During the Jurassic Period, hundreds of dinosaurs inhabited this area along the Purgatoire River when it was a vast lake. As they moved through mud along the lakeshore, they left trails of footprints in the soupy muck. The muddy flats later baked hard in the sun, were buried, and turned to stone. Today, more than 1,300 dinosaur footprints are exposed across nearly a quarter mile of bedrock, preserving evidence of dinosaur behavior from 150 million years ago. The footprints were made by several kinds of dinosaurs, mainly huge long-necked plant-eating sauropods traveling in herds and prowling meat-eaters. The Purgatoire River, which unearthed the site, has also begun eroding it away, and the U.S. Forest Service preserves it with erosion-control structures made from displaced blocks of the site itself.