Field Trip - Caldecott Tunnel

To apply students’ understanding of the rock cycle and basic principles of stratigraphy, I brought my students to the Caldecott Tunnel to investigate the local geology and piece together the geologic history of their backyard. The east side of the tunnel has an easily accessed road cut that displays a gorgeous example of a contact between older sedimentary rock layers and a more recent volcanic layer. The whole thing has been folded and faulted by the actions of the Hayward Fault, and thus the layers are no longer horizontal but at a sharp diagonal. My students drew pictures of the northern cliff face on the Orinda side of the tunnel then each student was assigned a rock layer to study in detail. When we got back to the classroom, we reassembled the data on the whiteboard, and made theories about the sequence of events that would bring about the rock layers we observed in the cliff. Finally, students drew pictures of what the area must have looked like at different parts of the timeline. This field trip led gracefully into the next segment of the unit on geologic time.

Caldecott Tunnel - north roadcut
Photograph of the northern roadcut face at the Caldecott Tunnel from the field trip “Caldecott Tunnel between Oakland and Orinda” by Russell W. Graymer in "The Geology and Natural History of the San Francisco Bay Area: A Field-Trip Guidebook", edited by Philip W. Stoffer and Leslie C. Gordon.


Can describe the environments in which different sedimentary rocks are formed
Can apply Steno’s 3 laws of stratigraphy to rock layers in the real world
Can apply the laws of stratigraphy to describe the relative age of rock layers, even when the layers have been disturbed
Can use field data to recreate the geologic history of the Berkeley hills
Can make hypotheses about the probable cause of transitions between 1 rock layer and another

Law of Original Horizontality
Law of Superposition
Law of Lateral Continuity
Depositional Environment

Attachment Size
trip_caldecott_v2.doc 67.5 KB

Caldecott Tunnel - Logistics

5-10 min classroom introduction to the field trip
35-45 min collect data at Caldecott Tunnel
travel time varies
45-55 min synthesize data, draw conclusions, and imagine the past through student drawings of the region

Individual or in pairs.


  • Lab notebooks
  • Rulers
  • Optional: hand lenses
  • Optional: hardness testing materials (penny, butter knife or glass, and steel file)

Part 1: Caldecott Tunnel on Highway 24 at Fish Ranch Road
Part 2: classroom

Caldecott Tunnel - Background

Teacher Background
The road cut on the east side of the Caldecott Tunnel provides a fantastic example of stratigraphy that students can use to assemble the geologic history of the East Bay Area using their own observations and basic knowledge of geology. Logistically, there is decent parking for a busload of students in the loop of the onramp heading west on Highway 24 from Fish Ranch Road. There is a wide barrier between the freeway and the area to conduct your geologic explorations so you and your students are reasonable safe from the rushing traffic.

North roadcut  South roadcut 
Caldecott Tunnel - north roadcut
Caldecott Tunnel - south roadcut

As to the geology, the first thing you will notice are clear rock layers at a steep diagonal to the horizontal. Clearly, something happened to turn the originally flat layers, according to the law of original horizontality, on their sides (more on the tilting of the rock layers in a moment). The second thing you will notice are two distinct rock types. As you look at the north face of the road cut, to the upper right are indistinct layers of dark brown rocks while to the lower left are much more clearly delineated grey-green rock layers of an entirely different origin.

Moraga Volcanics  Orinda Formation
Moraga Volcanics
Orinda Formation

Upon closer examination, the dark brown rocks are a volcanic basalt, part of what geologists call the Moraga Volcanics. Their hardness and uniform, microscopic crystallization pattern give these rocks away as igneous rocks. A high iron and magnesium content gives them the distinctive red-brown coloration, as opposed to the more traditional black basalt of other volcanic areas. Among the eroded rock pieces along the base of these volcanics you might also find holey basalt that looks like sea sponge, evidence that some of the basalt contained many gas bubbles that were trapped in the magma as it cooled. There are also great veins of pagioclase crystals that formed as the magma cooled slowly, deep below the surface. The plagioclase was carried to the surface by lava during major eruptions.

The basalt layers are clear evidence that this area was once peppered with active volcanoes. As you look across the highway to the south side of the road cut, the thickness of these lava layers is evident, indicating extensive volcanic eruptions that covered the region in thick lava flows for many thousands of years. Radiometric dating has determined that these Moraga Volcanics are about 10 million years old.

