Field Trip - Marin Headlands

Marin Headlands: photograph of Marin Headlands from the Golden Gate Bridge by Christopher BelandMarin Headlands: photograph of Marin Headlands from the Golden Gate Bridge by Christopher BelandSummary
The Marin Headlands contain the geologic record of a great deal of plate tectonic action that can be used to piece together the history of the formation of California. Briefly, around 180 million year ago, the North American plate collided with a now subducted plate called the Farallon plate. As the Farallon plate dove under the North American plate, bits and pieces of the Farallon plate were scraped off. These bits and pieces can be found in the Marin Headlands in several distinctive rock formations: pillow basalts (at the Point Bonita Lighthouse), chert (near Rodeo Lagoon), and sandstone (at Rodeo Beach). By closely observing these rocks and figuring out how they formed, an understanding of how California itself was formed may be inferred.

Note #1: If you are intimidated by the trip as described here and prefer to have park rangers lead your field trip and geology investigations, consider participating in “Rocks on the Move”. This free program provides teacher training, pre- and post-visit curriculum, as well as a very knowledgeable ranger to lead the field trip portion of the visit.

Note #2: The geologic investigations undertaken in this field trip require students to have a good understanding of the rock cycle and the geologic time scale (see Geology Box) as well as exposure to the theory of plate tectonics. It is designed as a culminating field trip to tie lots of ideas together into a cohesive theory.

Can make observations about the types of rock in the Marin Headlands.
Can model convergent, divergent and transform plate boundaries.
Can understand the conditions under which different types of rock form.
Can use evidence from rocks to piece together a theory of how California formed.
Can model the formation of California using sand castles.

Mid-ocean ridge
Tectonic plate
Seafloor spreading
Convergent boundary
Divergent boundary
Transform boundary
Island arc
Farallon plate
North American plate
Pacific plate
Sedimentary rock
Metamorphic rock
Igneous rock
Pillow basalt
Law of Original Horizontality
Law of Superposition
Law of Lateral Continuity

Attachment Size
headlands_trip.doc 63 KB
ca_timeline.pdf 15.66 KB

Headlands - Logistics

2 hours at Point Bonita Lighthouse
10 minutes travel to Rodeo Beach
2 hours at Rodeo Beach and Lagoon
travel time to and from Marin Headlands varies

45-50 minutes back in classroom the following day to discuss, review, and consolidate field observations

Whole class - preferably no more than 32 students on the trip at a time. For some activities, student divide into groups of 3-4 for discussion.


  • Small, lunch-sized ice chest
  • Ice
  • 2-3 cups of water
  • Clear plastic cup
  • Magic Shell® or other quick-hardening chocolate topping for ice cream
  • Hand lenses to observe radiolaria fossils
  • 1 package Oreo Cookies® or Nutter Butters®
  • Raised relief or shaded relief map of California (available from Hubbard Scientific and American Educational Products, $35.75 (

Marin Headlands, part

Headlands - Background

Teacher Background
The Marin Headlands offers an exquisite opportunity for students to consolidate everything they have learned about geology and plate tectonics. Students (with a lot of guidance and help) find and piece together evidence concerning the geologic history of California. Students observe 3 different rock types and learn about how each one formed. Then, they take these observations back to the classroom to consider several possible explanations for how these rock types all ended up in the same place.

Pillow Basalt

Pillow Basalts at Point Bonita: image courtesy of the US Geological SocietyPillow Basalts at Point Bonita: image courtesy of the US Geological Society Pillow Basalts forming at Loihi: newly formed pillow basalt at the understaer volcano, Loihi, in Hawaii, image courtesy of the US Geological SocietyPillow Basalts forming at Loihi: newly formed pillow basalt at the understaer volcano, Loihi, in Hawaii, image courtesy of the US Geological Society

The first rock formation students observe are the pillow basalts at the Point Bonita Lighthouse. The best observation point is past the tunnel, on the left side of the 2-man suspension bridge, before you cross over to the lighthouse itself. If you look down, there is a picture perfect outcrop of pillow basalt right near the water. Another place to observe pillow basalts is on the cliffs on the right side of the trail, before reaching the tunnel. These pillows have been cut in half and may be seen exposed on the cliff as oval-shaped pockets of basalt.

