Much of the western United States was once blanketed in hundreds of feet of sand. The unforgiving sun beat down on the landscape for 20 to 30 million years during the early Jurassic period. Thin layers of rock allowed water to collect even in the dry desert, though sometimes it was hidden a few inches below the surface. Dinosaurs and other animals were able to survive the harsh conditions, and as the sand slowly turned to sandstone, traces of these animals were caught and preserved in the rock, creating fossils.

More than 150 million years later, a man named Earl Douglass was born in Medford, Minnesota in 1862. He didn’t know it yet, but his fate was already entwined with the dinosaurs that once roamed the earth.

This week on America’s National Parks: Earl Douglass and Dinosaur National Monument.