The formations of this region are separated by major unconformities into three divisions of equal areal extent: (1) Basement, composed of metamorphosed sediments intruded by granite and more basic rocks, (2) Tertiary, including middle Eocene stream gravel, overlain by rhyolitic, andesitic, and basaltic deposits, of middle Eocene, Miocene, and Pliocene age, and (3) Quaternary, made up of glacial, landslide, and alluvial materials.
The Tertiary volcanic deposits are separated by lesser unconformities. These, and the major post-basalt unconformity, resulted from episodes of compressive deformation which recurred from middle Tertiary to post-Pliocene time along the Donner zone, which extends north-northwest for 15 miles or more along the crest of the range. Median in this zone, but not persisting throughout its length, is its dominant element, a west-dipping reverse fault, on either side of which are genetically related faults and folds.
Bounding the Donner zone on the east is a belt of east-dipping normal faults, some 2 miles wide, which originated in late Quaternary time, after the last compressive deformation.
The Donner zone originated as a fold in metamorphic rock. With progressive deformation, the surrounding granite accommodated itself by reverse faulting which was thereafter the dominant activity. During compressive episodes there was folding of both Tertiary and basement where the latter is metamorphic rock, and faulting of both where the basement is granite. However, where younger Tertiary rests unconformably on previously faulted older Tertiary, the younger formation was deformed by folding as well as faulting.