DURHAM, ENGLAND--Geoarchaeologist Nicholas Felstead of Durham University has conducted a new analysis of human footprints discovered in northeastern Mexico in 1961. The tracks, uncovered during highway construction, had been preserved in travertine, a sedimentary rock that contains traces of uranium. Uranium decays into thorium at predictable rates, allowing scientists to measure the ratio of uranium to thorium and to determine the age of the specimen. Felstead dated the footprints to 10,500 years ago, making them the oldest human footprints in North America. Another track way of 11 prints found in a nearby quarry was dated to 7,250 years ago.