These Oregon Hills Display 35 Million Years of Climate History in Colorful Layers

Painted Hills, Oregon

Oregon’s climate shifted from near-tropical to temperate over millions of years, and these hills recorded every change in their soil chemistry.

The result is a landscape where each colored stripe tells you exactly what the weather was like when that layer formed. Here’s the timeline and where to find these spectacular formations.

Ancient Volcanic Origins

Volcanoes in the early Cascade Mountains erupted violently 35 million years ago, sending huge clouds of ash into the air. Winds carried this ash eastward where it settled in thick layers across the John Day Basin.

The ash contained tiny bits of volcanic glass, rock, and minerals. Over time, more eruptions added new layers on top of old ones.

From Tropical Paradise to Cooler Climate

Between 40 and 20 million years ago, Oregon’s climate changed dramatically. Early on, during the Eocene period (55.8-33.9 million years ago), the region was tropical with 33-51 inches of rain each year.

Palm trees, bananas, and cinnamon trees grew in this warm, wet environment. Animals similar to those in today’s jungles lived here.

Over time, the Earth cooled as it entered the Oligocene period (33.9-23 million years ago). The area became drier and cooler, changing from dense jungle to more open woodland and grassland.

Red Soils From Tropical Periods

The bright red layers in the Painted Hills formed during warm, wet times. Heavy rain caused iron in the volcanic ash to rust, creating the deep red colors.

These red bands are called laterites, soils rich in iron and aluminum. They form in places with rainy seasons followed by dry ones, which concentrates minerals into distinct layers.

The rusting happened faster in warm, wet weather, turning the soil redder. Water flowing through the landscape carried these red soils onto ancient floodplains.

Yellow and Tan Layers From Drier Times

The yellow and tan bands show times when the climate was cooler and drier, with less standing water. During these periods, the region received about 27-37 inches of yearly rainfall, much less than during tropical times.

In these conditions, iron formed different minerals that created yellow colors instead of red ones. Many of these layers kept some of their original ash color.

Cycling Climate Conditions

From 34 to 28 million years ago, the climate switched back and forth between warm/wet and cool/dry periods. These changes created the striped pattern we see in the hills today.

Each stripe records a specific time, preserving evidence of that period’s temperature and rainfall. The cycle continued for about 6 million years as Earth gradually cooled.

The growing Cascade Mountains also affected the local climate by blocking rain clouds, creating drier conditions east of the range.

Black Streaks and Dark Layers

The black bands and streaks in the hills formed from ancient plant material that turned into lignite, a type of soft coal. Plants grew along rivers and in swamps, later buried by ash falls.

Without oxygen, dead plants didn’t fully rot. Instead, they turned into carbon-rich layers. Areas with lots of plants created the darkest deposits.

Black dots and streaks also came from manganese, a mineral that plants concentrated while they were alive. As water bodies dried up, dissolved manganese formed dark patches. These dark layers show where ancient wetlands once existed.

Mineral Composition of the Colorful Bands

The rainbow colors in the hills come from various elements in the volcanic ash: aluminum, silicon, iron, magnesium, manganese, and others. Iron mainly creates the reds and yellows, while manganese forms blacks and purples.

The ash was light-colored at first but changed as it mixed with soil and weathered over time. Green layers contain minerals that formed in cool conditions with moderate rainfall.

Different minerals formed depending on temperature, water content, and acidity. Each layer’s unique mix of minerals tells the story of the environment when it formed.

Visiting Painted Hills, Oregon

You’ll find the Painted Hills unit of John Day Fossil Beds National Monument 9 miles northwest of Mitchell, Oregon, off Highway 26. The unit is open from sunrise to sunset daily with no entrance fee.

The Painted Hills Overlook Trail offers the best overall view of the colorful hillsides formed by 35 million years of volcanic ash and climate changes. At Carroll Rim Trail, climb to an elevation of 3,100 feet for panoramic views of the striped layers.

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