Field Trip 4: Vasquez Rocks Natural Area

Introduction

Cerritos College students in front of the "Star Trek" outcrop. 

Vasquez Rocks Natural Area Park (Vasquez Rocks) is a local geologic gem in that it offers perhaps the best continuous sequence of early Miocene terrestrial sedimentary rock in southern California – vividly colored, spectacularly tilting out of the ground, and all this within an hour from wherever you are! Because of its geology, Vasquez Rocks has been used as the backdrop for countless movies, television shows, and commercials, including The Lone Ranger, Bonanza, Maverick, Blazing Saddles, several Star Trek movies, and TV episodes, Cars, Holes, Mighty Morphin Power Rangers, John Carter Westworld, the Flintstones, and most importantly, Bill and Ted’s Bogus Journey. Recently, Jeep was running a commercial where rock climbers appeared to be repelling down from the upturned strata into the parking area... Nothing bogus about that!  

Long before Vasquez Rocks was discovered by Hollywood, though, this area was settled by Native Americans. Archeological evidence indicates that people lived here as far back as 2300 BCE, perhaps even earlier. While little is known about the earliest occupants of Vasquez Rocks, more is known about its most recent inhabitants, the Native American Tataviam people. Tataviam, or “people facing the sun”, settled in this sunny region now known as Vasquez Rocks about 2000 years ago. They subsisted mostly by hunting and gathering. They made their homes by assembling cottonwood and Juniper branches around deeper sections of overhanging strata and then covering the framework with native grasses. The Tataviam are grouped with native peoples in the Uto-Aztecan language family, although their language was difficult to understand. Spanish missionaries arrived in this area in the 1700s, pressuring the Tataviam to adopt the Spanish language and culture. The last full-blooded and native-speaking Tataviam died in 1916.  

A more notorious dweller at Vasquez Rocks was Tiburcio Vasquez, an infamous bandito who utilized the complicated topography for his hideouts. Legend has it that he was Mexican Robin Hood, stealing money and livestock from the well-off and giving to needy Mexican families. His hideout was eventually discovered, and he was chased out of his Vasquez Rocks refuge, tracked down, captured then tried and executed.

This field trip could be done in a half-day or paired with other field trip itineraries covered in this book: The San Gabriel Mountains and the San Andreas Fault.

A video introduction:
 

Geographic Setting

Vasquez Rocks is located on the northern side of the central Transverse Ranges Links to an external site. at the eastern end of the Soledad Basin (a.k.a. Soledad Canyon). The Santa Clara River separates this hilly area from the San Gabriel Mountains to the southeast. Being shadowed by the San Gabriels, Pacific Ocean-borne moisture has a difficult time reaching this region, making the climate dry, resulting in vegetation typical of arid terrains of southern California like yuccas, California juniper, scattered scrub oak, and manzanita.  

Geology

The Vasquez Formation, which is principally exposed at Vasquez Rocks, is an exceptionally thick sequence of mostly sandstone, conglomerate, and breccia, that were deposited within a relatively short period, 20-23 million years ago. The size, angularity, and sorting of the grains represent braided stream, alluvial fan, and debris flow deposits. The Santa Clara River, which is crossed en route to Vasquez Rocks, is a braided stream. Comparisons can be made between the sediment in its channel and the texture of the grains in the Vasquez Formation.

The Vasquez Formation represents the lower portion of about 25,000 feet of sediment that accumulated in the Soledad Basin. The Soledad Basin was one of many basins forming about 23 million years ago when the plate boundary in southern California began changing from a convergent to a transform plate boundary. Consequently, tensional stresses started stretching the crust, resulting in blocks of crust rapidly down-dropping along faults, forming deep basins (structural depressions) like the Soledad basin. This basin was filled with sediment eroded from the ancestral San Gabriel mountains, an earlier version of the San Gabriel mountains that have since eroded. We know this because the clasts of anorthosite, gabbro, and the Lowe Granodiorite that make up some of the grains in the Vasquez Formation match the bedrock of the San Gabriel Mountains. Oxidation of the ferromagnesium minerals biotite and hornblende gives the Vasquez Formation its vibrant red-brown hue. During the past 5 million years, this stratigraphic sequence has been deformed by compressional stress into a wide syncline, uplifted, and differentially weathered. The northern limb of this east-west trending syncline is now exposed as the hogback ridges that make up the skyline and backdrop for the aforementioned television commercials, programs, and movies at Vasquez Rocks.    

