Glen Canyon Park - Geology and Rock Outcrops

Geology and Rock Outcrops

The scenery of Glen Canyon Park is also distinguished by numerous large outcrops of rock. The most striking of these consist of reddish, layered "Franciscan" chert. These outcrops have clearly visible banding (see photo) which is due to the different weathering of the layers.

The bedrock of the canyon is made up of rocks of the Marin Headlands terrane, which is a large packet of rock that extends diagonally from the Marin Headlands (just north of the Golden Gate), through the Twin Peaks and Glen Canyon area, and on to the southeast. This terrane is from 100 to 200 million years old (i.e. the Cretaceous and Jurassic periods). The bedrock of the lower slopes of the canyon - largely hidden under slope debris/ravine fill - is pillow lava or greenstone; these erupted from fissures in the deep ocean floor when the terrane was located hundreds of miles southwest of its present location.

The upper slopes and cliffs are of layered chert, which hardened into rock from the ooze of remains of countless radiolarian creatures that accumulated on top of the lava. The ooze was colored red by iron from hydrothermal springs. Both lava and chert were formed in the deep ocean near the equator, and were rafted northeast on the gradually-moving ocean floor toward California. Near shore, greywacke accumulated on the chert in some areas, including a small part of the southeast slope of Glen Canyon. Subduction then squeezed the terrane against the continent, and it eventually became part of the Franciscan formation that makes up much of coastal California. During subduction - 160 to 80 million years ago - and the subsequent uplift, the terrane was twisted, broken and disrupted, and the chert was deformed into the tight folds now visible in road cuts along O'Shaughnessy Boulevard (see photo).