The Pre-Cretaceous Bedrocks

THE pre-Cretaceous formations of southwestern Oregon and northern California, collectively referred to as the "bedrock series," consist of a wide variety of metamorphic rocks, chiefly mica schists, metavolcanics, argillites, and quartzites, that were intruded during the late Jurassic by a complex of igneous rocks ranging in composition from granite to serpentine. These bedrocks form the rugged Klamath and Siskiyou mountains to the west of the Cascade Range. From Weed, at the foot of Mount Shasta, northward to Medford, the Pacific Highway follows the depression separating the Cascades from this bedrock province. From Medford north to Roseburg, the highway leaves the border zone and makes its circuitous way across the bedrock mountains, leaving the Cascades far to the east (see map, plate 2).

Plate 2

Although not a single fragment of bedrock has ever been found among the ejecta blown out by the volcanoes of the southern Cascades, rocks similar to those of the Klamath and Siskiyou mountains must underlie the Cascade Range, for in the Blue Mountains and elsewhere in eastern Oregon such rocks reappear from beneath the cover of younger deposits.