Painted Hills is named after the colorful layers of its hills corresponding to various geological eras, formed when the area was an ancient river floodplain. The black soil is lignite that was vegetative matter that grew along the floodplain. The grey coloring is mudstone, siltstone, and shale. The red coloring is laterite soil that formed by floodplain deposits when the area was warm and humid.Painted Hills, John Day Fossil Beds National Monument, Painted Hills Unit, Oregon, USA.