On the other hand, the grey-green rocks are clearly not volcanic. They form easily identifiable layers, alternating between chunky conglomerates, crumbly mudstone, and rough sandstone, collectively known as the Orinda Formation to geologists. These are clearly sedimentary rocks, formed from sediments deposited and then compacted into rock. The conglomerates contain a wide variety of rocks trapped within a matrix of sand and mud. These trapped rocks are rounded and worn, just like river rocks because they are the remnants of an ancient river that once flowed through the area. The sandstones and mudstones are evidence that this river changed over time, changing course so this spot became part of the surrounding flood plain or becoming part of the river delta as sea levels rose and fell. Although these rocks cannot be radiometrically dated, it is clear that the Orinda Formation is relatively older than the 10 million year old Moraga Volcanics that lie on top of them, the law of superposition.

The junction between the Orinda Formation and the Moraga Volcanics is called a contact – a place where 2 distinct geologic formations meet. A close look at the junction between the two layers shows a red layer of mudstone. Unlike the grey-green mudstone elsewhere in the Orinda Formation, the red color is evidence of the mudstone being baked by the red hot lava that flowed across its surface, just as grey clay turns red after it has been fired in a kiln.

To summarize so far, the East Bay was at one time a great river valley with a river coursing through it, changing course and its identity as the ocean levels rose and fell and as sediments from hills being eroded upstream were deposited. Then, 10 million years ago, there was a burst of volcanic activity, flooding the river valley, not with water and transported sediments, but with magma.

So how did these layers get tilted? Sometime after the period of volcanoes marked by the Moraga Formation, the Hayward fault came into existence, causing the Berkeley hills to be pushed upward and the rock layers here to become folded and tilted out of their original, flat orientation. The conglomerates that once lay in a river valley and were then covered in layers of lavarock, were pushed skyward by tectonic forces, lifting them into the cliffs that tower above the roadside today. In the 1930’s construction began on the Caldecott Tunnel proper. As the hillside was cut open and the tunnels bored through the mountains, these beautiful rock layers were revealed.

East Bay Rock Layers

Student Prerequisites
Essential to this field trip are: a solid understanding of the rock cycle (see Crayon Rock Cycle lesson), previous experience identifying the individual rocks that will be encountered in the field and deducing the history of their formation (see History of Rock lesson), and a good grasp of the major principles of stratigraphy (see Layers Upon Layers lesson).

Caldecott Tunnel - Getting Ready

Getting Ready

  1. Get permission from CalTrans to take your students to the site. The contact person is Brigetta Smith, CalTrans Public Information Officer, 510-286-5820, [email protected]
  2. You may wish to take the directions detailed in the lesson plan below and print them out as a checklist/handout for the students to carry with them as they complete their observations at the tunnel. I had my students write the general directions in their lab notebooks before departing from the classroom but then realized that many would have benefited from a more detailed checklist.
  3. Each student needs a ruler. Bring a class set or make sure each student brings their own.
  4. Optional: prepare rock study kits to take to the field with hand lenses and hardness testing equipment.
  5. Lay down a strip of masking tape along the wall or the edge of the board as a timeline showing the sequence of events in the formation of the rock layers at the Caldecott Tunnel. The tape should be about 1 foot long for every 2 students in your class.

Caldecott Tunnel - Lesson Plan

Lesson Plan
Classroom Introduction

  1. There are 2 pieces of information that are useful to review with students in the classroom before going on this field trip.
  2. Review the types of rocks students will encounter. This is a good opportunity to take out samples of the rocks and the summary tables created in the History of Rock lesson. Ask students to identify the rocks and describe the depositional environments that created them
  3. Review Steno’s 3 laws. In particular, review how to tell which layer is the oldest using the law of superposition, even when the layers have been tilted.
  4. Describe the observations students will be expected to make at the tunnel and set up the lab notebooks so that students know what they need to do.
  5. Go to the Caldecott Tunnel!

