Developed in the mid-twentieth century by Willard Libby and Jim Arnold, radiocarbon dating (used for dating organic materials)brought the chronology of North American prehistory into sharp focus. The method provided accurate ages for the deposits of organic material left behind by the last great ice sheets that covered the continent during the Pleistocene Ice Age that began approximately 1.6 million years ago. One such deposit was determined to be 11,557 years old. Radiocarbon was also used to date the traces(indications of their presence) left by America's earliest human inhabitants, enabling scientists to map out the movements of people and glaciers and to investigate the interrelationships between the two. Libby's first attempts to analyze an ancient North American site included what archaeologists call the Folsom culture.

Folsom is a small town in northeastern New Mexico. In 1926-27, arrowhead-like stone points were found there, mixed together with bones from a now-extinct type of bison, a discovery that caused great excitement because it placed humans in New Mexico during the last glacial period, when bison were abundant. Beyond that general observation, however, there was no way to date the site. Eventually, additional"Folsom"sites were discovered in other regions, all characterized by the same distinctive stone points. A few of these were in places that could be correlated with specific glacial deposits, which, through a fairly weak line of reasoning, were thought to be between 10,000 and 25,000 years old.Most workers favored the older end of the range.

Arnold and Libby included a charcoal sample linked to the Folsom culture in their first published list of radiocarbon dates. The result was a surprise: 4,283 +/- 250 years B.P. (before the present). This was clearly much younger than any of the earlier estimates suggested, and, if the date held up, it would mean that what appeared to be one of the oldest Native American cultures was actually quite recent. Although they were confident about their analysis procedures, Libby and Arnold were suspicious of the result and wondered if the sample had been contaminated with young carbon, or if there was some other difficulty they were unaware of. In the end, it turned out to be a classic case of improper sampling, and an example of the importance of careful field documentation. When the charcoal was collected in 1933 (it had been stored away from then until the analysis), it appeared to be lying within a soil layer that contained both animal bones and the distinctive Folsom stone points. But the unexpectedly young age prompted a reexamination of the site, and it was determined that the charcoal came from a channel that cut into and through the older layers. Although it appeared to be at the same level as the bones and stone points, it was actually much younger. Once this problem was recognized and new samples from this and other sites were analyzed, it became clear that the most reliable Folsom ages fell in the range of 10,000 to 11,000 B.P.

However, it was also discovered that Folsom sites are not the oldest evidence for humans in North America. At some localities, slightly different varieties of stone hunting points occur; initially it was thought that these were simply regional variations, or perhaps weapons used for hunting different types of animals. But, in some places- -notably at Clovis, New Mexico- -they appear in layers that lie beneath the typical Folsom points. This indicated that they were older, and soon archaeologists began to distinguish between Clovis and Folsom cultures. Obviously, Clovis sites became another target for radiocarbon dating, and the results confirmed their antiquity. Clovis sites consistently gave dates that were a few hundred years older than those characterized by the Folsom artifacts, and there seemed to be little or no overlap between the two cultures.

With these results, the radiocarbon dates of both glacial deposits and archaeological sites in North America seemed to be painting a consistent picture. As the last severe glaciation of the Pleistocene Ice Age lost strength, early people spread into the United States. Clovis people were the first widespread hunters, making distinctive stone points for their weapons and hunting large game such as mammoths. Within a few hundred years, however, a new culture appeared, making smaller and finer stone points and apparently taking over from its Clovis predecessors as the dominant hunters in North America.