Field Trip 7: Palos Verdes Peninsula

Introduction

Photograph Palos Verdes and Pacific Ocean.
View from Del Cerro Park. 

The itinerary suggested below could be completed in about 3 hours, making for an easy half-day trip. It includes just 2 vehicle stops, but due to limited parking on weekends it is recommended that this trip be done during a weekday or early morning if going on a weekend. In either case, limiting the number of vehicles in your group or using a charter bus is advised.   Any time of the year is good for a visit.

The trip starts at the top of Del Cerro Park, which provides a perfect vantage point to observe and discuss the Portuguese Bend landslide and marine terraces, two big attractions for geologists. From there, you’ll drive across and down a few of the marine terraces, then across the Portuguese Bend landslide, eventually arriving at White Point Park. At White Point, you will start out on the youngest of the Palos Verdes Peninsula (PVP) marine terraces, upon which you will have a better vantage point to observe and discuss common coastal landforms and a landslide: the recent Paseo Del Mar landslide. Afterward, one can walk or drive (but be prepared to pay to park) down to the beach to see a superb recumbent (overturned) fold, a wave-cut platform, and riprap to mitigate beach erosion.

If you are interested in adding content to your field trip, there are several other excellent locations in the immediate area for further geologic study. Descriptions of some of these locations can be found in the National Association of Geoscience Teachers – Far West Section, Spring 2004 Conference field trip guidebook, and in the online field trip guidebook offered by Brendan McNulty at California State University Dominguez Hills: CSUDH geology field trip guide to Palos Verdes. Links to an external site.

Additional materials

Provide the group with a copy of Google Maps “terrain view” of the field trip area: Google Map terrain view of the southern PVP. Links to an external site.

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 active and passive continental margin
  4. Identify a landslide on a topographic map
  5. Identify evidence of mass wasting in the field
  6. Identify coastal erosional landforms: marine terrace, wave-cut platform, cliff, cove, and headland
  7. Describe how a marine terrace forms
  8. Identify sedimentary rocks and strata
  9. Distinguish sedimentary rocks in the field
  10. Identify igneous rocks in the field
  11. Explain why folds violate the Law of Original Horizontality

Key Vocabulary

  • Active Continental Margin – the transition from continental to ocean crust is a plate boundary; narrow continental shelf
  • Cliff – a steep slope adjacent to a beach caused by wave erosion
  • 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
  • Headland – a protrusion of land into the ocean caused by erosion on either side
  • Landslide – a general term describing the mass movement (mass wasting) of weathered rock and/or soil downslope under the influence of gravity
  • 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 continental to ocean crust is not a plate boundary; wide continental shelf
  • Recumbent fold (overturned fold) – strata folded into a “U”-shaped and tipped on its side, like a taco shell; these structures are formed by compressional stress
  • Rock slide – a type of landslide involving rock sliding down a distinct slip (slide) surface
  • Scarp – the exposed slip surface at the head of the landslide
  • Slide block – a cohesive mass of material that has slid down a landslide slip surface
  • Strata – a vertical sequence of layers of sediment considered to be the definitive characteristic of sedimentary rocks
  • Translational slide – a landslide where a cohesive mass of material (slide block) slides down a surface (slip surface) that is roughly parallel to the ground surface above
  • Wave-cut Platform – the gently sloping surface extending from the beach out into the ocean caused by wave erosion

Pre-Field Trip Questions

  1. Using the following link, CSUDH field trip guide about landslides Links to an external site., describe (in your own words) why landslides are common on the PVP.
  2. Draw a simple diagram of a wave-cut platform, marine terrace, cove, and headland.
  3. How do marine terraces form?

Field Trip Stops

All stops listed in this chapter are accessible by large charter bus, van, or 2-wheel drive car.

Stop 1 – Del Cerro Park, Palos Verdes

Directions: from I-110 south, exit PCH/CA-1, and head west for about 7 miles. Turn left (south) on Crenshaw Blvd and proceed nearly 4 miles to Del Cerro Park.

Addresses learning objectives:
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 landslides on a topographic map
5. Identify evidence of mass wasting in the field and principal landslide features: scarp and slide mass/block
6. Identify coastal erosional landforms: marine terrace, wave-cut platform, cliff, cove, and headland
7, Describe how a marine terrace forms

Activity 1: Ask students to organize into groups of 2 or 3 and take out their Google “terrain view” maps and mark current location.

Activity 2: Ask students to compare what they are observing in “real-life” to what they see on their maps; specifically, ask them to trace out top of the horseshoe-shaped depression in front of them (the head of the Portuguese Bend landslide) and then point out where it is in real-life.

