Field Trip 1: Joshua Tree National Park

Overview

Joshua Tree National Park (JTree) is situated within the little San Bernardino Mountains Links to an external site. at the easternmost extent of the Transverse Mountain Range. Lying in the rain shadow of the San Jacinto and San Bernardino Mountains, JTree contains parts of two deserts, the Mojave and Colorado.  Yucca brevifolia, a.k.a. Joshua trees, roughly delineate the Mojave Desert in the northern part of the park from the Colorado Desert to the south. It is an excellent destination for observing intrusive granitic igneous rocks, common rock-forming minerals, metamorphic rock (gneiss), the effects of weathering on rock, intrusive structures like veins, the San Andreas fault, common desert landforms, including inselbergs and alluvial fans, desert ecology, and historic mining operations. It’s also probably unlike any landscape most students have seen making this eye-opening experience reason enough to expose young minds to Los Angeles’s closest National Park.

When planning your trip consider the season and weather.  It can be dangerously warm during the summer, early fall, and late spring, and surprisingly cold during the winter.  Plan your visit for the late fall, winter, or early spring months to enjoy comfortable temperatures and blue skies.  However, this is also the peak season for visitors, so arrive early if you visit on a weekend.

Human History

JTree was first inhabited by Native Americans 7000-5000 years ago, based on artifacts found in the Pinto Basin area.  More recently, ancestors of the Cahuilla, Chemehuevi, and Serrano people occupied other parts of the park, including the Oasis of Mara, which later became a popular stopover for miners working in the area due to the freshwater source.  Mining operations in the park have produced millions of dollars in gold and silver.  Coincident with mining was cattle raising due to the relative abundance of grassy meadows found in the park when the climate of this region was milder and wetter just 150 years ago.  In the early part of the 20th Century, the park’s natural beauty drew in visitors, many of whom found the warm, dry air therapeutic.  However, with more people came a more significant impact on the environment, prompting the protection efforts that led to the establishment of Joshua Tree National Monument in 1936 and then JTree NP in 1994.

Geology

Two rock types constitute most of the JTree landscape, granite and gneiss. Technically, the “granite” in JTree does not fully meet the specific chemical criteria for granite. Nevertheless, it will be called granite here for the sake of simplicity. The granite is comparable to the rock of the Sierra Nevada and Peninsular mountain ranges, having formed through subduction of the Farallon Plate Links to an external site. as the North American continent broke free of Pangea and moved westward. The Pacific Plate is now situated where the Farallon Plate once was.
In contrast, the distribution of the gneiss is not as widespread and is more local to the JTree region. It is the byproduct of the metamorphism of ancient sediments through the collision of continents some 1.7 billion years ago.  

The granite that provides the backdrop for many of the points of interest in the park is an assemblage of several separate masses of solidified magma, known as batholiths Links to an external site.. Each batholith formed at different times with different chemistries; therefore, each has distinctive mineral compositions and weathering characteristics. Consequently, these distinct granitic bodies vary in appearance and are given different names, including Palms granite, White Tank monzogranite, Twentynine Palms quartz monzonite, and Queen Mountain monzogranite. At a distance, these granitic bodies appear buff to tan to brown as they weather into inselbergs and boulder piles. With some training, the eye can distinguish between the different batholiths. Upon closer inspection, one can observe the mineral crystals of quartz, plagioclase, orthoclase feldspar, and biotite and muscovite mica that constitute granite. In Rattlesnake Canyon, at the southeast end of the Indian Cove campground, the granite contains impressive “megacrysts” of feldspar: blocky crystals several inches across. Visiting this area is not part of this chapter’s itinerary but is a worthwhile side trip if you have enough time in your day.

Spheroidal Weathering

Weathering is the action of rock physically and chemically weakening and breaking down at or near Earth’s surface. One of the main reasons millions visit JTree each year is its uniquely beautiful landscape, one that contains numerous isolated rounded hills that are surrounded by boulders of granite. Many of these hills qualify as inselbergs Links to an external site., the bedrock remnants of mountains, which have been weathered and eroded down to resistant, boulder-mantled knobs rising above the desert plain; aptly been described as “highly eroded mountains drowning in their own debris”.

Photograph of an inselberg. 

