The bedrock of the canyon is made up of rocks of the Marin Headlands "terrane". The terrane 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.