USGS1969
In an attempt to date stone artifacts of Early Man excavated from several sites at the Valsequillo Reservoir, a few kilometers south of Puebla, Mexico, Szabo applied the uranium-series method on bone samples known to be either from the same geologic formation as the sites or in direct association with the artifacts. The geologic context of the bones was studied by Malde, and the archaeological sites were excavated by Irwin-Williams. A date determined for bone associated with an artifact (Caulapan sample M-B-6, see below) agrees with a radiocarbon date for fossil mollusks in the same bed and indicates man's presence more than 20 000 years ago. However, some of these bone dates exceed 200 000 years. Because such dates for man in North America conflict with all prior archaeological evidence here and abroad, we are confronted by a dilemna — either to defend the dates against an onslaught of archaeological thought, or to abandon the uranium method in this application as being so much wasted effort. Faced with these equally undesirable alternatives, and unable to decide where the onus fairly lies (if a choice must be made), we give the uranium-series dates as a possible stimulus for further mutual work in isotopic dating of archaeological material. A sample from the Lindenmeier archaeological site north of Fort Collins and another from a Pleistocene terrace along the Arkansas River, both in Colorado, were also dated.