Forming Inselbergs

1. Before reaching the earth’s surface, stresses acting on a batholith cause joints or sets of roughly parallel fractures oriented in different planes. These intersecting fractures can dissect the body of rock into cubic blocks.
2. The granite is brought close to the earth’s surface through uplift and erosion.
3. Acidic water infiltrating from Earth’s surface flows down and along with the fractures, chemically reacting with feldspar in the granite, changing it into clay.

Rock with joints. - CC. 

4. Because the fracture pattern created cube-like blocks of granite, the corners of the cubes are preferentially weathered faster than the rest of the block – just like sucking on an ice cube will result in the cube becoming rounded – and the cubes of granite become more spherical.
5.  Once erosion exposes granite on the earth’s surface, the clay is easily washed away, leaving behind rounded hills and cliffs of granite. The stripping away of clay and loose mineral grains is particularly effective when warming and drying climates reduce the density of vegetation, leaving soil and weathered rock more exposed to water erosion.

Rock undergoing spheroidal weathering. –CC

Cliffs of rounded and fractured granite bedrock.

Learning Objectives

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

1. Name the type, and the specific name of the plate boundary crossed to get to JTree
2. Identify phaneritic (coarse-grained) igneous rock texture in the field
3. Identify granite in the field
4. Identify common igneous minerals in rock samples in the field
5. Describe the role played by subduction in forming granite
6. Identify foliation and metamorphic rocks in the field
7. Distinguish between igneous and metamorphic rocks in the field
8. Identify spheroidal weathering in the field
9. Explain the process of spheroidal weathering and how it contributes to the look of the JTree landscape
10. Identify veins and explain how and when they form
11. Use linear landforms, such as the Indio Hills, to locate faults, like the San Andreas
12. Identify a roof pendant and explain or illustrate the structural relationship to the batholith
13. Identify common desert landforms, including inselbergs, alluvial fans, and pediments
14. Describe the climate and geography of JTree and briefly discuss how the climate has changed over time
15. Identify a “soil notch” in the field and briefly explain its significance

Key Vocabulary

• Alluvial Fan – a fan-shaped pile of sediment that was deposited intermittently by flash floods/debris flows at the mouth of a mountain canyon
• Batholith – a very large mass of crystalized magma; often granitic
• Bedrock – solid rock of the crust that makes up the geologic foundation of most landforms
• Gneiss – metamorphic rock with distinctive wavy bands of lighter and darker minerals
• Granite – intrusive igneous rock, rich in silica and potassium, light-colored
• Inselberg – “rock islands” that are bedrock remnants of prehistoric mountains
• Joints – sets of fractures in the bedrock
• Pediment – gently sloping surfaces of bedrock leading up to the base of desert mountains or inselbergs
• Pinto Gneiss – metamorphic rock unit representing the oldest rock exposed in the park
• Roof Pendant – a geologically older bedrock body of rock “capping” a relatively younger intrusive igneous rock unit
• Soil Notch – eroded “dents” or “hollows” in granite outcrops
• Spheroidal Weathering – chemical weathering process that develops in the subsurface along joints, resulting in creating spherical boulders that are later exposed on Earth’s surface
• Vein – relatively thin, tabular bodies of minerals cutting across masses of bedrock
• White Tank Monzogranite – granitic unit that makes up the most easily observable landscape of JTree NP

Pre-Field Trip Questions

1. Using the National Park Service (NPS) webpage on geologic formations in JTree Links to an external site. (video below), What plate tectonic process was needed to make the rocks of JTree NP?
   Geologic Formations of JTree video:
   
   
   
   1. What type of rock makes up the landscape of JTree NP (igneous/sedimentary/metamorphic)?
2. Using the NPS webpage on Deserts Links to an external site., Describe why JTree is situated in a desert. What type of desert is this?
3. Read “Fracturing and Faulting Links to an external site.”, Define what a joint in rock is and describe, in your own words, how joints form.

Directions to JTree

From Cerritos College, 605 north to the 10 east, to CA-62 north. From Cerritos College, 605 north to the 10 east, to CA-62 north. You should follow along as your professor discusses the en-route talking points.