Supplementary materials

• Compasses
• Vasquez Rocks USGS 7.5 minute topographic map (2 scanned and modified sections of this map are included in this chapter, one with locations corresponding to suggested learning activities)
• Free digital topo maps can be accessed through the USGS Links to an external site. (Enter "Vasquez Rocks Natural Area Park" in the address bar). 

Learning Objectives

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

1. Identify sedimentary rocks and strata
2. Identify clastic/detrital texture in rocks
3. Explain how and why the orientation of the strata of the Vasquez Formation violates the Law of Original Horizontality 
4. Locate positions and interpret topography using a topographic map
5. Recognize braided stream channels
6. Identify channel bars within a braided stream channel
7. Discuss some of the human history associated with field trip stops

Key Vocabulary

• Alluvial Fan – fan-shaped pile of sediment that was deposited intermittently by flash floods/debris flows at the mouth of a mountain canyon
• Braided stream – a stream channel made up of several interlacing individual channels, separated from each other by channel bars (piles of alluvium)
• Breccia – detrital sedimentary rock composed of angular cobble to boulder-sized grains
• Clasts – fragments of rock or mineral within detrital sedimentary rocks  
• Conglomerate – detrital sedimentary rock composed of rounded cobble-sized grains
• Detrital – texture of sedimentary rock composed of clasts or grains of rocks and minerals
• Differential Weathering (Erosion) – different rates of weathering for different types of rock, based on variations in texture, composition, and structure
• Hogback ridges – adjective used to describe a landscape with beds of tilted strata separated by gaps where the rock was more easily weathered
• Law of Original Horizontality – sediment is always deposited as strata that are horizontal or very nearly horizontal
• Megacycle – an unofficial term referring to a repeating succession of similar textures in strata
• Strata – a vertical sequence of layers of sediment and is considered the definitive characteristic of sedimentary rocks
• Sandstone – detrital sedimentary rock composed of sand-sized grains
• Syncline – a “U”-shaped structure caused typically caused by compressing of strata

Pre-Field Trip Questions

1. What physical characteristics make sedimentary rocks easily recognizable in the field?  
2. What characteristic makes a detrital (clastic) sedimentary rock, look detrital?
3. Using this video: forming sedimentary rocks Links to an external site., answer the following questions (note, you need only watch up to the 3:36 mark in the video):
   1. What is meant by a clastic sedimentary rock? Describe the relationship between detrital and clastic sedimentary rocks.
   2. Where can clastic sedimentary rocks form? 
4. About how old are the rocks that make up the Vasquez Rocks landscape? 
5. Where did the clasts found in the strata at Vasquez Rocks come from? 
6. What caused the strata at Vasquez Rocks to be tilted? 

Directions to Vasquez Rocks

From Cerritos College, I-605 north, I-210 west, and CA-14 north. 

Field Trip Stops

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

Stop 1 (optional) – Placerita Canyon

Addresses learning objective:
7. Discuss some of the human history associated with field trip stops

Exit Placerita Canyon Road (exit 3) from CA-14, turn right onto Placerita Canyon Road and drive east for about 1.5 miles to Placerita Canyon Nature Center. Here you’ll find restrooms, a vulture, hawk, and raven (at the time of this writing) in outdoor cages, interesting information about the local nature, and a trail system that includes a trail to the Oak of the Golden Dream.

