<P> The site was listed on the National Register of Historic Places in 1978 . In 1999, the Pennsylvania Historical and Museum Commission installed a historical marker noting the historic importance of the site . It was designated a National Historic Landmark in 2005 . It is designated as a historic public landmark by the Washington County History & Landmarks Foundation . </P> <P> Native Americans left the site during the American Revolutionary War . It was not re-discovered until many years later, when, in 1955, Albert Miller found the first artifacts in a groundhog burrow . Miller delayed reporting his findings until he contacted James M. Adovasio, who led the first excavations of the site in 1973 until 1979 by the Cultural Resource Management Program of the University of Pittsburgh . Further University of Pittsburgh field school excavations were conducted through 1989 . Since the 1990s, more recent work has also been undertaken by Adovasio through the Mercyhurst Archaeological Institute . The methods of excavation used at Meadowcroft are still seen as state - of - the - art . It is viewed as one of the most carefully excavated sites in North America . </P> <P> Radiocarbon dating of the site indicated occupancy beginning 16,000 years ago and possibly as early as 19,000 years ago . The dates are still controversial . A recent survey carried out by the Society for American Archaeology reported support from 38% of archaeologists, with 20% rejecting the early dates . Criticism of these early radiocarbon dates has focused on the potential for contamination by ancient carbon from coal - bearing strata in the watershed . The samples, tested by an independent third party geomorphologist, concluded that the samples showed no evidence of groundwater activity . Tests performed via accelerator mass spectrometry also support the earlier dates . If authentic, these dates would indicate that Meadowcroft was used in the pre-Clovis era and, as such, provides evidence for very early human habitation of the Americas . Meadowcroft Rockshelter may be the oldest known site of human habitation in North America, providing a unique glimpse into the lives of prehistoric hunters and gatherers . Paleoindian, Archaic, and Woodland remains have all been found at the site . </P> <P> Meadowcroft Rockshelter has yielded the largest collection of flora and fauna materials ever recovered from a location in eastern North America . The arid environment provided the necessary and rare conditions that permitted excellent botanical preservation . In total, animal remains representing 149 species were excavated . Evidence shows that people gathered smaller game animals as well as plants, such as corn, squash, fruits, nuts and seeds . </P>

When did the oldest human occupation of meadowcroft rockshelter occur