Location of Joshua Tree National Park. –CC

Directions into JTree

The park can be accessed from CA-62 using the West entrance, by turning south on Park Blvd from the arty community of Joshua Tree, or via the North entrance, from Twentynine Palms, by turning south onto Utah Trail road, which is 16 miles east of Park Blvd. While the west entry is more picturesque, the northern entrance will likely be less crowded and has a larger visitor center with more restrooms. In either case, arrive as early as possible and be prepared for a line of cars at the entrance kiosk, especially on the weekends. You will need to present your academic fee waiver for free entrance and be sure to request park brochures to help you and your students navigate. Alternatively, one can access JTree from the south by exiting I-10 at Cottonwood Springs Road/Box Canyon Road and driving north to the South entrance. This route is not recommended because it demands the longest drive time (from the greater LA area) and is furthest from the stops outlined in this chapter.

Field Trip Stops

All stops listed in this chapter are accessible by large charter bus, van, or 2-wheel drive car, and are written as if entering the park from the West entrance. “Zero” odometer at West entrance Kiosk, to use mileages listed (note, some mileages may refer to other starting points).  

Activity 1: As you wait to enter the park at the West entrance kiosk, instruct students to interpret the landscape as you wind your way through the park. Specifically, you may ask them to identify the different rock types making up the landscape (granites and gneiss) and desert landforms, like inselbergs.

Activity 2: Help students develop map skills by having them locate and label each field trip stop on a JTree National Park map, #1, #2, etc. (Maps can be requested at Kiosk, visitors center, or through the NPS website: Joshua Tree brochure map Links to an external site.)

Cartoon map of Joshua Tree National Park. Numbered stops shown.

Stop 1 – Inselberg and Introduction to JTree NP

Addresses learning objectives:
13. Identify common desert landforms, including inselbergs, alluvial fans, and pediment
14. Describe the climate and geography of Joshua Tree and briefly discuss how the climate has changed over time

Roughly 3.5 miles from the West entrance kiosk, park on the road shoulder where it is safe to observe an excellent example of an inselberg off to the right (west) Links to an external site.. Note how loose boulders rest on the flanks and around the base, earning these landforms the nickname, boulder pile mountains. Behind the inselberg, off in the distance, you may notice the darker rock making up the relatively smoother slopes of the mountains. The lighter rock of the inselberg and slopes across the road are made of the 75 million-year-old Palms granite, while the darker rock is just a bit older: 1.7 billion years old Pinot Gneiss (Trent, et. al., 2015). These are some of the oldest rocks in the western United States.

If you couldn’t park at 3.5 miles in, look for parking at other road-shoulder pullouts or the Quail Springs picnic area, Boy Scout Trailhead, or Hemmingway parking lots. In any event, try to find a good spot to “set the scene” for the day, where you can also discuss:

• The geographic location of JTree NP
• Explanation of the rain shadow effect
• The human history of JTree NP
• Timing of protecting this area as a National Monument and National Park; why protection is needed
• Define and discuss the formation of inselbergs

Stop 2 - Hidden Valley parking area and loop trail

Addresses learning objectives:
2. Identify phaneritic (coarse-grained) igneous rock texture in the field
3. Identify granite in the field
4. Identify common igneous minerals in rock samples in the field
5. Describe the role subduction plays in forming granite

Some logistical considerations:

• Commit 1-1.5 hours.
• There may not be space for the bus (if you’ve arrived by charter bus) to park. If so, ask the driver to drop off the group and then return at a designated time (cell phone service may not work).
• Remind students to bring the necessary gear (water, note-taking materials, etc.) and to watch for cars while exiting the bus and crossing the parking lot.
• Direct students to a meeting point – I recommend the 3^rd picnic table, about 200 feet south of the Hidden Valley trailhead. This area makes for a convenient natural classroom, where one can conduct an in-field lecture and student activity.
• There are 2 bathrooms – one just north of the trailhead and another south of the picnic table classroom mentioned above.

Activity 3 – Organize students into 2-3 person field teams and ask them to sketch a small, maybe 6”x 6” inch section of one of the boulder outcrops (I use this area Links to an external site., ~100 yards south of the trailhead) and to label the common granite forming minerals: quartz, feldspar(s), and biotite mica. If possible, bring along some hand lenses or magnifying glasses – 1 per team works well.

