In 2008, a large Apatosaurus bone was unearthed from a quarry in Picket Wire Canyonlands. Apatosaurus was among the most massive land animals ever to exist and was a long-necked, plant-eating sauropod. About half of an Apatosaurus skeleton was preserved in the quarry, along with partial skeletons of several other sauropod dinosaurs. The bones were buried in mudstone deposits of an ancient system of rivers and lakes, now exposed as red and green soils on nearby slopes and as limestone layers at the dinosaur tracksite. These rock layers, called the Morrison Formation, are known to produce dinosaur remains across the Rocky Mountain West. The bones are 148 million years old and date to the Jurassic period, before the uplift of the Rocky Mountains, when this area was dominated by conifer forests and long before formation of the modern Purgatoire River Valley. The shoulder blade is nearly 8 feet in length, and comparison with similar skeletons suggests this Apatosaurus was roughly 30 meters, or 98 feet, long and weighed over 88,000 kilograms, or roughly 40,000 pounds. Animals such as these left many of the prints at the dinosaur tracksite.