This type of basalt forms when magma spills into cold ocean water. The outer layer of basalt rapidly hardens when it contacts the ice cold water, forming a round, pillow shaped shell. As more magma pushes from behind, part of the shell bursts and more magma rushes out. This pattern of magma release leads to a formation similar to pillow stacked one on top of another, or similar to the pattern made (in miniature) when Magic Shell® chocolate topping is dropped into ice water.

Two places one might expect to find pillow basalts forming is 1) at a volcanic island arc near a convergent plate boundary or 2) at a mid-ocean ridge where magma from the mantle pours out from between the gap between two diverging plates. While the pillow basalt found elsewhere in the Marin Headlands is consistent with a mid-ocean ridge origin, based on chemical analysis of the titanium and iron content, these pillow basalts at Point Bonita seem more consistent with a volcanic island origin.

Rodeo Beach and sandstones: image courtesy of nickoneill ( Beach and sandstones: image courtesy of nickoneill ( short drive from the lighthouse takes you to Rodeo Beach. You can park at the north end of the beach by the restrooms.

The second rock formation students observe are the sandstone cliffs at the northern end of Rodeo Beach, near the parking area. These sandstone cliffs are typical of sandstones formed from the sedimentary remains of underwater landslides at the edge of a continent, generally near a subduction zone. One can tell that these sandstones were laid down in large, tumultuous landslides because they closely resemble a hugely magnified version of a soil separation test (see Soil Analysis lesson) – where sediments of various grain sizes are shaken in water and the large pebbles settle near the bottom and the smallest particles settle near the top. The sandstone formations similarly show a pattern of layered beds several meters thick. In each bed, large pebbles are found near the bottom and tiny clay particles may be found at the top, suggesting that the entire layer collapsed off the edge of the continent underwater, and sorted by particle size before being compacted and cemented together into a sandstone.

Ribbon Chert in the Marin Headlands: image courtesy of the US Geological SocietyRibbon Chert in the Marin Headlands: image courtesy of the US Geological SocietyTake the students south down the beach and hike around Rodeo Lagoon, a beautiful 1 mile walk. About 3/4 of the way around the lagoon, there is a tall, exposed chert cliff face on the opposite side of the road, with a nice open area at the base for students to gather and observe the rocks.

Chert is composed of several centimeter thick layers of radiolaria fossils. Radiolaria are tiny oceanic creatures whose shells drift down to the ocean floor when they die. Over many millions of years, their shells pile up on each other and form layers, which, as specified in the law of original horizontality, were originally laid down flat.

How then did the layers get so torturously folded? To illustrate the process, take an Oreo Cookie or Nutter Butter and carefully twist off the top cookie, leaving the filling in a nice layer on top of the bottom cookie. The top cookie represents a continental plate. The filling represents the chert, laid down in nice flat layers. The bottom cookie represents an oceanic plate, which will subduct under the continental plate. Hold the continental plate cookie still and gradually allow the oceanic plate cookie to dive below the edge, scraping the chert filling off of the oceanic plate cookie as it goes. You should get a wrinkled, folded pile of filling on the edge of the continental plate cookie. This same process explains how the originally flat layers of chert became so wrinkled and folded – as the oceanic crust subducted, the chert was scraped off the surface and piled up on the continental crust.

Putting it all together
Knowing the stories of these three rock types, it becomes clear how this piece of California formed. Around 200 million years ago, Pangaea began to break up. The North American Plate moved westward, away from what is now Europe and Africa. At that time, California did not exist. The edge of the North American continent was in Nevada. As the North American Plate traveled west, a now almost entirely subducted plate called the Farallon Plate dove under the edge of the North American Plate. This subduction began around 160 million years ago. This subduction zone resulted in oceanic rock being scraped off the Farallon plate and piling up against the edge of the North American continent. Gradually, California grew as the North American Plate pulled off chunks of ocean floor – pillow basalts, cherts, and entire island chains the size of Japan – like a continent sized bulldozer.

