Field Trip 6: Crystal Cove State Park

Overview

This field trip takes us to the southern terminus of the Los Angeles Basin, across the coastal plain of southern Los Angeles County and northern Orange County, and up and over the northernmost extent of the Peninsular Ranges, the San Joaquin Hills.

Beach at Crystal Cove. 

The beach at Crystal Cove State Park is not only beautiful but also one of the best places in southern California to study coastal geology. It offers excellent examples of erosional coastal landforms like marine terraces, wave-cut platforms, cliffs, coves, and headlands, as well as superb exposures of the Monterey Formation, one of the most important Miocene rock formations in California. It also serves as a nice opportunity to discuss the differences between an active and passive continental margin, using the west and east coasts as examples. The steep San Joaquin Hills and precipitous cliffs along the beach at Crystal Cove provide good examples of active continental margin geography.

The shoreline at Crystal Cove represents the base level for local urban runoff, resulting in small streams working their way across the beach to the ocean, providing an opportunity to observe and discuss stream-related landforms. There is also an opportunity to observe igneous rocks in the form of rip-rap (imported boulders used to stabilize slopes) and columnar-jointed cliffs. Development along the beach has helped contribute to mass wasting with recent landslide scars visible, especially in the Crystal Cove Historic District area; there are also instructive examples of slope stability efforts. Finally, while strolling along the beach one can observe fossils clams, clam-drilled holes in rocks, concretions, badland topography, folded strata, an unconformity, and groundwater seeps.

This excursion requires a half to 2/3 of a day and its proximity to Los Angeles and Orange Counties make it a relatively short drive. Consequently, this is a destination that lends itself to carpooling using personal vehicles, which makes for easier planning and less cost – no bus to hire. A visit mid-week might also be doable, especially for groups originating in south Los Angeles or north Orange counties. Any time of the year works for a visit, although it can be surprisingly warm on the beach, where there is no shade and the sun reflects from the sea and sand.    

Note: The beach at Crystal Cove is best experienced at low tide when exposure to the wave-cut platform is best. At high tides, access along the beach will be limited and could make for hazardous trekking in spots if coupled with high surf conditions.

Learning Objectives

Through participation in this field trip students should be able to:

1. Compare and contrast the topography of the east coast vs. west coast
2. Compare and contrast the geology of an active vs. passive continental margin
3. Describe the topographic differences between an active and passive continental margin
4. Identify coastal erosional landforms: marine terrace, wave-cut platform, cliff, cove, and headland
5. Describe how a marine terrace forms
6. Identify sedimentary rocks and strata
7. Identify igneous rocks
8. Identify soft-sediment deformation and describe how these types of folds can form
9. Explain why folds violate the Law of Original Horizontality
10. Discuss the origin of sand
11. Identify topographic evidence of mass wasting
12. Identify and explain the origin of concretions
13. Identify and explain the formation of columnar jointing
14. Identify stream related landform features, i.e. stream terraces, channel bars, distributaries
15. Identify and explain the significance of an unconformity

Key Vocabulary

• Active Continental Margin – the transition from continental to ocean crust is marked by a plate boundary; narrow continental shelf
• Cliff – a steep slope adjacent to a beach caused by wave erosion
• Columnar Jointing – vertical fractures caused by contraction in a thick lava flow that cools slowly overtime, resulting in polygonal shaped columns
• Constructive Geological Forces – geologic processes that build up landforms and/or add new rock to the crust; e.g. volcanic activity
• Continental Margin – the transition zone from continental crust to oceanic crust
• Cove – a convex indention into the shoreline caused by erosion
• Destructive Geological Forces – geologic processes that wear down landforms and take away rock, e.g. erosion
• Detrital – texture of sedimentary rock composed of clasts or grains of rocks and minerals
• Headland – a protrusion of land into the ocean caused by erosion on either side
• Law of Original Horizontality – sediment is always deposited as strata that is horizontal or very nearly horizontal
• Marine Terrace – a wave-cut platform elevated above the beach by tectonic uplift and/or a drop in sea level
• Passive Continental Margin – the transition from continetal to ocean crust is not marked by a plate boundary; wide continetal shelf
• Strata – a vertical sequence of layers of sediment, which is considered the definitive characteristic for sedimentary rocks
• Stream Terrace – topographically flat surface elevated above the active stream channel representing the past position of the stream bed or flood plain for the stream  
• Unconformity – contact between older rock below and significantly younger rock above, representing a gap in geologic time caused by prolonged erosion and/or lack of deposition
• Wave-cut Platform – the gently sloping surface extending from the beach out into the ocean caused by wave erosion

