Rocks on the Move Investigation Sites

Eight investigation sites along the Point Bonita trail highlight evidence of geologic change and are part of the Rocks on the Move program. The location of each investigation can be found on the Point Bonita Geologic Investigation map. We also offer PDF versions of all of the investigations in English and Spanish.

Historic photo of path to the Point Bonita Lighthouse, circa 1940s.
Path to Point Bonita Lighthouse, circa 1940s

GGNRA Archives

Investigation A

Photo: Path to Point Bonita lighthouse, circa 1940s.

Location: #12 on the Point Bonita map. Near the southeastern end of the suspension bridge, facing nearly west.

  • The metal tower in front of the lighthouse is the old radio/radar, used before the radar was built near #2 on the map.
  • Lighthouse keepers used this path day and night, in all kinds of weather, including storms with high winds.
  • What geologic process is occurring under the suspension bridge?
  • What are the advantages of building a suspension bridge, instead of the path shown in this photo?
Side by side images of the arch at Point Bonita, each with two different sea levels.
A side by side comparison of the pillow basalt arch at Point Bonita.

NPS, Roxi Farwell

Investigation B

Photo: Pillow basalt arch at Point Bonita

Location: #12 on the map. Near information panel at southeast end of suspension bridge, looking southwest.

  • Compare the two photos of the pillow basalt arch. What has changed? How can you explain this?
  • The maximum daily difference between high and low tide for the Bay Area is approximately 3 meters (9 feet).
  • The yellow line in the picture represents a height of about 16 meters (50 feet).
Looking north from the Point Bonita bridge at a marine terrace at the top of the hillside.
A marine terrace at Point Bonita.

NPS, Roxi Farwell

Investigation C

Photo: Marine terrace, 2004

Location: #14 on the map. Standing near the northeast end of the suspension bridge, facing north. The marine terrace looks like a sand dune on top of the cliff

  • A marine terrace is an ancient beach. Marine terraces are common along the California coast. They often look flatter than the one you see here. Parts of the Coast Highway are built on top of marine terraces.
  • This terrace is now about 50 meters (150 feet) above sea level.
  • How did the beach end up this high above sea level?
A photograph of the lighthouse keeper's house, formerly the fog signal building, circa 1924.
Fog signal building, 1924.

GGNRA Archives

Investigation D

Photo: Fog signal building, 1924.

Location: Between #11 and #12 on the map. Looking southeast from southern end of plastic bridge.

  • This building was built in 1874 for the stream siren fog signal. When the Lighthouse Service moved the fog signal to the cliff west of the lighthouse in 1903, this building was used for storage.
  • The San Francisco earthquake in 1906 destroyed a small house at the top of the trail, leaving the families of the lighthouse keepers homeless. For a few years after the earthquake, this building was used as a keeper’s home.
  • Keeper Alexander Martin lived here with his family. He was very nervous about letting his children play outside the house, near the cliffs. Keeper Martin made harnesses for them. Whenever the Martin children played outside, they wore the harnesses and were tied to the house.
  • The U.S. Coast Guard tore down this building in the 1960s. What geologic processes do you think contributed to this decision?
    HINT: It was not due to an earthquake!

Inclined tram and boat landing. Photo circa 1943.
Inclined tram and boat landing. Photo circa 1943.

GGNRA Archives

Investigation E

Photo: Inclined tram and boat landing. Photo circa 1943.

Location: #11 on the map. Looking north, from cut off pole near the southern end of the old generator building.

  • The tram and boat landing were first built in 1871, so that the Lighthouse Service could bring supplies to the Point Bonita fog signal.
  • A horse walked in circles at the top of the tram, pulling the carts of supplies up the hill. In 1902, an engine (called a “donkey engine”) replaced the horse.
  • What kind of rock was the 1871 boat landing built upon? Look at the condition of the landing. Does this tell you anything about the strength of the rock?

A photograph of the wooden walkway circa 1876, that was used before the construction of the tunnel on the Point Bonita trail.
The “Gallery” at Point Bonita, circa 1871.

Eadweard Muybridge, GGNRA Archives

Investigation F

Photo: The “Gallery” at Point Bonita, circa 1871.

Location: #7 on the map; Facing southeast, looking at northern tunnel entrance.

  • The wooden walkway was built to provide access to the steam fog signal (see photo for Investigation D).
  • From 1855 to 1877, the lighthouse was located where the Coast Guard radar is today (near #2).
  • Rockslides damaged the wooden walkway in 1872, 1874 and 1876.
  • The tunnel was dug with hand tools. It was completed in 1877. It is 39 meters (118’ long). There has never been a rock fall inside the tunnel. What does this tell you about the stability of the rock?
  • The elevated, concrete bridge you see in front of the tunnel entrance was built after a landslide in the 1950s.
Photograph of roadbridge built to replace a portion of the Point Bonita trail washed out during the 1982-3 El Nino.
Bridge along the Point Bonita trail, 2004.

NPS, Roxi Farwell

Investigation G

Photo: Road bridge along Point Bonita trail, 2004.

Location: #4 on the map, facing south, southeast.

  • The roadway at #5 on the map was washed away during the El Niño storms in 1982-3.
  • What geologic process is occurring under the roadway as a result of the El Niño storms? What will this area look like in the future?
  • Look at the area where the trail turns to the left (between #5 and #6 on the map).
  • What is happening to the slope below the trail? Look for evidence of earth movement. What will this area look like in the future?
Photograph of small thrust fault that crosses the Point Bonita trail.
Historic Point Bonita lifeboat station circa 1940.

GGNRA Archives

Investigation H

Photo: Lifesaving Service boathouse, circa 1940.

Location: Between #1 and #2 on the map, facing southeast.

  • The boathouse was first built in 1911. It was destroyed in a landslide in 1912. It was rebuilt and again destroyed by another landslide in 1929. Then it was rebuilt again in 1929. It burned down in the 1950s.
  • Was this a good location to build a boathouse? Why or why not?
  • How did the geologic feature at investigation H (shown on the map) affect the boathouse?

Last updated: February 28, 2015

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