The story of the badlands begins over 65 million years ago during the Paleocene Epoch. The dinosaurs had just become extinct at the end of the Cretaceous Period. The western half of North America was buckling and folding to create the Rocky Mountains. Large amounts of sediments were forming as water, wind, and freezing worked to break down the mountains. These sediments, mostly sand, silt, and mud, were carried off the eastern slopes by ancient rivers and deposited here in layers. Volcanoes in South Dakota, Montana, Idaho, and across the west were also erupting during this time, spitting out huge amounts of ash. Some of this volcanic ash was blown or carried by rivers into North Dakota and accumulated in standing water. Over time, the sediments turned into the sandstone, siltstone, and mudstone layers now exposed in the park, while the ash layers became bentonite clay.