Which Types of Rock Straddle the Fault Line?

A small thrust fault between greenish diabase and weather greywacke sandstone.

Kirke Wrench

The relative ease of building the first lighthouse on the lightly slopped hill, made way for what was to become an engineer's nightmare. To reach the new location, at the end of this spit, much preparation was required. Notice how the path winds its way among jagged rock. It is re-enforced here and there with retaining walls to combat erosion and interspersed with bridges over precipices dropping to the sea. (Sound of rockslides) And all this needed to be in place before construction could begin on the new lighthouse.

Please walk carefully across the cement bridge. Look up and locate the radar tower. Please stop when you are standing below the radar tower.

There is a vertical swath of dirt running down from the top of the radar tower. The Point Bonita Lighthouse trail is built on two very different kinds of rock, both well represented here. On the left side of this small thrust fault line, there is a wall of diabase, an intrusive igneous rock that forms beneath basaltic volcanoes. This thrust of greenish rock braces Point Bonita against the pounding storms and crashing sea.

On the right side is weathered graywacke sandstone, the crumbly light-colored sedimentary rock. It has begun to break down, and will eventually become sand and soil. Here, the graywacke is unstable-prone to erosion by the powerful forces of wind and sea. And there are many small pieces, which have broken off and fallen to the ground in front of this rock outcrop. Although the diabase at this location appears much more durable than the graywacke, along the path there are many places where the iron-rich diabase and pillow basalt have weathered, contributing to Point Bonita's landslide history.

This fault line extends to the cove below, near the old lifesaving station. The engineers building the lighthouse at the time could never have foreseen the extent of the geologic calamity, which was to occur just three decades later.

Now continue down the trail.

1906 EARTHQUAKE

Official Point Bonita Log Entry: April 18th, 1906
Terrible earthquake occurred at 5:12 a.m., doing considerable damage to the assistant's quarters. Shaking the gables' ends out, and cracking it so badly, that it was with difficulty that the families were taken out without injury... the doors were all jammed, so the keeper had to take his family out the kitchen window. (Sound of crashing, glass breaking, screaming, etc.)

This destruction overwhelmed the keeper's house, a half-century old structure with unreinforced walls, at the top of the hill.

Now stop and face San Francisco across the strait.

The '06 earthquake shook the city to the core. The devastating quake ignited an even more devastating fire. (Fire and fire engine bells clanging in the distance) Half the city was in flames, and the residents of Point Bonita could only watch helplessly. (Sound of flames crescendo)

Please walk down the trail to the white bench that faces the Golden Gate Bridge. You can sit on the bench when you reach it.