AVELLA, Pa. -- Beer cans crumpled round a dead campfire, signs of late-night partying scorched into the sandstone. In a cut-away 15 feet below the modern fire circle, there's more charred stone, flecked with the shells of Ohio River mussels and the bones of passenger pigeons -- both long extinct.

Between the two fire rings? The oldest site of human habitation in North America, at least 16,000 years old.

That's the surprise of Pennsylvania's Meadowcroft Rockshelter, an inviting sandstone overhang in a tributary valley of the Ohio River that's been welcoming anglers, hunters and travelers since the Paleo-Indians. The site in Avella, 30 miles southwest of Pittsburgh, is now a National Historic Landmark, set amid towering sycamore and pawpaw trees. Its 52 carbon dates, in almost perfect stratigraphic order, reflect a continuous human record for 16,000-plus years.

''It was like a Paleo motel,'' guide Eleanor Crowe said. ``People would come along Cross Creek, seven miles from the Ohio River, and stay here from the earliest Paleo-Indians to the time of European settlement.''

Closed in 2007, the landmark has reopened with a new shelter of its own, a $2.3 million enclosure that's bolted into the bedrock and raked at an improbable 17-degree angle. A new roof protects the archaeological dig, and new platforms allow more visitors to see the excavated levels and start piecing the timeline together for themselves.

''Until we completed the new structure, there was just a temporary wooden structure built by the archaeologists to protect the site,'' said director David Scofield. ``Ten people was a crowd.''

WORK BEGINS

Archaeologists began digging and sifting in 1973, led by James M. Adovasio, a University of Pittsburgh professor. He and his students held six consecutive field schools there. Later, Adovasio founded the Mercyhurst Archaeological Institute at his new school, Mercyhurst, in Erie, Pa., and brought Mercyhurst archaeology students to Meadowcroft.

For now, the dig is quiet, but millions of bone fragments, plant materials and cultural artifacts such as basketry fragments are being studied at Mercyhurst.

Thirty-five years ago, however, Meadowcroft seemed like just a good spot for field study, a ''closed site'' unexposed to the elements. Rockshelters ''often served as veritable magnets for prehistoric and historic populations,'' Adovasio wrote.

Meadowcroft was south of the last Ice Age glaciers. When an ice dam broke between 40,000 and 22,000 years ago, the racing water scooped out the softer sandstone and left the overhang, its ceiling about 43 feet above the modern surface of the shelter. From about 21,000 years ago, when the water subsided -- long before the Egyptian pyramids and the Greek Parthenon were glimmers in anyone's eyes -- the rockshelter was ready for visitors.

Facing south for warmth, with a good east-west breeze, this spot 50 feet above the north shore of Cross Creek would stay dry and well ventilated. It remained 93 miles south of the ice front. Permanent springs to the east and west made it ideal for hunter-gatherers to stay for a few days or to set up a fall hunting camp.

We can still see a deer rib bone sticking out of the rock, proving that Indians butchered their kill here about 400 years ago. But what of the first inhabitants?

As Adovasio and his students bore down into the layers of silt, the cultural evidence kept getting older and older. By the time they hit bedrock about 15 feet down, Adovasio was sending specimens for carbon dating and the word back was staggering: at least 16,000 to 17,000 years old.

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