Pre-Field Trip Questions

1. What physical characteristic makes sedimentary rocks easily recognizable in the field?  
2. Refer students to the excellent coastal geology overview of Crystal Cove by Dr. Merton Hill at Saddleback College, Crystal Cove coastal geology Links to an external site. (then click on: Click here for Crystal Cove Geology Coast Guide).  Organize students into pairs or small groups, with each group being made responsible for teaching one of the numbered sections in this document to the others in the class. These short (perhaps 5-minute) lessons could be done during the class before the field trip or in the field at the start of the field trip.

3. Draw a simple diagram of a wave-cut platform, marine terrace, cove, and headland.
4. What is the difference between a passive and an active continental margin?  

Crystal Cove State Park Directions

From Cerritos College, I-605 south to I-405 south to CA-73, exit Newport Coast Drive and proceed south to Pacific Coast Highway (PCH a.k.a. CA-1). Turn left (south) onto PCH then left at the first signal into the Los Trancos Parking lot. Show preapproved fee waiver at the Kiosk. Note that there are other parking lots that allow access to most of the stops in this chapter. 

Note that the CA-73 is a toll road. Charter buses assume the responsibility of the toll, but non-commercial vehicles will need to pay the toll, which can be done through The Toll Roads website https://www.thetollroads.com/ Links to an external site..

Crystal Cove can also be accessed via PCH/CA-1. However, this route takes longer and does not take students up and over the hills, which might make it more difficult for them to conceptualize the learning objective about “active vs. passive continental margins” (see below).

Field Trip Stops

This field trip entails only one vehicle stop, the Los Trancos parking lot (or one of the other parking lots serving Crystal Cove), which is accessible by a large charter bus, van, or 2-wheel drive car and requires a $15 parking fee. This fee may be waived by requesting an academic fee waiver from Crystal Cove officials ahead of the field trip: Crystal Cove State Park fee waiver Links to an external site.. If students are carpooling be sure to specify this in your fee waiver request.  

From the Los Trancos parking lot, groups will walk down to the beach, walk along the beach, and then return to the parking lot. Instructors should address the aforementioned learning outcomes at each of the instructional stops listed below, as they deem appropriate.

Suggested Itinerary

1. In the parking lot
   • East Coast vs. West Coast Geology
   • Prompt: As you may have noticed on the drive here, the road goes over some fairly steep hills. Here, in the parking lot, have we yet reached the beach? Where is the beach relative to the parking lot?
   • Possible questions:
     • What was the topography like from the freeway to where we are now? –Hilly; definitely not flat.
     • What is the topography like along the east coast of North America? – More subdued; gentle, low hills; flat.
     • Why is the west coast hilly and mountainous and the topography relatively flat along the east coast? – Active vs. passive continental margins; tectonic uplift is happening here, but not along the east coast.
   • Discuss active vs. passive continental margins.

• • • Continental margin: the transition zone from the shoreline to the abyssal plain of the ocean floor; including the continental shelf, continental slope, and continental rise
    • Active continental margins: a plate boundary is crossed from continental to ocean crust; narrow continental shelf
      • Interaction of the plates along the plate boundary drive constructive geologic processes, such as tectonic uplift, making hills, mountain, and even volcanoes

Active continental margin. –CC.

• • • Passive continental margin: no plate boundary between continental and ocean crust; wide continental shelf.
      • No plate boundary means no constructive geologic forces. Instead, destructive geologic processes dominate and the landscape is worn down by erosion

Passive continental margin. The continental margin spans the continental shelf, continental slope, and continental rise.   

Note: Utilizing Google Earth imagery can effectively demonstrate the geographic differences between the west and east coasts. I’ve brought an iPad into the field with me to do this. Students could also use their phones.

