United States

The Northern Rockies contain the most varied mountain landscapes of the Cordillera, reflecting a corresponding geologic complexity. The region’s backbone is a mighty series of batholiths—huge masses of molten rock that slowly cooled below the surface and were later uplifted. The batholiths are eroded into rugged granitic ranges, which, in central Idaho, compose the most extensive wilderness country in the conterminous United States. East of the batholiths and opposite the Great Plains, sediments have been folded and thrust-faulted into a series of linear north–south ranges, a southern extension of the spectacular Canadian Rockies. Although elevations run 2,000 to 3,000 feet (600 to 900 metres) lower than the Colorado Rockies (most of the Idaho Rockies lie well below 10,000 feet [3,050 metres]), increased rainfall and northern latitude have encouraged glaciation—there as elsewhere a sculptor of handsome alpine landscape.

The western branch of the Cordillera directly abuts the Pacific Ocean. This coastal chain, like its Rocky Mountain cousins on the eastern flank of the Cordillera, conceals bewildering complexity behind a facade of apparent simplicity. At first glance the chain consists merely of two lines of mountains with a discontinuous trough between them. Immediately behind the coast is a line of hills and low mountains—the Pacific Coast Ranges. Farther inland, averaging 150 miles (240 km) from the coast, the line of the Sierra Nevada and the Cascade Range includes the highest elevations in the conterminous United States. Between these two unequal mountain lines is a discontinuous trench, the Troughs of the Coastal Margin.

The apparent simplicity disappears under the most cursory examination. The Pacific Coast Ranges actually contain five distinct sections, each of different geologic origin and each with its own distinctive topography. The Transverse Ranges of southern California are a crowded assemblage of islandlike faulted ranges, with peak elevations of more than 10,000 feet but sufficiently separated by plains and low passes so that travel through them is easy. From Point Conception to the Oregon border, however, the main California Coast Ranges are entirely different, resembling the Appalachian Ridge and Valley region, with low linear ranges that result from erosion of faulted and folded rocks. Major faults run parallel to the low ridges, and the greatest—the notorious San Andreas Fault—was responsible for the earthquake that all but destroyed San Francisco in 1906. Along the California–Oregon border, everything changes again. In this region, the wildly rugged Klamath Mountains represent a western salient of interior structure reminiscent of the Idaho Rockies and the northern Sierra Nevada. In western Oregon and southwestern Washington the Coast Ranges are also different—a gentle, hilly land carved by streams from a broad arch of marine deposits interbedded with tabular lavas. In the northernmost part of the Coast Ranges and the remote northwest, a domal upwarp has produced the Olympic Mountains; its serrated peaks tower nearly 8,000 feet (2,440 metres) above Puget Sound and the Pacific, and the heavy precipitation on its upper slopes supports the largest active glaciers in the United States outside of Alaska.

South Fork Kings River, Kings Canyon National Park, California, U.S.Josef Muench

East of these Pacific Coast Ranges the Troughs of the Coastal Margin contain the only extensive lowland plains of the Pacific margin—California’s Central Valley, Oregon’s Willamette River valley, and the half-drowned basin of Puget Sound in Washington. Parts of an inland trench that extends for great distances along the east coast of the Pacific, similar valleys occur in such diverse areas as Chile and the Alaska panhandle. These valleys are blessed with superior soils, easily irrigated, and very accessible from the Pacific. They have enticed settlers for more than a century and have become the main centres of population and economic activity for much of the U.S. West Coast.