Geology 

  • Lithology: The bedrock of the PVP is comprised of two rock units, the Catalina Schist, and the Monterey Shale.
    • Catalina Schist
      • The approximately 75 million years old Catalina Schist is comparable to the Pelona Schist (mentioned in Chapter 5), representing marine sediments that were deposited on top of the subducting Farallon Plate and metamorphosed into the metamorphic rock, schist, during subduction. Because the Farallon Plate is subducted under the North American Plate, the Catalina Schist is referred to as a “lower plate” rock unit and is only sporadically exposed as “tectonic windows” in southern California. On the PVP, there is only one small exposure, which is unconformably overlain by significantly younger sedimentary rock units.  
      • The Catalina Schist was exhumed (brought towards Earth’s surface) when the crust in southern California was stretched and ripped open during rotation of the Transverse Range
        • Stretching and thinning of crust triggered volcanic activity; some of the lava flows associated with this event are exposed on the PVP
    • Monterey Formation
      • 12 to 6 million-year-old thinly bedded shale, mudstone, and chert from 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. See previous chapter for more details on the Catalina Schist.
      • Sediments accumulated directly on top of the Catalina Schist.
  • Mass Wasting (Landslides)
    • PVP is a popular place for geologic study because of the size and number of landslides
    • The composition and structure of rock making up the PVP drives mass wasting
      • The tectonic uplift of PVP has caused strata to dip (tilt) gently to the southwest
      • Mass wasting is more common on the southwest side of the PVP
      • Ash layers have been converted to bentonite, a clay that becomes very weak when wet, acting as slip surfaces for landslides
    • One of the largest landslides in the country, The Portuguese Bend landslide complex
    • Support for landslides continually removed by wave erosion, contributing to instability
    • Portuguese Bend Landslide complex
      • Prehistoric Portuguese Bend landslide (PPBL) active from 375,000 to 120,000 years ago
      • Modern Portuguese Bend landslide (MPBL) started in 1956 and is sliding at an average of a ¼ inch per day
      • Abalone Cove landslide (ACL) portion started moving in 1974
        • Water from septic tanks saturated bentonite layers within the Monterey Formation, reducing the internal friction of these layers and initiating slope failure
        • Geologists designed a pumping system to remove water, which stopped the slidingIllustration of Portuguese Bend landslides.Map showing the approximate Locations of landslides observable from Del Cerro Park.
  • East Coast vs. West Coast Geology
    • Prompt: Here, in Del Cerro Park are we at sea level? – No, we are over 1000 feet above the ocean surface.
    • Questions:
      • If we were standing this close to the Atlantic Ocean along the east coast of North America, would we be this high above sea level? – No, because the topography is more subdued; with low hills and flat coastal plains.
      • Why does the west coast have steep, tall hills right next to ocean, while it’s 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 margin: a plate boundary is crossed from continental to ocean crust; narrow continental shelf.

Cross section illustration of an active continental margin.
Active continental margin. – CC

      • Interaction of the plates along the plate boundary drive constructive geologic processes, such as tectonic uplift, making hills, mountain, and even volcanoes
    • Passive continental margin: no plate boundary between continental and ocean crust; wide continental shelf.

Illustration of a passive continental margin.
Components of a passive continental margin: the continental shelf, continental slope, and continental rise. - CC  

      • No plate boundary means no constructive geologic forces. Instead, destructive geologic processes dominate and the landscape is worn down by erosion
  • Marine Terraces
    • Ancient wave-cut platforms (where the beach once was) that have been elevated above sea level
    • 13 terraces have been mapped, recording the episodic uplift of PVP over the past 1.5 million years (Sylvester and Gans, 2016)
      • 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 terrace will be the closest to sea level

Before loading vehicles to head for stop 2, instruct the group to look for marine terraces as they descend down towards the beach – they should be easily visible off to the east, which will be out the left-hand side of the vehicles. Also, ask them to look for signs (literally), where the road crosses over the active portion of the Portuguese Bend slide.

Stop 2 – White Point Park, Palos Verdes

Departing Del Cerro Park, turn left on Crenshaw Blvd, left on Crest Rd., left Hawthorne Blvd., and left on Palos Verdes Drive. Turn right on Western then veer left where road splits on to Paseo Del Mar and park on road adjacent to the White Point Park parking lot.

