Samples from the floor of a rock shelter in Pennsylvania were dated by analyzing the carbon they contained  The dates assigned to samples associated with human activities formed a consistent series, beginning with the present and going back in time, a series that was correlated with the depth from which the samples came  The oldest and deepest sample was dated at 19, 650 years before the present, plus or minus 2, 400 years
Skeptics, viewing that date as to early and inconsistent with the accepted date of human migration into North America, suggested that the samples could have been contaminated by dissolved old carbon carried by percolating groundwater from nearby coal deposits
Not every application of the carbon-dating procedure has led to results that have been generally acceptable to scientists.