At the same time, magma rose up from beneath the descending plates, causing the formation of large volcanoes on the North American Plate, such as Mount Lassen and Mount Shasta. Some of the magma never broke through the crust as a volcano. Instead, the balloons of magma cooled gradually beneath the surface, formed huge granite mountains, and pushed up the Earth’s crust. These granite mountains form the bulk of the Sierra Nevada.

By 100 million years ago, these newly formed mountains had begun to erode heavily. Vast amounts of sediment washed down off the mountains and extended the edge of the continent further west, creating the Sacramento Valley. The bulldozer action of the North American Plate continued. The sandstone formations (like those on Rodeo Beach) provide evidence that the Marin Headlands is the edge of the continental plate. The pillow basalts (like those at the Point Bonita Lighthouse originally created near mid-ocean ridges and far away volcanic islands) and chert (like those near Rodeo Lagoon originally laid down in the ocean in nice flat layers) were unceremoniously scraped off the Farallon Plate. These pillow basalts and chert are the last remaining remnants of the Farallon Plate. By 28 million years ago, the Farallon Plate had been entirely consumed.

Student Prerequisites
Before undertaking this field trip and investigation, students should have a thorough understanding of the rock cycle (see Rock Cycle lesson), basic geology (see the History of Rock lesson), stratigraphy (see Layers Upon Layers lesson), geologic time (see Geologic Timelines lesson), soil separation tests (see Soil Analysis lesson), seafloor spreading (see Seafloor Spreading lesson), and plate tectonics (see Evidence for Plate Tectonics lesson).

Headlands - Getting Ready

Getting Ready

  1. The Point Bonita Lighthouse is only open on weekends and Mondays from 12:30 to 3:30. To have a docent or ranger open the lighthouse tunnel for you on other days and times, contact the National Park Service at (415) 561-4754.
  2. Arrange transportation to the Marin Headlands and around the park.
  3. Gather materials:
  • small ice chest, ice, water, cup and Magic Shell for the pillow basalt demonstration
  • hand lenses and cookies for chert observations
  • map of California for putting it all together

Headlands - Lesson Plan

Lesson Plan

  1. Before going on this trip, review any of the science concepts that students should have fresh in their minds: rock cycle, basic geology, stratigraphy, geologic time, seafloor spreading, and plate tectonics
  2. Go to the Point Bonita Lighthouse in the Marin Headlands.
  3. Investigate the pillow basalts:
    • Begin by asking students to draw or photograph the pillow basalts.
    • Model the formation of pillow basalt with ice water and Magic Shell. Fill a cup with ice water and then pour Magic Shell into the water – the cold water makes the outside of the chocolate syrup harden and the chocolate soon piles up in pillow shaped piles.
    • Discuss the model with the students, pointing out how the syrup represents magma welling up on the ocean floor.
    • Ask students where magma might well up from the ocean floor: volcanic islands and mid-ocean ridges.
  4. Travel to Rodeo Beach.
  5. Investigate the sandstones:
    • Ask students to draw or photograph the sandstones. Point out the borders between each large layered sandstone bed.
    • At the water’s edge, observe how sand particles act in the water. Draw connections to the soil separation test and the tiny particles remain suspended in water but the larger particles rapidly settle to the bottom.
    • Point out the sorting of the sediments in each bed.
    • Discuss how much sand would need to be dumped at one time to create each bed.
    • Ask students what might cause this much sediment to be dumped at one time: under-water landslides.
    • Point out how large landslides are known to occur at the edge of a continent, particularly near subduction zones.
  6. Lead students on a counter-clockwise walk around Rodeo Lagoon.
  7. Stop at the chert outcrop and investigate the chert:
    • Ask students to draw or photograph the chert. Pass out hand lenses and encourage students to look for fossils.
    • Review the law of original horizontality. Ask students how these layers must have been laid down originally. How did the bodies of these fossils originally form layers? (By millions of years of accumulation on the ocean floor.)
    • Ask students for initial hypotheses about how these layers could have become so folded.
    • Pass out cookies and model the subduction of an oceanic plate with cookies. Carefully remove the top cookie, leaving the filling on top of the lower cookie. Hold the top cookies still while gradually moving the lower cookie so that it subducts unter the top cookie, scraping off the filling as it goes.
    • Discuss the model, pointing out what the cookies and filling each represent. Observe the similarities between the now wrinkled filling and the folded layers of chert.
    • Collectively piece together the story of how this chert was originally laid down and how it became wrinkled.
  8. Return to the parking lot and go back to school (or play on the beach a little first).
  9. At school, review the stories of the 3 rocks that were investigated. Discuss how each type of rock must have formed, specifically focusing on where it must have formed.
  10. Spend at least half an hour piecing together the story of how all 3 types of rock, sandstone, chert and pillow basalt, came to all be found in one place. Once the story is filled out, diagram it on the board.
  11. Use the relief map of California to show the 3 major geologic zones in California – the Sierra Nevadas, the Central Valley, and the Coast Range mountains.
  12. Tell the story of how each of the 3 zones formed. Create a timeline to show students what was happening at different periods.