Once the lecture and field exercise is complete, collect the magnifiers and head up the trail into the hidden “valley”. If you wish to add a little cultural flavor to the hike, discuss how this area came to earn its name:

• When white men first settled in the region of JTree, it was wetter and cooler than today, and grassy meadows blanketed the valleys, making them well-suited for grazing livestock. In fact, the grass was described by an Army officer in 1852 as being “belly-high to a horse”. Along with the livestock came cattle rustlers who discovered that the cliffs surrounding what is now the Hidden Valley trail offered an excellent hiding place for their stolen livestock, and so it came to be known as the “hidden valley”.

For a virtual field trip experience Links to an external site., watch the video from 1:37 to 4:33, shot by The Table along the Hidden Valley loop trail.

Once up the short switchback section and into Hidden Valley proper, head left and clockwise around the trail. Depending on your time constraints and depth of curriculum, there are many talking points along the trail, but two are particularly beneficial for teaching about spheroidal weathering.   The first is about ¼ mile along the trail, just before the first moderately steep downhill section and just after a section of the trail bracketed by two cliffs; here, located around 34°00'51.3"N 116°10'21.6"W, one can observe a partially “sphered” block of granite. The second recommended lecture stop could be somewhere along the northern section of the trail (where it begins to climb gently uphill); the south-facing cliffs of granite exhibit the jointing needed for spheroidal weathering to happen.

Partially sphered block of granite. 

I find that just the walk alone, allowing students to soak up their surroundings, is enough to have them buzzing and talking about coming back with their family, significant others, etc. To tailor the walk to your students, I’d encourage you to visit before your first field trip and note spots that would best enhance your course curriculum.

Stop 3 – Keys View and the San Andreas Fault

Addresses learning objectives:
1. Name the type and specific name of the plate boundary crossed to get to JTree
11. Use linear landforms, like the Indio Hills to locate faults, like the San Andreas

Logistical notes:

• Accessible by any vehicle, with parking typically available
• There is a bathroom at the bottom of the parking lot
• Remind students to not leave any trash behind and to look both ways when exiting vehicles  

Although it lacks designated picnic areas, the end of Key’s View Road, at Key’s View Links to an external site. makes an excellent lunch spot. Even with larger groups, there is plenty of room to spread out and enjoy the impressive vistas. After lunch, students can be directed to meet at the top of the hill along the south side of the parking lot, this is Key’s View. From Key’s View, the Salton Sea and Salton Trough can be observed and discussed, the Peninsular Ranges and Mt. San Jacinto, and the Indio Hills and the San Andreas fault.

The Salton Trough is a pull-apart basin, a product of the same tensional forces peeling Baja California away from mainland Mexico and forming the Gulf of California. The Salton Sea is California’s largest lake at about 35 miles long and 10 miles wide, but it’s been as long as 100 miles long and 35 miles wide during its 2.5-million-year history (Sylvester and Gans, 2016). The water currently occupying California’s second-lowest landscape was put there by accidental flooding of the Colorado River when engineers were diverting water for agriculture.

The Indio Hills are the low, straight hills to the south of Keys View, between the Little San Bernardino Mountains and Palm Springs at the base of the San Jacinto Mountains. They are a squeeze-up structure or a “squeeze block Links to an external site.” caused by compressional forces, created where the San Andreas fault diverges into two branches, the Banning and Mission Creek faults.

Indio Hills and the San Andreas fault. Animated version of the illustration.  Links to an external site.

Linear features like the Indio Hills are atypical in nature – nature typically doesn’t like straight – and is, therefore, a good indicator of a fault-controlled structure. Keys View offers one of the best places to see the topographic expression of the San Andreas fault.  

Activity 4 – Have students sketch the Indio Hills and the San Andreas fault splitting into 2 branches, the Mission Creek fault, and the Banning fault. Sketches could include arrows indicating the relative movement of the crust on the south and north sides of the fault and the names of the tectonic plates on either side, the Pacific Plate and North American Plate, respectively.

