The sky was spring blue and the air mercifully dry on the day I visited the Gault School of Archaeological Research, about fifty miles north of Austin. Filmmaker Olive Talley spread out a cloth near Buttermilk Creek, and our little band, which included archaeologist and this site’s champion Michael Collins, sat in the shade of the pecan trees for lunch. With an odd sort of electric contentment, I realized that our tableau—the water trickling as we talked and laughed, sharing food, admiring particularly well-crafted stone tools, and pointing out wild turkeys—was one people had replicated in that very spot for 20,000 years.The flakes of rock at our feet told a story, but there’s no substitute for a good interpreter. Talley’s documentary, The Stones Are Speaking, covers…