South of La Junta, where Bent's Old Fort stands on the Arkansas, the plains look as if nothing could be hidden in them. Then the ground breaks open into the canyon of the Purgatoire River, some 250 to 400 feet deep, and on its floor, in a bed of gray limestone the river has scoured clean, are the footprints of dinosaurs.
A lakeshore in the Jurassic
About 150 million years ago, in the Late Jurassic, this was the edge of a broad, shallow freshwater lake. The rock is part of the Morrison Formation, the same layer that holds the bones at Dinosaur Ridge near Denver and the quarries of Wyoming and Utah. Animals walking the soft lime mud along the shore left prints that dried, were buried and turned to stone. Ripple marks in the same beds show where the water lapped.
The Picket Wire Canyonlands tracksite holds more than 1,300 prints in about a hundred separate trackways across a quarter mile of bedrock, the largest known dinosaur tracksite in North America. Most were left by two kinds of animal: big plant-eating sauropods of the brontosaur kind, whose round, tub-sized prints cross the rock in long lines, and the three-toed meat-eater Allosaurus.
Walking together
What makes the site matter is the pattern. Several sauropod trackways run parallel, side by side, the same size and pace and heading the same way, as if a group of animals was moving along the shore together. To paleontologists it is strong evidence that sauropods traveled in herds. The Allosaurus tracks, by contrast, wander singly.
The river keeps changing the picture. Floods bury stretches of the bedrock in sand and gravel and uncover others. In the early 2010s about sixty-five volunteers working with Forest Service paleontologists cleared modern sediment from a stretch of the riverbed and re-exposed more than forty-five tracks in three trackways, made by three young sauropods walking together.
Getting there
The tracksite lies on the Comanche National Grassland, much of it farmland that failed in the Dust Bowl and was bought back by the government. From the Withers Canyon trailhead a trail drops about 250 feet into the canyon and follows the river upstream, roughly eleven miles round trip, open to hikers, cyclists and riders but closed to camping. About four miles in it passes the ruins of the Dolores Mission and its cemetery, the center of a small Hispano community in the late nineteenth century; the tracks lie about a mile and a half beyond, on both banks, and seeing the far side means wading the river. In spring and fall the Forest Service runs guided auto tours by reservation, the only way to drive in.
The river's name is its own story. Spanish tradition calls it the river of souls lost in purgatory, after an expedition said to have perished there; French trappers shortened that to Purgatoire, and English-speaking cattlemen turned it into Picketwire.