Nineteenth century Americans knew the Pecos River as the boundary of the West. And the westering traveler today can feel that transition. In the canyonlands at the confluence of the Pecos and the Rio Grande, the Hill Country gives way to the Chihuahuan Desert. Mountains appear on the western horizon. Ocotillo and yucca stud a harsh landscape.

These canyonlands of the Lower Pecos are also a world-class archeological site. Caves and shelters here preserve one of the richest records we have of hunter-gatherer life in North America. Now, a group of Texas archeologists is taking a new look at that 13,000-year record.

A half-mile from Langtry, Texas, Eagle Nest Canyon is luminous in midday l...