Some of the largest dinosaur trackways are called megatrack sites, where footprint-bearing rock can extend for hundreds or even thousands of miles. Several Jurassic and Cretaceous periods sites in North America have such trackways. For example, tracks in the Entrada sandstone beds in eastern Utah (Middle Jurassic) cover about 116 square miles (300 square kilometers); the density of the prints is estimated to be between 1 and 10 per 10.8 square feet (1 square meter). Another is a 150-million-year-old dinosaur trackway in the Purgatoire River Valley, Colorado. The area was once home to a large freshwater lake, allowing dinosaurs to walk through the mud along the edge, and leaving behind vast trails of footprints. Today, over 1,300 of these now-solidified-in-rock footprints are exposed at the Picketwire Canyon-lands dinosaur trackways.