 96 p. 

LeTourneau, P.M., 2010, The stone that shaped America in the 19th century―

The geology and history of the Portland brownstone quarries, chap. 3 

of LeTourneau, P.M., and Thomas, M.A., eds., Traprock, tracks and 

brownstone―The geology, paleontology, and history of world-class sites 

in the Connecticut Valley: Geological Society of Connecticut Field 

Trip Guidebook No. 1, p. 17‒30. [Also available at https://www.

geologicalsocietyct.org/uploads/3/0/5/5/30552753/gsc_guidebook1final.pdf.]

Figure 6. Photograph of part of the Cockspur Island 

core recovered at a depth of 385.5 to 393.4 feet 

(117.5 to 119.9 meters; fig. 3) at Fort Pulaski National 

Monument during February 2010; color charts and 

scales are also shown. Photograph by Wilma AlemánGonzález, U.S. Geological Survey.

Figure 5. Photograph of the U.S. Geological Survey (USGS) drill rig at the Cockspur

Island drill site at Fort Pulaski National Monument during February 2010. Photograph 

by Colleen T. Durand (USGS).

Ryan, M., 2008, Survey of Fort Pulaski research material at the National 

Archives and Carlisle Barracks: U.S. National Park Service report, 

Fort Pulaski National Monument, Chatham County, Georgia, 4 p. 

Weems, R.E., and Edwards, L.E., 2001, Geology of Oligocene, Miocene, 

and younger deposits in the coastal area of Georgia: Georgia Geologic 

Survey Bulletin 131, 124 p. [Also available at https://www2.usgs.gov/

water/southatlantic/ga/publications/ggs/bull-131/.] 

Weems, R.E., and Olsen, P.E., 1997, Synthesis and revision of groups within the 

Newark Supergroup, eastern North America: Geological Society of America 

Bulletin, v. 109, no. 2, p. 195‒209. [Also available at https://pubs.

geoscienceworld.org/gsa/gsabulletin/article/109/2/195-209/183209.]

Williams, L.J., and Kuniansky, E.L., 2016, Revised hydrogeologic framework 

of the Floridan aquifer system in Florida and parts of Georgia, Alabama, 

and South Carolina (ver. 1.1, March 1, 2016): U.S. Geological Survey 

Professional Paper 1807, 140 p., 23 pls., accessed December 2017, at 

https://pubs.usgs.gov/pp/1807/.

Young, R.W., 1936, The construction of Fort Pulaski: The Georgia Historical 

Quarterly, v. 20, no. 1, p. 41‒51. [Also available at http://www.jstor.org/

stable/i40024843.]

Mercer Parker

U.S. Geological Survey

12201 Sunrise Valley Dr., MS 926A

Reston, VA 20192

Telephone: 703-648-6924

E-mail: mercerparker@usgs.gov

Ellen L. Seefelt

U.S. Geological Survey

12201 Sunrise Valley Dr., MS 926A

Reston, VA 20192

Telephone: 703-648-6006

E-mail: eseefelt@usgs.gov

Christopher S. Swezey

U.S. Geological Survey

12201 Sunrise Valley Dr., MS 926A 

Reston, VA 20192

Telephone: 703-648-6444

E-mail: cswezey@usgs.gov

Contact Information Image and Photograph Credits

Aerial view of Fort Pulaski—Edibobb, Creative Commons 3.0

Fort Pulaski Civil War drawing—Public Domain

Cannon above wall—Bubba73 at English Wikipedia, 

Creative Commons 3.0

Moat and bridge—Billy Hathorn, Creative Commons 3.0

Oyster shell wall—Marc Smith, Creative Commons 2.0, 

https://creativecommons.org/licenses/by/2.0/

USGS photograph attribution included in captions. All other 

photographs courtesy of the National Park Service.

https://doi.org/10.3133/fs20183011

Cockspur Island Drill Site