Jad D'Allura

The Cascade-Siskiyou National Monument is dominated by volcanic rocks.
These rocks overlie a sequence of Jurassic crystalline basement rocks, Upper
Cretaceous marine rocks, and Upper Eocene continental river deposits. The
oldest, and slightly altered, volcanic rocks are part of the Western Cascade
Volcanic Series (WCVS; 35.9-19.5 Ma). From SW to NE, and older to younger, the
WCVS is comprised of: distal to medial volcaniclastics with minor lava; medial
to proximal facies shield and composite volcanic lavas and volcaniclastics; an
interval of silicic ash falls, ash flows, and andesite to basalt lava flows; and
younger basaltic andesite lava. All are tilted to the northeast with the tilt
decreasing in progressively younger units. In the eastern portion of the
Monument, a succession of High Cascade (5.77-0.33 Ma) shield volcanoes of
basalt and basaltic andesite lava flows and vents unconformably overlie
WCVS rocks. Steeply-dipping NE-trending faults affect rock units older than a
26.5 Ma tuff. One of the major faults, the Siskiyou Summit Fault, shows oblique
movement including right lateral motion. Steep NW-trending normal faults
affect all younger rocks, a few of which show right-lateral motion.