Stop 4 – Roof Pendant from Oyster Bay parking lot

Addresses learning objective:
12. Identify a roof pendant and explain/illustrate the structural relationship to the batholith

This quick, 10-minute stop is useful for observing and discussing a roof pendant. From the parking lot Links to an external site. observe that Ryan Mountain, immediately east, is composed of lighter rock, Palms granite (also referred to as White Tank monzogranite), and darker rock, the Pinto Gneiss. From this perspective, we see the Palms granite is topographically lower than the Pinto Gneiss. To create this juxtaposition, we need to go back to about 75 million years ago, when magma rose through the crust, intruding into the overlying host rock. The host rock and ancient “roof” to this body magma is the 1.7 billion-year-old Pinto Gneiss. Thanks to erosion, we can see the relationship between these two rock bodies, which has exposed a cross-sectional perspective of the Pinto Gneiss, the roof pendant, capping the Palms granite. Note: this is a better stop in the afternoon when the sun is illuminating the mountainside than in the morning.

Ryan Mountain roof pendant.

Illustration of a roof pendant. 

Activity 5 – Ask students to sketch the mountainside and label the White Tank monzogranite, Pinto Gneiss, and the roof pendant. Additionally, you could have students draw step by step: (1) the magma being made through subduction; (2) the magma rising up into the Pinto Gneiss; and (3) the magma solidifying into granite.  

Stop 5 – Veins near Skull Rock

Addresses learning objective:
10. Identify veins and explain their emplacement

Parking for this stop can be found on the road shoulder just after the “Reduce Speed” sign Links to an external site., about a 15-minute drive from the Oyster Bar parking lot. The Reduce Speed sign is ~1/8 mile east of the entrance to the Jumbo Rocks Campground. If parking is not available, which is quite possible considering the proximity to the very popular Skull Rock, proceed another mile to “Plan B”, the Live Oak picnic area. The Live Oak picnic area will likely have ample parking and a bathroom, although fewer examples of veins.

If you did find parking, exit vehicles and proceed about 200 feet south up the slope to the granite outcrops. Here is an offering of excellent, easy-to-see examples of veins cutting through the White Tank Monzogranite.

Veins like these probably formed as the pluton cooled, solidified, and contracted, then cracked in response to tensional stresses. Residual, mineral-rich fluids filled the cracks, where minerals like quartz and feldspars crystallized.   After completing Activity 6 (see below), you may wish to join the madness at the very popular photo-op spot, Skull Rock.

“Plan C” could be, instead of going to Live Oak, to the Split Rock trail, opposite the Live Oak access road. From the west side of the parking lot, walk about ¼ mile west along the trail. Eventually, you’ll notice abundant veins cutting across the granite.

Activity 6 – Have students sketch a section of the granite containing a vein and label the relative age of each unit.

After returning to vehicles, continue in the same eastward direction to complete the loop of the park. If time permits, make a last, quick stop to observe the Pinto Gneiss firsthand.  

Stop 6 – Pinto Gneiss

Addresses learning objectives:
6. Identify foliation and metamorphic rocks in the field
7. Distinguish between igneous and metamorphic rocks in the field

An easily accessible outcrop of Pinto Gneiss Links to an external site. is just northeast of the junction with Pinto Basin Road that leads to Pinto Basin, Cottonwood, and I-10. Proceed towards 29 Palms about 500 feet and park in the turnout. Exit the vehicles and proceed a few tens of feet onto the small knob of black Pinto Gneiss, pointing out the foliation to students, which helps to identify this rock as metamorphic. From the turnout, head north to the northern park kiosk to exit the park. Be prepared to show your fee waiver to a park ranger once again at the kiosk as you leave.  

Pinto Gneiss

Follow-up Questions

1. What type of rock makes up the inselbergs (small-boulder pile hills) throughout the park?
   1. What are 3 minerals contained in this rock?
   2. Considering plate tectonics, what process was initially needed to make the magma that crystallized to form this rock? About how long ago did this happen?
2. What type and what is the name of the oldest rock observed in JTree NP?
3. What process makes the outcrops of granite so rounded?
   1. Describe or illustrate, step by step, how this type of weathering gives the rock its “bouldery” appearance.
4. What is a roof pendant? How old is the rock that makes up the roof pendant? Illustrate the relationship between the roof pendant and the rock that intruded into it with a simple, labeled diagram.
5. What type of climate exists in Joshua Tree?
   1. Was the climate of JTree wetter or dryer in the past?