• Introduce coastal landforms.  Preparing illustrations and/or hand-outs ahead of time can help students conceptualize:
  • wave base – the depth at waves no longer erode
  • wave cut cliffs – cliffs bordering the beach made by wave erosion at base of cliffs (shoreline angle)
  • headlands – narrow cliffs jutting out into the ocean, commonly separating one cove or bay from another
  • cove – a narrow bay or indention along the shoreline
  • wave-cut platform – gently sloping erosional surface extending from the base of wave cut cliffs out the wave base
  • marine terraces – wave-cut platform elevated above sea level

Cross-sectional drawing of the continental margin and coastal landforms. Not to scale.

2. Bathroom break. Restrooms are located at the top of the trail to the beach.

Map view of Los Trancos parking lot and trail down to the beach. –Google Earth Image

3. As students congregate on the path near the bathroom, you could point out the extensional cracks in the path. These are from slope creep, the slowest form of mass wasting.
4. Descending path to the beach; note additional cracks and any mitigation efforts.
   • Looking up at the surrounding slopes, ask students to consider if mass wasting could affect these slopes.
5. Storm Drain at the bottom of the trail.
   • This makes for a good opportunity to discuss the “fate” of precipitation and irrigation.
     • Precipitation and urban runoff (typically polluted) eventually flow down to base level, the ocean.
   • Suggested questions:
     • How do growing populations and development influence runoff? – Increase.
     • Is there anything that could be done to reduce runoff? – Answers will vary.
6. Rocks before tunnel
   • Rip-rap for slope stabilization; Suggested questions:
     • What type of rock are we looking at (igneous, sedimentary, metamorphic)? – Plutonic igneous (gabbro).
     • Are these rocks from here? – No (from local mountains).
     • How can we tell? – Don’t match local geology; the arrangement of the boulders; background research on Crystal Cove, etc.
     • Why are they here? – Slope stabilization.

7. Rocks after tunnel – as above, but granitic.
8. River channel, if present, point out and discuss stream-related landforms:
   • type of stream channel (meandering/braided)
   • channel bars
   • stream terraces
   • distributaries
   • ripples
9. Beach cottages – Crystal Cove Historic District
   • Why are they abandoned? - Mass wasting; beach erosion.
   • Mass Wasting events (landslides) are common in the Monterey Formation.  Clay layers interbedded within the strata can become very weak when wet and unable to resist the force of gravity acting on the strata above.  For example, heavy rains can add enough water to the clay beds that they are transformed into a mud-like consistency, creating a plane of weakness or landslide slip-surface that triggers a rockslide.  Wave erosion constantly removing rock and support from the base of a cliff is an additional trigger for coastal mass wasting.
   • Behind the southernmost cottage is an obvious landslide scar from a past rock slide.
     • Fun fact! This southernmost cottage is also the cottage in which the movie Beaches was filmed.

From the cottages walk southward along the beach eventually reaching the impassible headland (Abalone Point)

10. Origin of sand (at any place along the beach).
    • Discuss sand “basics”: what it is, how it’s made, where it came from, where it’s going, etc.
    • Suggested questions:
      • Where does sand come from? – Weathering of rock.
      • What is sand mostly made of? – Quartz.
      • What happened to the dark minerals that were part of the original (parent) rock from which this sand was weathered? – Chemically weathered away.
      • Now that the sand is here, will it stay? – No. Longshore drift.

11. Monterey Formation (at any place along the beach)
    • 12-6 million-year-old thinly bedded shale, mudstone, and chert form a mix of fine terrestrial sediment (from destruction of rock on land) and diatomaceous ooze (remains of microscopic planktonic creatures with silica shells called diatoms). Interbedded (interlayered) with these sediments are thin layers of volcanic ash.

Photograph of diatoms. 

