The Painted Hills is part of the John Day Fossil Beds National Monument
and is named after the colorful layers which correspond to various
geological eras, which formed when the area was an ancient river floodplain.
The black soil is lignite that used to be plants that grew along the banks.
The grey coloring is mudstone, siltstone, and shale. The red coloring is
laterite soil that formed by deposits when the area was warm and humid.
(Information courtesy Wikipedia)