Suggested Citation: Kerr, Andy. 2000. Oregon Desert Guide: 70 Hikes. Seattle: The Mountaineers Books. pp. 42-43.
Geology does not hide well in the desert. The vegetation is sparse enough (or nonexistent) that one can often see the hare rock in all its color, shape, size, form, and texture.
If you are quizzed about a random rock in the Oregon Desert and you have no idea as to its kind, guess basalt. You would be right more often than not. The second guess should be rhyolite. Both are volcanic.
Basalt is a fine-grained dark-colored igneous rock that often has a smooth texture. It is black when "fresh." If weathered or otherwise altered, it tends to greenish black or shades of rusty brown. On occasion, it is red as a brick.
Rhyolite is light-colored pastels of yellow, gray, and pink.
Essentially all of the Oregon Desert rock is volcanic. Some rock is quite old, while other deposits have been identified as having been molten just 100 years ago. Some of the most ancient rocks in the Oregon Desert, between 215 and 190 million years old, are in the Pueblo Mountains. They are the rare nonvolcanic formations.
Many forces and events have shaped the desert to date, the most significant being volcanism, uplifting and faulting, and erosion by wind and water.
Nearly everywhere you look at the desert, if you know what to look for, you see lava flows, cinder cones, volcanic plugs, the ghosts and corpses of old calderas, and other evidence of volcanism.
To see representative samples of the freshest volcanic activity, start at the Newberry National Volcanic Monument southeast of Bend in the Deschutes National Forest and then travel to the Fort Rock Lava Beds, Diamond Craters, and finally Jordan Craters (all proposed national monuments).
The Basin and Range ecoregion is a series of long and narrow, north-south-trending fault-block mountain ranges separated by sediment-filled basins. Steens Mountain is a classic example, sloping gently up from the Catlow Basin on the west in a 1-mile rise over a distance of 18 horizontal miles. From the summit, the mountain drops dramatically downward another mile over a horizontal distance of but 3 miles to the Alvord Desert. From the east side, one can easily see the numerous layers of volcanic deposits. Steens Mountain is high enough to have been glaciated during the four ice ages that have come and gone in the Oregon Desert. Classic U-shaped glacial valleys are still clearly visible.
Between the uplifts in the Oregon Desert are flat valleys that often have no outlet to the sea—hence the descriptor Basin and Range.
The Great Basin is the name given both to a huge area of land that has no outlet to the sea and to one of the four major types of American desert. It should have been called the Great Basins or Countless Basins, because it is actually a great number of basins with either a lake or a dry lake (due to evaporation) at their bottom. Alkali salts remain after evaporation in the dry season, leaving playas.
In earlier times, the climate was tropical and subtropical, and the land was covered with oceans. Mountain ranges have come and gone. In "recent" (geologically speaking) times, during the Pleistocene (2 million to 10,000 years ago), large pluvial lakes filled between the ranges. Christmas and Fort Rock Valleys were covered with water; Lake Abert and Summer Lake were one and called (later by us) Lake Chewaucan (CHEE-wah-CAN). Upper Klamath Lake hadn't yet found its way to the Pacific. The largest lake was in the Catlow Valley It dried out enough to have the general appearance it has today. The ancient lake shore terraces can often be seen 70 to 300 feet above the valley floors.
Just as the desert isn't all sagebrush vegetation, it is not all basalt and rhyolite. Hot springs abound in many areas along fissures or breaks caused by uplifts. Boiling mudpots can be found. Glass Butte is a solid mountain of mostly black, but some reddish, obsidian.
The bright reds and oranges at the Honeycombs and Leslie Gulch (in the proposed Lower Owyhee National Conservation Area) resemble the canyonlands of Utah. For stark layers of reds, blues, greens, yellows, tans, oranges, grays, browns, and blacks, check out the John Day country.
More local geology is described in appropriate wilderness sections.