Placerita Canyon earned its name and fame for the placer deposits discovered here in 1842, six years before the Gold Rush began in the western Sierra Nevada Mountains. As the story goes, Francisco Lopez found gold flakes in the soil while digging up wild onions. Gold deposited as sediment by rivers is called placer gold, hence the canyon's name. Look for signage for the Oak of the Golden Dream, the tree under which Francisco found the gold. Rumor has it that one can still find flakes of gold in the streambed after heavy rains have turned over boulders (Prothero, 2011).

Return to vehicles and continue northbound in CA-14.

Stop 2 (optional) – Santa Clara River

Addresses learning objectives:
5. Recognize braided stream channels
6. Identify channel bars within a braided stream channel

Exit Sand Canyon Road; turn right onto Sand Canyon and drive over the bridge. Make a right on the first street after the bridge, Lost Canyon Road, and immediately park on the road shoulder opposite the education center (GLC SB/SC). Exit vehicles and backtrack over the west side of the bridge (be sure to remind students to stay on the sidewalk) to about 2/3 of the way across for the best views of the Santa Clara River.

Santa Clara River

Santa Clara River, looking westward. 

• Longest undammed river in southern California (Prothero, 2011) and significant in that its channel hasn’t been modified by human construction
• Braided stream, common in the foothills of mountains
  • Multiple intertwining channels, weaving around channel bars
• Ephemeral stream channel in that it is dry unless it has just rained
• The dry channel is deceiving because water is flowing in the subsurface as groundwater, which will eventually be utilized by communities downstream (Sylvester and Gans, 2016)

Return to CA-14 North and proceed 2 exits to Agua Dulce Canyon Road.  

Stop 3 – Vasquez Rocks

Addresses learning objectives:
1. Identify sedimentary rocks and strata
2. Identify clastic/detrital texture in rocks
3. Describe how the orientation of the strata of the Vasquez Formation violates the Law of Original Horizontality and explain why this has happened
4. Locate position and interpret topography using a topographic map

Exit Agua Dulce Canyon Road from CA-14, turn left at the bottom of the off-ramp onto Agua Dulce Road, and drive a little over 2 miles to the entrance, marked by the sign “Vasquez Rocks Natural Area”. If traveling by charter bus, alert the driver that they will need to make a hairpin turn. Proceed past the visitor’s center, veering left towards the hogback ridges and onto the unpaved road. Take the road all the way down to the “bottom”, which is a huge parking area where any type of vehicle can easily turn around. I recommend parking along the eastern margin. There are three port-o-potties at the south end of the parking area that some students may need to visit before embarking on a field trip.

Note: the visitor’s center has interesting historical information and a little about the local biology and geology, but perhaps not enough content to make it educationally worthwhile for an earth science field trip.

From the eastern margin of the parking area, you could start by pointing out some of the nice examples of rockslides on the south-facing slopes a few hundred feet to the north. 

Rockslide blocks can be seen resting on the dip slope of a hogback ridge above the hiker.

After doing so, walk east up onto the outcrops (opposite the tallest tilted strata upon which people are probably climbing, hereafter referred to as the Star Trek outcrop) and across the “hogbacks” of tilted strata for about 100 yards until you reach a ledge that cannot be navigated without climbing. You should be standing on a very coarse breccia bed, which is at approximate point “A” on the map (see below). Look back westward over the stratigraphic sequence between you and the parking area.

Tilted strata at Vasquez Rocks as viewed from location A on the topographic map below. 

Talking Points: 

• Hogback ridges formed by the tilting of the strata of the 20-23 m.y.a. Vasquez Formation
• Dip slope and scarp slope (escarpment)
• Differential weathering
• Rock slides  
• Geologic description of the Vasquez Formation (see Geology section at the start of this chapter)
• Point out the sizes, shapes, and compositions of the grains contained in the strata here 
• These strata exemplify detrital sedimentary rocks, as the grains are visible in outcrops
• Point out how the grain size controls how easily beds are weathered – generally, the coarsest beds are most resistant, while finest-grained beds are more easily weathered and eroded

Activity 1: Ask students to identify the sandstone stratum (layer of sedimentary rock), conglomerate, and breccia stratum. What clasts make up the pebbles and cobbles in the conglomerate and breccia?

