Painted Hills

The Painted Hills began to form 35 million years ago when pumice and ash from volcanic eruptions in the Cascade Mountains traveled 100 miles east and settled over the area.[4][5] Once there, the ash and other sediments were mixed by natural processes including the flow of water, growth of plants, and the movement of animals. Over time, this led to oxidation of the ash on the surface. Buried under new layers and deposits, the ash turned into soils by way of compaction and cementation. With more time and weathering, the exterior surfaces of The Painted Hills were worn into clay. Now, they are primarily made of hard claystone layers.[6]

The colors of the Painted Hills come from the combination of elements including Aluminum, Silicon, Iron, Magnesium, Manganese, Sodium, Calcium, Phosphorus, Titanium, Potassium, Oxygen, and Hydrogen, among others, which arrived with the initial ash fall. Originally light in color, the ash mixed with soils and other elements to form the minerals that give the hills colors such as red, yellow, and black.[6]

The alternating red and tan layers within the hills are the result of cycling climate conditions, which were alternately warm and wet or cool and dry for a period of time between 34 and 28 million years ago.[2] As the mild, humid Eocene epoch gave way to the Oligocene, the tropical climate shifted, becoming cooler and drier.[3][4] Red soils came from floodplain deposits during the more tropical periods, and are made up of laterites, or soils rich in iron and aluminum.[7] These rust-colored layers are full of iron and oxygen in the form of iron oxide.[6] The yellow layers are from a drier and cooler time[7] and take on their yellow hue from iron and magnesium oxides.[6]

The darker, black soil is lignite that was once vegetative matter that grew along the floodplain. The grey coloring is mudstone, siltstone, and shale.[8] The black marks on the hills get their color from manganese oxide.[6]

Fossils are rare in the Painted Hills. The original deposits of ash arrived irregularly, creating poor conditions for fossilization, and most organic matter decayed. Still, it is possible to find leaf fossils from ancient lake beds throughout the hills.[6] Many of the fossilized leaf impressions that have been found indicate the presence of plants and trees including redwood, cinnamon, fig, and alder, which correspond to the warmer, more humid climate that predominated during the early formation of the hills.[4]

An abundance of fossil remains of early horses, camels, and rhinoceroses in the larger Painted Hills unit makes the area particularly important to vertebrate paleontologists.[9]