Many millions of years ago, a sea covered the middle of the North American continent. As the sea receded, this area became a vast river delta where streams and rivers fanned out and emptied into the sea. Sediment washed down from the Rocky Mountains came to rest in large, flat sheets across the river delta. The layers built up over time, creating the Great Plains, and the colorful layers of the badlands can be traced for miles across distant buttes. Scanning the canyon walls from top to bottom reveals progressively older layers. Plants and animals trapped in sediment were sometimes preserved as fossils, and fossils and other geologic evidence in the exposed rock layers indicate a warm, wet habitat similar to today's Florida Everglades when these rocks formed. The visible badlands rocks are estimated to have formed between 55 and 65 million years ago, making them too young to contain dinosaur fossils; any such fossils would lie deeper in rock layers not yet revealed.