Geological Formations and White Ash Layer

Stop on Fort Funston Historic & Scenic Tour, a free self-guided tour.

The cliffs at Fort Funston provide a fascinating geological cross-section revealing millions of years of Bay Area history. The most striking feature is a prominent white layer of volcanic ash embedded in the sandstone cliffs. This ash traveled nearly 200 miles from ancient eruptions in the Lassen Peak area and was deposited here thousands of years ago. The layered sedimentary rocks tell the story of ancient seas, rivers, and coastal environments that preceded today's landscape. Ribbons of different colored sediments mark distinct geological periods and depositional environments. The cliffs are actively eroding, constantly revealing new geological details while also making them hazardous to approach. Visitors should observe from safe distances and stay on designated trails. The erosion itself is part of the ongoing geological story, as wind and waves continue to shape this dynamic coastline. The exposed geology makes Fort Funston an outdoor classroom for understanding earth science and the deep time scales over which landscapes form and transform. Each layer represents a chapter in the long story of how the Bay Area came to be.