“Painted Hills is one of the three units of the John Day Fossil Beds National Monument, located in Wheeler County, Oregon. It totals 3,132 acres and is located 9 miles northwest of Mitchell, Oregon and 75 miles east of Bend. 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. The yellows, golds, blacks, and reds of the Painted Hills are best seen in the late afternoon. Tones and hue may appear to change from one visit to another, as the claystones differ with ever-changing light and moisture levels.
—-from Wikipedia and the NPS