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).