Topographic map of Vasquez Rocks with locations, A, B, and C. Map from USGS National Map. 

Topographic Map of Vasquez Rocks without labels. Map from USGS National Map. 

Activity 1 continued… 

Confirm students' rock identification. Ask: Is there any sort of pattern to the occurrence of the coarsest beds? 

Point out that within a sequence of strata, the grain size changes from fine to coarse then back to fine. In other words, if you were to start with the most coarse-grained bed of an outcrop and then look stratigraphically upward, each higher bed is finer-grained than the bed below until reaching the next coarse-grained bed and the fining-upward sequence repeats. This stratigraphic pattern reflects the episodic mountain building happening while these sediments were being deposited. During a period of mountain building, stream channels steepen and streams flow with greater potential energy and carry larger clasts. When tectonic uplift slowed, streams would erode their channels to lower gradients and in turn, would flow slower and with less energy, transporting progressively finer sediment. This historic mountain building is recorded in the Vasquez Formation. Coarse beds of breccia beds fine upwards to conglomerate then sandstones, or finer stratum, as the streams eroded their channels downward over time. When mountain building was renewed the gradient of streams increased and transported and deposited larger grains. A megacycle is a general term that can be applied to describe this cycle of episodic mountain building as recorded by the repeated pattern of fining-upward sequences of strata.   

Cartoon showing a cartoon of a megacycle: coarse to finer, back to coarser sediments being shed from a fault-controlled mountain front.

 
A labeled photograph of tilted strata records an example of a megacycle. 

Activity 2: Have students sketch an outcrop showing a megacycle sequence. Label drawings with “megacycle”, “breccia”, “conglomerate”, “sandstone”, and “siltstone/shale” (if observed).

Activity 3: Questions: (1) “How do the strata at Vasquez Rocks violate the Law of Original Horizontality?” (2) “How did this happen?” and (3) “How does grain size control the rate of weathering?” 

From here, walk southwestward across the parking area, so you skirt the southern terminus of the Star Trek outcrop, around the yellow road gate, then northwest along the trail at the base of the Star Trek outcrops. When you arrive at the trail junction, turn left and walk southwest to the next trail junction, where you’ll turn left again, walking up the steep ridge to point “B”. See the map below.

Google Earth Links to an external site. map of Vasquez Rocks Natural Area showing field trip stops A, B, & C, with suggested hiking route.

The next activity offers an opportunity to introduce interpreting contour lines and locating positions on topographic maps.

Activity 4: Once assembled at stop B, ask students to take out compasses or use the compass utility on a smartphone to orient themselves and their map so they face north. Next, ask them to locate themselves, reminding them to keep their maps pointing north. As they are working, ask them to point out the “Star Trek” peak, which lies between “B” and “A”, which may help them in pinpointing their location (which is around the “6” in 2600).

Question: What about our current location makes determining our position easier?  

Activity 5: At location B the outcrops offer more good examples of the previously discussed megacycles.  

From location, B walk down to point “C” and repeat Activity 4.  From point C, return to your vehicle(s).  Alternatively, if you have an enthusiastic and physically fit group, you may want to extend your visit, as there are abundant hiking trails throughout Vasquez Rocks.   The map below highlights some of the many trail options.  

   
A Google Earth image of Vasquez Rocks area with hiking trails highlighted.  

Follow-up Questions

1. How did Placerita Canyon earn its name?
2. Where’s the water in Santa Clara River? 
3. Why is the Santa Clara River unique compared to other streams in southern California? 
4. What was the depositional setting for the Vasquez Formation, and when did this happen? 
5. What are the texture and names of the sedimentary rocks at Vasquez Rocks? 
6. Describe or illustrate how a megacycle sequence is formed, including the geologic conditions needed to produce a megacycle and the types of sedimentary rock formed as the depositional environment and geologic conditions change over time.
7. If you found yourself lost in the wilderness but were equipped with a compass and topographic map of the area, what steps would you take to try to locate yourself on the map?