The Marin Headlands are composed of ancient ocean-floor rocks -- cherts, basalts, and sandstones -- that were scraped off the Pacific Plate and plastered onto the North American continent over millions of years. The resulting terrain is steep, unstable, and spectacularly scenic. The ridges rise to over 900 feet above the strait, dropping steeply to the ocean on the west and more gradually to the bay on the east. The headlands form the northern jaw of the Golden Gate, creating the narrows through which the Pacific and the bay exchange their waters twice daily.

The strategic importance of the headlands -- they command the approaches to the Golden Gate from the north -- made them a military priority from the Civil War through the Cold War. Battery emplacements, gun positions, bunkers, and a Nike missile site dot the ridges and coastal cliffs. Fort Barry, Fort Cronkhite, and Fort Baker anchored the defensive network. After the military left, the National Park Service took over, and the fortifications became hiking destinations. Concrete bunkers open onto Pacific views. Missile silos sit empty in the fog.

Despite their proximity to a metropolitan area of seven million people, the Marin Headlands support remarkable wildness. The Golden Gate Raptor Observatory monitors one of the West Coast's premier raptor migration corridors from Hawk Hill. Harbor seals haul out on the beaches below. Coyotes trot across the trails at dusk. The headlands' west-facing slopes catch the full force of Pacific weather, creating conditions that are beautiful, dangerous, and wildly variable -- sunshine and sixty-five degrees on one side of a ridge, fog and fifty on the other.