California’s northern boundary passes through three major geomorphic regions; the Klamath Mountains, the Cascade Range, and the Modoc Plateau. The rugged topography, generally poor soils, and challenging climates of all three have severely restricted economic development. Timber and mining industries have played important roles in their economies in the past, but in recent decades, these have declined, also. Today, fewer than 50,000 people reside in all three regions, and no town has a population as large as 8,000 people.

To the West, the Klamath Mountains have rugged topography with prominent peaks and ridges reaching 6,000-8,000 feet above sea level. In the western Klamath, an irregular drainage is incised into an uplifted plateau called the Klamath peneplain. The Klamath River follows a circuitous course through the mountains. The uplift has left successive benches with gold-bearing gravels on the sides of the canyons. The province is considered by some geologists to be a northern extension of the Sierra Nevada. Rocks include pre-Cretaceous metamorphic, abundant serpentine, and granitic. Volcanic rocks of the Cascade Range lie to the East, Cretaceous sediments lie to the Southeast, and Franciscan and younger Coast Range formations lie to the West.

The Cascade Range, a chain of volcanic cones, extends through Washington and Oregon into California. It is dominated by Mt. Shasta, a glacier-mantled volcanic cone, rising 14,162 feet above sea level. The southern termination is Lassen Peak which last erupted in the early 1920s. The Cascade Range is transected by deep canyons of the Pit River. The river flows through the range between these two major volcanic cones, after winding across the interior Modoc Plateau on its way to the Sacramento River.

The Modoc Plateau is volcanic table land (elevation 4,000-6,000 feet above sea level) consisting of a thick accumulation of lava flows and tuff beds with many small volcanic cones. Occasional lakes, marshes, and sluggishly flowing streams meander across the plateau. The plateau is cut by many north-south faults. The province is bound indefinitely by the Cascade Range on the West and the Basin and Range on the East and South.

Collections of unique aerial panoramas of these three regions may be seen in the
Klamath, Southern Cascades, and Modoc Plateau sections of the California Atlas of Panoramic Aerial Images.