Geology of the Western Siskiyou Mountains

Abstract

The results of the research in the western Siskiyou Mountains may be grouped in three principal categories, all of which represent original investigation. The first category deals with the land forms or physiography. Three distinct provinces, the coastal belt, the Klamath oldland, and the Siskiyou upland, are herein described in detail. The coastal province near the California-Oregon line exemplifies emergence and in this process the large terrace, on which Crescent City is situated, was formed. The Klamath oldland, a sub-aerial erosion surface, is found to have been developed in late Pliocene time and later uplifted 1000 to 3000 feet. The Siskiyou upland is fretted with mountain peaks of igneous rocks and has been subjected to Pleistocene glaciation. The second phase of the study, stratigraphy (and petrology) was concerned with the physical characters of the rocks exposed and with defining their position in the geologic sequence of the northwestern Klamath Mountains. The third major problem attacked is the crustal structure of this mountainous region. The principal fault, known as the Orleans fault, is reverse in nature and of large throw. By movement along it the Siskiyou upland has been lifted above the Klamath oldland to the west. The Crescent City fault has outlined the coastal pattern south of that community. By displacement on it the Crescent City terrace has been depressed with respect to the Klamath oldland to the east. The results of folding, intrusion, metamorphism, and faulting give eloquent witness to Klamath Mountains orogeny and indicate the circumferential compression of the group.