11. • Sediments accumulated on the deep ocean floor, about a mile below sea level, where they were compacted and cemented together over time.
    • During the lithification process, the ash layers were converted to clay. These clay layers become very weak when wet, resulting in mass wasting. Landslides are common in the Monterey Formation.
    • One of the most economically important geologic formations in the country.
      • Yielded 8 billion barrels of oil since 1880
        • Enough to fill 4,000 football stadiums
        • At one time, accounted for 20% of the world’s oil production
        • 8 million cubic feet of natural gas
        • The Monterey Formation could contain another 8 billion barrels
12. Concretions (at any place along the beach)
    • Originally land-derived wood that sank and was buried in diatomaceous ooze of the deep sea
    • Wood became “infected” in the ooze, filling and swelling these particles with silica, resulting in the petrified concretions
13. Springs/groundwater seepage (at any place along the beach)
    • Groundwater flowing out of the cliff
      • Surface water infiltrates permeable sections of the Monterey Formation, eventually encountering impermeable clay layers, along which the water flows until exiting the cliff
      • The colors associated with the springs are iron oxide minerals: goethite limonite 
14. Magnetite in sand (many places along the beach)
    • Magnetite is a mineral associated with mafic rocks, like basalt that make up the ocean floor
    • Will stick to a magnet!
15. Badlands (many places along the beach)
    • Deeply gullied landscape formed by runoff over the 120,000 years old weakly consolidated sediments atop the marine terrace
    • Unconformity
      • 120,000 years old sediment atop 6 million (at least) years old Monterey Formation
      • Represents an erosional surface and 6 million years' worth of rock, and geologic time, that is missing
16. Fossils in rock (many places along the beach)
    • Clams embedded in marine terrace deposits
    • Look very similar to modern-day clams, because 120,000 years is a very short time ago (geologically speaking)
17. Clam-drilled rocks
    • Bivalves
    • Teeth on shell can burrow into rock
    • Clams grow as they burrow and get trapped
    • ~ 8 year lifespan
    • filter feeders
18. Beach erosion
    • wave base – the depth at waves no longer erodes
    • wave cut cliffs – cliffs bordering the beach made by wave erosion at the base of cliffs (shoreline angle)
    • headlands – narrow cliffs jutting out into the ocean, commonly separating one cove or bay from another
    • cove – a narrow bay or indention along the shoreline
    • wave-cut platform – gently sloping erosional surface extending from the base of wave-cut cliffs out the wave base
    • marine terraces – wave-cut platform elevated above sea level
19. Columnar jointing on the headland
    • Formed as a thick lava flow or lava intrusion like a dike or sill, cooled and contracted, fracturing the mass into columns
    • This exposure is andesite
    • More famous examples include Devil’s Postpile and Giants Causeway

Retrace your steps to the Crystal Cove Historic District. From here you can walk back through the tunnel and up to the Los Trancos parking lot or continue walking northward to the next trail leading up the cliff where you can address the topics under 20 below.

20. Ascend cliff
    • Folds – walking up the trail, look for folded strata within the Monterey Formation. Folded strata are typically associated with deformation due to compressive stresses. However, these folds are more likely the result of soft sediment deformation that occurred when unconsolidated sediment slid down the continental slope and was crumpled upon colliding with ocean floor.
      • Activity: ask students to make a cross-sectional sketch of the folded strata
    • Overlook – Continuing up the trail a bit further, shortly after it switches back, there is a small observation area that provides an excellent vantage point to observe and discuss the coastal landforms mentioned earlier: wave-cut platforms, marine terraces, coves, headland, etc.
      • Marine Terraces: ancient wave-cut platforms (where the beach once was) that have been elevated above sea level
        • Form during a pause in uplift, when wave action erodes a very gently sloping surface into the bedrock at sea level called a wave-cut platform
        • Tectonic uplift elevates this surface above sea level, resulting in a marine terrace
        • Over tens of thousands of years, the process then repeats, making a new wave-cut platform that will eventually be uplifted to make new marine terrace; in this way the oldest terrace will be the highest and the youngest marine terraces the closest to sea level
      • Activity: ask students to make a cross-sectional sketch of the coastal landforms above

Follow-up Questions

1. Describe the differences between active and passive continental margins.
2. Explain why the west coast of North America is hilly and mountainous, while the east coast is relatively flat.
3. Explain how a marine terrace is formed.
4. What is the economic significance of the Monterey Formation?
5. Why do landslides happen along beaches like Crystal Cove?