Paseo Del Mar landslide overlook and coastal landforms

Walk through parking lot towards the ocean and past the buildings to the southeastern end of the dirt trail atop the sea cliff (don’t walk down the road to the beach yet). The baseball field should be on your left as you face the ocean. Walk around railing at the end of the path and along the trail (Don’t get too close the edge!) between the top of cliff and the baseball field. Walk around to the southeast side of the field, where you’ll have an excellent view of the Paseo Del Mar landslide. From this vantage point one can also observe marine terraces, wave-cut platforms, headland, and cliffs.

Map of White Point and Paseo Del Mar landslide.
Map view of White Point and Paseo Del Mar landslide.

Addresses learning objectives:
5. Identify evidence of mass wasting in the field
6. Identify coastal erosional landforms: marine terrace, wave-cut platform, cliff, cove, and headland
7. Describe how a marine terrace forms

Activity 3: From the overview, ask students to point out the landslide and landslide features like the scarp and slide mass.

Activity 4: From the overview, ask students to point out marine terraces, wave-cut platforms, and sea cliffs. Directing their attention to the east, ask students to make a simple cross-section drawing of the wave-cut platform, cliff, and marine terrace.

Cross-section drawing landslide.
Cross-section view of landslide.

Paseo Del Mar landslide

  • Early 2011 cracks were noticed in road
  • November 2011 an approximately 400 foot long section of Paseo Del Mar road slid over 50 feet southward towards the ocean
  • The rock unit that failed is the Monterey Shale, which is dipping about 10-20o to the south, making for a rough “dip slope”, which could be considered a trigger for this mass wasting event
  • This rock slide could be classified as a “translational block slide”, because the slide mass slid as a cohesive block with little rotation
  • Probable mitigation (repair efforts) includes building a bridge over the slide area or removing the unstable material and filling up this area with rock and soil (engineered fill) then supporting the base with a retaining wall. Cost estimates range from about 25-45 million dollars.  

Return the way you came, perhaps stopping along the west side of the headland over which you are walking to observe other examples of marine terraces and wave-cut platforms. Walk back to Paseo Del Mar and go east (right) down to the end of the road. If the fence is open, go through and walk about 100 yards to the head scarp of the Paseo Del Mar landslide, exercising extreme caution with your group to prevent any falls. From here you have a closer look at the translational slide, the scarp, and the slide block.

Activity 5: Ask students to identify landslide features like the scarp and slide block.

Stop 3 – White Point Beach

Addresses learning objectives:
8. Identify sedimentary rocks and strata
9. Distinguish sedimentary rocks in the field
10. Identify igneous rocks in the field
11. Explain why folds violates the Law of Original Horizontality

White Point Beach

  • Originally made famous as a natural spa, fed by hot, sulfur-rich water venting out near the base of the cliff
  • Several million years ago lava rose up to Earth’s surface along a fault that intersects the cliff
  • Residual heat from the lava delivered the heated sulfur-rich waters
  • The magnitude 6.3 Long Beach earthquake in 1933 disrupted the natural plumbing causing the hot water to virtually stop flowing
  • Surprisingly, the earthquake did not close the spa; sadly though, internment of its Japanese owners during World War II did

Access White Point Beach via the sidewalk or 2-lane road at the west end of the parking area. Note: if you choose to drive down to the beach, you may be required to pay to park.

Activity 6: Before starting the descent, ask students to observe the rock making up the cliff face to your right as they descend. What type of rock makes up the cliff? – Monterey Shale

How are the rock layers oriented/tilted? – Tilted/dipping south

Recumbent fold

  • At the bottom of the beach access road you’ll see a beautiful example of recumbent or overturned fold.
  • Structures such as these are formed due to compressive stresses. In this case, this fold might have formed as tectonic stresses squeezed and uplifted the PVP.

Activity 7: ask students to sketch the fold in their field book and use arrows to indicate the direction of stress needed to fold the strata. Ask them how to explain how a fold violates the Law of Original Horizontality.

Riprap has been placed along the beach in order to slow erosion of the beach by wave action.

Activity 8: ask students if they think that the rock making up the riprap was collected from the PVP? Why not? – No. Lithology does match the bedrock of the PVP.

Activity 9: ask students to identify the rock type of the riprap (granodiorite, diorite, and gneiss).

Follow-up Questions

  1. Explain in words and/or with drawing how marine terraces form.
  2. What is the relationship between a marine terrace and a wave-cut platform?
  3. Considering what was discussed during the field trip, explain why landslides are common on the PVP, especially on the southwest side.
  4. What roll do clay layers play in triggering landslides?
  5. Draw a picture of folded strata, name the type of fold, and use arrows to show the type of stress needed to make such a structure.