Oregon includes parts of four major physiographic provinces (natural land regions) of the United States. The Pacific Border province occupies the western part of the state and encompasses the Oregon Coast Range, the Klamath Mountains, and the Willamette Valley. The Sierra-Cascade province is dominated by the Cascade Range, which parallels the Pacific Border province. The Columbia Plateau lies in the northeastern and north central part of the state. It is subdivided into the Blue Mountains, the Deschutes-Umatilla Plateau, the Harney Desert, and the Payette Section. In the south is Oregon’s fourth physiographic region, a subdivision of the Basin and Range province called the Great Basin.

The Oregon section of the Coast Ranges is typical of such systems, with ridges running parallel to the coast and narrow valleys between the ridges. The crests of the ridges reach an average height of 1,800 ft in the north. In the south they are higher, reaching an average height of nearly 3,600 ft. The high peaks and rugged headlands along the coast are formed of igneous rock intruded into sedimentary formations. The highest of the mountains is Marys Peak 4,097 ft.

Abutting the Oregon Coast Range are the higher and more rugged Klamath Mountains, which extend southward into California. Their highest peaks exceed 7,000 ft. The ridges and peaks are made up of a complex mass of relatively old marble and limestone as well as serpentine, shale, and hard sandstone. This mountain structure has yielded considerable quantities of gold, chromite, and nickel. There are a few alluvial basins in the Klamath Mountains, notably the Rogue River Valley around the cities of Medford and Ashland, a smaller alluvial lowland near Grants Pass, and another in the vicinity of Cave Junction. These lowlands provide favorable locations for farms and cities and are densely populated.

The Willamette Valley is the only large alluvial lowland in Oregon. The heart of the state, it contains the largest cities and a majority of Oregon’s population. The Willamette River meanders back and forth across this valley, which is more than 150 mi long and up to 30 mi wide. Some hilly areas emerge from the lowland, such as the Salem and Eola hills in the middle of the valley. Elsewhere the lowland is level, and in places artificial drainage is required to make it habitable.

The Cascade Range, containing some of Oregon’s most magnificent scenery, extends the entire length of the state. The general level of the high plateau of the Cascades is 5,000 ft, but numerous sharp volcanic peaks lie above the plateau level of this mountain range. Among them is Mount Hood 11,239 ft, the highest peak in Oregon. Other peaks are Mount Jefferson and a series of three peaks called the Three Sisters, all of which are more than 10,000 ft high. Passes traversed by east-west highways provide fairly easy travel through these mountains, except in winter, when they are sometimes blocked by snow.

The Columbia Plateau, which extends into Washington and Idaho, is primarily the result of great extrusions of lava from fissures and vents that once covered a wide area. One of the major divisions of the Columbia Plateau is formed by the Blue Mountains, which might be more accurately described as a large plateau with some steep and rugged areas. The Wallowa Mountains, which lie in the extreme northeastern part of the state, are sometimes considered a part of the Blue Mountains. In the Wallowas, erosion has exposed the granite underlying the lavas, and the highest part of this region, which is severely glaciated, contains many spectacular peaks and glacial lakes.

Good farmland is found in small alluvial basins in the northeast, around La Grande, Baker City, and Enterprise. These basins are well drained by rivers and receive water for irrigation from the adjacent mountain areas. One of the most dramatic features of this northeastern area is Hells Canyon of the Snake River, the deepest gorge in North America, running along the Oregon-Idaho border. The Snake River has cut a narrow gorge into the lava flows of the nearby mountains, forming a canyon, which in places lies 6,000 ft or more below the adjacent uplands.

The Deschutes-Umatilla Plateau extends eastward from the northern Cascades to the vicinity of Pendleton. The Deschutes-Umatilla is a lava plateau dissected by the canyons of the Deschutes, John Day, Umatilla, and other rivers. The Harney Desert, or High Lava Plains, extends from the central Cascades eastward to the vicinity of Malheur Lake. It consists of relatively young lava flows, covered in places by ash and pumice. Only the irrigated part near the Cascades is very productive. The Payette Section, or Owyhee Upland, consists of old lava plateaus that have been cut up, or dissected, by the action of streams. The lower canyon of the Owyhee has been dammed to produce a long reservoir lying between rugged multicolored cliffs that irrigates a large area near the Snake River.

Oregon’s fourth physiographic region, the Great Basin, is found in south central Oregon, east of the Cascades. Here north-south ranges alternate with broad basins. Some of the basins contain intermittent lakes. An extensive irrigated area near Klamath Falls is supplied with water from the Cascade Range.

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