The Klamath Mountains are the dominant geological feature of northwestern California and southwestern Oregon. Topographically they are part of the set of coastal ranges that run roughly parallel to the Pacific Ocean in California and Oregon, but the Klamath Mountains differ in history, complexity, and geological character. In fact, the Klamath Mountains’ closest geological connection is to the Sierra Nevada. While most of northwest California can geologically be referred to as the Klamath Mountain Province, the region also presents a coastal area called the North Coast Range. In California, the lands west of the South Fork Mountain – the west boundary of the Klamath Mountains – are best considered part of the North Coast. From the city of Eureka, on Humboldt Bay, north this band of rock extends inland 40 miles and tapers to a sliver near Crescent City, CA. This region is composed of rocks from the Franciscan Complex—which is the major component of the coastal ranges south to San Francisco.

The geology of northwest California is as much responsible for the plant diversity as is the climate of the area. The complex mountain topography and composition, in turn linked with the wide array of soils, makes the range unique. Ancient soils have formed during the repeated uplift and wearing away of mountains. Granitic batholiths were formed under the western part of the range—these are the connection to the Sierra Nevada—and have slowly worn away. Terrane accretions also occurred. Terrane accretions are caused by the heavier Pacific being subducted under the North American Plate and in this process, pieces (known as ophiolites) scrape off the top of the Pacific Plate and are deposited, or obducted, onto the North American Plate. The Mendocino triple junction, where the Pacific, North American and Gorda plates all meet  just offshore from Petrolia, CA, has facilitated these accretions through plate movement. This is one of the most active tectonic regions of the San Andreas Fault transform system; earthquakes occur regularly. The geological formation of northwest California has been a complex process.