Headlands - Sources and Standards

All the models - chocolate pillow basalt and cookie chert – were introduced to me by Eric Muller of the Exploratorium Teachers’ Institute. For detailed information about the chocolate pillow basalt demonstration, see his write up “Chocolate Lava” on his website.

The best overview of the geology of the San Francisco Headlands region is available in the book: The Geology and Natural History of the San Francisco Bay Area: A Field-Trip Guidebook, edited by Philip W. Stoffer and Leslie C. Gordon, published by USGS. The information you want is found in the third field trip, “Geology of the Golden Gate Headlands”, stop #2, 3, and 4. The entire guide with other excellent field trips throughout the Bay Area may be downloaded from. The USGS provides an online photographic tour of the Golden Gate National Recreation Area, including a great geologic map of the Marin Headlands.

Two excellent books describing the geologic history of California are:

  • California Geology, by Deborah Harden, 2003, Prentice Hal
  • Assembling California, by John McPhee, 1994, Farrar Straus Giroux

Some good websites for additional information about the history of California include:

  • The National Parks Service website has a great image of North America showing the various geological regions with a brief description of their formation.
  • This USGS bulletin carefully describes many important characteristics of the geology of the Bay Area.

Grade 6
Plate Tectonics and Earth's Structure
Plate tectonics accounts for important features of Earth's surface and major geologic events. As a basis for understanding this concept:
a    Students know evidence of plate tectonics is derived from the fit of the continents; the location of earthquakes, volcanoes, and midocean ridges; and the distribution of fossils, rock types, and ancient climatic zones.
e     Students know major geologic events, such as earthquakes, volcanic eruptions, and mountain building, result from plate motions.
f     Students know how to explain major features of California geology (including mountains, faults, volcanoes) in terms of plate tectonics.

Shaping Earth's Surface
Topography is reshaped by the weathering of rock and soil and by the transportation and deposition of sediment. As a basis for understanding this concept:
a     Students know water running downhill is the dominant process in shaping the landscape, including California's landscape.
b     Students know rivers and streams are dynamic systems that erode, transport sediment, change course, and flood their banks in natural and recurring patterns.
Investigation and Experimentation
Scientific progress is made by asking meaningful questions and conducting careful investigations. As a basis for understanding this concept and addressing the content in the other three strands, students should develop their own questions and perform investigations. Students will:
e     Recognize whether evidence is consistent with a proposed explanation.
f     Read a topographic map and a geologic map for evidence provided on the maps and construct and interpret a simple scale map.
g     Interpret events by sequence and time from natural phenomena (e.g., the relative ages of rocks and intrusions).

Grade 7
Earth and Life History (Earth Sciences)
Evidence from rocks allows us to understand the evolution of life on Earth. As a basis for understanding this concept:
a     Students know Earth processes today are similar to those that occurred in the past and slow geologic processes have large cumulative effects over long periods of time.
c     Students know that the rock cycle includes the formation of new sediment and rocks and that rocks are often found in layers, with the oldest generally on the bottom.