Learn about the mountain range that preceded the Rocky Mountains and how remnants of that long-lost range are still visible today—as jagged slabs of red sandstone—in this video from NOVA: Making North America: Origins. Long before the Rockies formed, the Ancestral Rockies stood. Over millions of years, these ancient mountains eroded into the sand and gravel that eventually compressed into horizontal layers of sandstone. Later, underground forces drove a large piece of the ocean floor under the North American continent, shattering the sandstone layer and forcing pieces of it to jut out of the ground. This resource is part of the NOVA: Making North America Collection.