At the Tunnel

  1. Park in the gravel loop within the onramp to the freeway. Gather the students and ask them to look at the north face of the cliff. Have them describe what they notice. The 3 key observations are:
    • There are distinct rock layers
    • The dark brown rock layers on the top right look different from the light grey-green rock layers on the bottom left
    • The layers are NOT flat but tilted
  2. Have students look across the freeway to the south side of the road cut. There, the contact between the dark brown basalt and the grey-green sedimentary rocks is much more obvious. The thickness of the basalt layer is also more distinct. Relate these observations to the rock layers created in the classroom in the Layers Upon Layers lesson. How are they the same? How are they different?
  3. Lead the students across the street, past the base of the cliff, and up a small trail on the left that climbs up to a ledge a little higher up on the cliff-side. Come back along the ledge towards the contact. Here, have kids make additional observations of the rocks layers they see. From this distance, students should be able to observe clear layers within the sedimentary rocks of the Orinda Formation.
  4. Have the students draw a full page picture of the cliff face, paying particular attention to the rock layers they can observe. If the students can, label each rock layer with the name of the type of rock (conglomerate, sandstone, mudstone, basalt, etc.).
  5. On the next page, write a short paragraph describing what you observe about the big picture in words (basically, the 3 key observations from step 1).
  6. As the students finish their drawings and big picture observations, assign each student a rock layer to study in detail. The first student to finish can work on the basalt in front of them. The next can examine the red, baked mudstone. The next can examine the wide conglomerate layer. And so on. If you have more than 20 students, in the interest of time when students are presenting their observations and theories back in the classroom, you may want to have students work on a rock layer in pairs rather than individually.
  7. Each student should draw a detailed picture of their rock layer and the layers that sandwich their layer. They should then make the following observations
    • Identify the type of rock
    • Measure the size of the grains in your rock in cm
    • Measure the width of the entire layer in cm
    • Identify the type of rock on either side of your layer  - be sure to indicate which one is  “above” your layer and which one is “below”
    • Propose a theory describing what the area might have looked like before your rock layer existed, what the area looked like during the formation of your rock layer, and what the area looked like after the formation of your rock layer.
  8. A good way to have students confirm their observations and rock identifications is to have them share their observations and theories with the students studying the neighboring layers. They can confirm that they identified the rocks correctly and see if their theory matches that of their neighbors.
  9. Students that finish early can write down their theory of the sequence of events that led to the formation of these rock layers.
  10. Return to school.

Organizing the Data and Drawing Conclusions

  1. Have students open their lab notebooks. Ask them to share their observations and instruct you on how to recreate their big picture drawings on the white board in as much detail as possible. Get them to give you as many details as they can reassemble – the number of rock layers, the type of rock in each, the relative width, etc. Focus on accurately representing the students’ measurable observations. Drawing conclusions about the depositional environment and sequence of geologic events will come later.
  2. When a pretty detailed drawing to work from, begin to reassemble the geologic timeline as the kids see it, starting with the oldest layer that was studied in detail by a student. Of course, that means the students have to decide which is the oldest layer, the sedimentary layers or the igneous layers. Most likely, and correctly, they will select the bottom-most sedimentary layer. Discuss whether these layers were originally laid down at a tilt or if they were laid down flat and then tilted and how they know.
  3. Once you know which is the bottom most layer, each student should have a chance to explain their theory about the environment present at the time their rock layer formed and what evidence they have to support that theory. Keep track of the events on a line drawn across the board with markers for each of the rock layers. As more students present their findings, the presentations will become shorter, leaving time to speculate on how a fast-flowing river might change into a lake or a delta or a sand bank. You could also speculate on the length of time it would take to accumulate each layer of sediments – is it hundreds or thousands of years?
  4. When all the students have gone, there is the remaining problem of how the layers become tilted. Have students propose their theories of what happened to tilt the layers. Most likely, someone will mention earthquakes and propose that the earthquakes folded the rock layers and pushed one end up and the other end down, causing the tilt. The key is to let the students come up with the theory, don’t tell them.
  5. Finally, with the remaining time, I asked each student to create an index card drawing of what they thought that spot would have looked like at the time the rock layer they studied was forming. Imagine standing there thousands of years ago. Would it have looked like a river? Would you have been standing next to a giant volcano spewing hot lava?
  6. Optional: Since there isn’t a “rock layer” that shows the tilting, you might want to draw an index card, or series of cards, showing the hills being pushed up and the tunnel being carved.
  7. Arrange the drawings along the masking tape timeline. You can come back to this timeline, adding additional information (such as the periods and epochs - Miocene epoch 5-24 mya, Tertiary period, Cenozoic era - and the types of plants and animals that might have roamed the ancient river valleys and volcanoes) as students learn about geologic time in the upcoming lessons.

Caldecott Tunnel - Sources and Standards

The best overview of the geology of the Caldecott Tunnel 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 second field trip, “A Geologic Excursion to the East San Francisco Bay Area”, stop #3, “Caldecott Tunnel between Oakland and Orinda”. The entire guide with other excellent field trips throughout the Bay Area may be downloaded from the USGS website.

Professor Steven Dutch of the University of Wisconsin, Green Bay has put together an excellent series of photos of the road cut near the Tunnel.

To learn more about the Caldecott Tunnel itself, the California Department of Transportation has a website with a historical timeline of the tunnel and information about current projects. In addition, engineers J. David Rogers and Ralph Peck describe the geologic engineering for the BART system (Bay Area Rapid Transit).

For an even broader discussion of the geology in the Bay Area, the USGS has assembled a treasure trove of information about the geology of the San Francisco Bay Area.

Finally, to pan back even further to view the geology of the entire state of California, legendary science writer John McPhee’s book Assembling California provides an in depth, highly accessible discussion of the geologic history of California. READ IT! And read McPhee’s other works such as Basin and Range. I personally disliked geology as a science until I read McPhee and suddenly fell in love with the field.

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:
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.