http://www.mnsu.edu/emuseum/archaeology/sites/northamerica/meadowcroft.html
[snip] Dr. James Adovasio is the man who meticulously excavated this large shelter between 1973 and 1977. The archeological remains were found as deep as 11.5 feet below the ground. Adovasio revealed no less than 11 strata, the lowest of which contained traces of human occupation... the lowest sublevel (which also is sealed by the pieces of rock from the walls and roof of the shelter) contains radiocarbon dates from 19,600 to 13,230 years ago... The site has yielded the largest collection of plant and animal remains in a single site in North America. [end]
http://people.delphiforums.com/MCCONAUGHY/meadowcroft/meadow.htm
[snip] The sample dated to 19,600 B.P. +/- 2400 years or 17,650 B.C. (SI-2060, uncorrected) and was recovered in an appropriate stratigraphic location relative to the other early dates from the shelter. If it was from a Paleoindian fire and was contaminated with coal, as some have suggested, how much coal would have to be added to the sample to make it date to 19,100 B.P.? Lets presume that the bark sample should have dated to ca. 11,000 B.P. (and the artifacts recovered from the lower levels at the least demonstrate a Paleoindian presence at the site, they were stratified below Early Archaic materials) and was contaminated by coal to produce 19,000 B.P. dates. Nearly 40% of the sample must have been composed of “dead” carbon to result in this date. Particulate coal contamination would have been readily noticeable to the Smithsonian Radiocarbon lab that dated this sample... particulate coal can be eliminated as a contaminate in this and all the samples from the site. [end]
“the lowest sublevel...is sealed by the pieces of rock from the walls and roof of the shelter...”
Are you saying that they dug all the way to the rock floor, or that they were stopped by a layer of rocks from the walls and roof? If they did not dig all the way to the bottom, perhaps the layer of rocks was caused by a major catastrophe/earthquake, and there might be some very interesting stuff underneath all those rocks that fell from walls and roof.