Just north of the Florida-Georgia border lies an island with no bridges; it has mansions with no people, a road but few cars.

Once privately owned, two of the mansions and most of the island are now part of Cumberland Island National Seashore. Its 16-mile, shell-speckled beach appears endless and is usually deserted. Its spartan salt marsh harbors uncountable fiddler crabs and other small, soft, barely glimpsed creatures. Dark green, low-lying live oaks hung with Spanish moss blanket the center of the island. Some of their branches, too long and too heavy, droop deeply and rest on the ground.

In this forest even an amateur can see wild turkey and deer. Alligators and shore birds congregate near Lake Whitney. Loggerhead turtles lay eggs on the island's beaches, warblers and woodpeckers infest its trees.

The island once belonged to the Carnegies -- not Andrew, but Tom, Andrew's younger brother and business partner. They built the mansions. Tom's granddaughter, Lucy Ferguson, still owns one -- Greyfield, which she converted into an inn in 1966.

Today, Greyfield has a jeep available for tours, but the best way to see Cumberland is to walk. And from Greyfield, a walk to the south end best samples the island's historical sights, its wildlife and its beaches.

I started my tour of the south end one morning with a knapsack sagging under the weight of Greyfield's picnic lunch. I headed south on Grand Avenue, the hard-packed, crushed-shell road that bisects the length of the island. Live oaks line this road; their overlapping branches provide a shady, green canopy that, in spots, transforms the road into a tunnel.

Since deer and feral pigs love live-oak acorns, I stopped when I heard the dry, crackling sound of an animal crashing about in the dead saw palmetto. Green fronds rippled as it came toward me.

It emerged from the underbrush four feet ahead. The slender head with its thin, powerful jaws swung from side to side as it gathered its legs under its heavily armored body for the final charge. It started toward me, malevolence gleaming from its tiny eyes. "Boo!" I said, and the armadillo, finally seeing me, turned and ran, grazing a small tree and almost falling as it dashed into the forest.

I continued south to Seacamp Visitor Center. The morning supply of backpackers and day visitors had just arrived. The National Park Service limits the number of visitors they allow on Cumberland to 300, 120 of whom can camp overnight. Seacamp, less than a half mile from the NPS ferry dock and visitor's center, has the only developed campsites, but backpackers can choose from four primitive campgrounds.

Just below Seacamp I stopped briefly at Dungeness Dock -- where the park service maintains a small museum in the old Carnegie icehouse -- and then went on to Dungeness, the ruins of the 40-room mansion built by the Carnegies in the late 19th century. The realities of the site and the ruin illustrate the island's history better than any museum could -- if you know a little of its past.

Cumberland's history started with the Timucuan Indians 2,000 years ago. Theirs was a primitive society of hunters and gatherers that roamed freely over the island, leaving behind only middens, huge piles of shells discarded by centuries of raw-oyster lovers. One Timucuan midden can be seen at the northern end of the island; though you can't spot it today, there was a similar one at Dungeness.

In 1566, the Spanish arrived to build forts and missions, to convert the Indians to Christianity. The forts have vanished, but the Franciscan monks were successful: They converted many Timucuans. Then the Timucuans vanished, victims of overwork and European diseases. Still, more people came, among them the English and the French.

They sparred with the Spanish for the island for 150 years -- until 1736, when James Oglethorpe, the English founder of Georgia, built two forts on the island, one on the northern end, one on the southern. He also built a hunting lodge on the south end, near the Timucuan midden, which he called Dungeness. (The brick-and-stone ruin that remains today stands on Oglethorpe's site.)

Revolutionary War hero Gen. Nathanael Greene bought the southern end of Cumberland soon after the war. He died in 1786, and his widow Catherine built a large four-story house, also called Dungeness, on the site of Oglethorpe's hunting lodge.

In the mid-1800s, America's industrialization was spearheaded by Andrew Carnegie, who towed younger brother Tom along with him. Tom bought much of Cumberland in 1881 and began building his Dungeness soon after, wanting to escape the business pressures of Pittsburgh. But he died five years later at age 42. His wife Lucy stayed on, and Dungeness flourished: In its heyday the 40-room mansion and its outbuildings required 300 servants to keep them running.

Many of these outbuildings are still standing, and one, a small white building with simple lines, remains in good shape. It was built about 1800, long before the Carnegies arrived, out of tabby -- a lime, sand and oyster-shell cement -- and is known as Tabby House. It is the oldest building on Cumberland, the only one on the Dungeness property not leveled when the Carnegies began to build. The Carnegies closed Dungeness in the 1920s, and it burned in 1959 in a fire some say was started by a disgruntled ex-employe.

If you approach the ruin by walking down Grand Avenue, you will first glimpse it through sabal palmetto fronds. Through the rusted wrought-iron arch over the main entrance, details become clearer. The "Danger -- Keep Out" signs are strung on ropes between jumbled piles of bricks and stone. Saw palmetto sprouts where the front door once opened, and wild horses graze on the back lawn. The edges of the ruin are soft, blurry against the bright Georgia sky. Vegetation creeps in; Cumberland is healing itself. In another century the ruin will be a low hump on the land -- a white-man's midden.

From the ruins I walked eastward, past old out buildings now used by the park service as equipment maintenance sheds. I crossed one row of dunes, passed through a meadow where I'd seen wild turkey on an earlier trip, then topped the last row of dunes. The bright beach spread out in front of me. No moody ghosts here; the present prevails.

I walked south on the beach. It was wide and hard and easy walking, and the birds saved it from becoming boring. Gulls, royal terns, black skimmers and sandpipers formed a living strip separating sand from sea. In the ocean rafts of ducks bobbed about.

After lunch with the birds on the beach, I continued south toward the jetty and Pelican Banks. I saw pelicans. And oyster catchers. And those crazy little sandpipers that hop around on one leg though they have two. And other shore birds I couldn't identify.

I passed the southern tip of the island and reached Beach Creek. After squishing around in the mudflats trying to find a direct route back to Dungeness, I gave up and headed back toward the beach. I walked five miles north on the hard white sand now cooling in the late afternoon sun before turning west toward Greyfield. I saw only two people, and they were too far away to identify.

At Greyfield I settled into a front-porch rocker and drank a pitcher of fine iced tea. I watch three deer come to graze on the front lawn, then let the slowly descending darkness envelop me.