Geology Tour Road

Joshua Tree National Park

Now that we have a new car, we can safely navigate dirt roads that were previously off-limits in our little purple eggplant of a vehicle.

Notice the CA license plate…

We take our new 2019 Honda HRV to Joshua Tree National Park and drive part of the Geology Tour Road that intersects Park Boulevard, the main road through the NP.

We drive as far as Squaw Tank before turning back. After Squaw Tank the road gets rougher and is only recommended for 4-wheel-drive vehicles.


Rocks have stories to tell. Geologists listen and speak their language, sharing with us the tales of the long, dynamic history of Earth. The 1.8-billion-year geology in Joshua Tree National Park is capsulized along this 18-mile round-trip road. (plaque on site)

As we drive and bounce along this unpaved route on a hot but beautiful clear-blue day, I will do my best to explain the history of today’s landscape in Joshua Tree National Park while mostly just sharing my pictures along the way.

According to the Geology Tour Road Guide available at the “trailhead” of this motor tour, the landscape we see today in Joshua Tree NP is the product of at least 2 widely separated episodes of mountain building. The latest of these episodes was followed by uplift, very deep erosion and then further uneven uplift along fault lines.

Erosion exposed two rock bodies originally formed deep below Earth’s surface: the 1.7-billion-year-old Pinto gneiss and the 85-million-year-old White Tank monzogranite that intruded the gneiss as molten magma.

Let’s stop here for a moment! Gneiss? Okay… gneiss is a foliated metamorphic rock.

A metamorphic rock has been modified by heat, pressure, and chemical processes, usually while buried deep below Earth’s surface. There are 2 basic types of metamorphic rocks: foliated and non-foliated. (

Foliation refers to repetitive layering in metamorphic rock. The word comes from the Latin folium meaning “leaf”, referring to the sheet-like structure. (

Here is an example of Pinto gneiss in Joshua Tree NP:, courtesy of Blizo

Monzogranite rock is formed when molten liquid, heated by the continuous movement of Earth’s crust, seeps upward and cools while still below the surface. (

According to, this molten liquid (magma) beneath Earth’s surface hardened into stone and cracked under pressure some 100 million years ago. As the ground above the surface eroded, exposing the hardened stone beneath, water seeped into the cracks slowly turning rock into soil. Finally, as this rocky soil slowly eroded away, what was left were towers of disjointed rock intrusions.

Here is an example of White Tank monzogranite:

These magnificent irregular boulders remain iconic symbols of Joshua Tree National Park!

This short, excellent, and animated video on the website explains the unique geology of Joshua Tree:

Geologic Formations…

Marker 1: Queen Valley (and Lost Horse Valley on the other side of Ryan Mountain) are formed by a difference in the rate of erosion between the rock underlying the valley itself and the rock composing the surrounding mountains. (

The rock making up these valleys is generally less resistant to weathering and erosion than the rock forming the surrounding mountains so it disintegrates more rapidly to form low-lying plains. (

As we travel through Queen Valley…

Marker 2: This knoll is the north-south drainage divide for Joshua Tree National Park., courtesy of Walter Feller

From here water drains either to the northwest via the Quail Springs Wash…

…or to the southeast via the Fried Liver Wash that empties into the Pinto Basin. (

Of course, I have no idea what I am really taking pictures of (except for the monzogranite rock piles, Mojave yucca, and Joshua trees, that is…) The desert, rocks, sky, and clouds are just beautiful, that’s all! And no other cars are in sight.

a juniper bush

Marker 3: Just ahead is a desert wash, aka nature’s gutter. Mineral grains, loosened from parent rocks are moved downslope during rainstorms. (Geology Tour Road Guide)

A desert wash is a dry intermittent stream channel where a sudden intense rainfall can produce flash floods. Mineral grains, loosened from rocks by weathering, are moved further down the wash by flooding. It may take a number of storms before the mineral grains deposit into an alluvial fan or dry lake.

The soil in the wash contains more moisture, allowing some plants to grow here more readily than in drier soil. (

I continue taking pictures as we drive along the unpaved road…

Marker 5: The monzogranite rock piles continue to amaze me. Once upon a time they were just a molten mass 15 miles beneath the surface of Earth. Then they cooled and crystallized to form solid rock that pushed upward and eroded into magnificent sculptures. (Geology Road Tour Guide)

Joshua Tree “Geology Tour Road” Guide

Erosion over the ages has stripped away the overlying Pinto gneiss, exposing the monzogranite outcrops in the pictures above. The mountains to the west are composed of darker gneiss, which is more resistant to erosion than monzogranite. (Geology Road Tour Guide)

Marker 7: The twin peaks of Malapai Hill rise about 400 feet above the valley floor. According to the Geology Tour Road Guide the Hill is mostly composed of black basalt, which is more resistant to weathering than monzogranite, and most likely resulted from a shallow intrusion of molten magma that did not quite reach the surface. Geologists speculate that the basalt formed within the last 2 or 3 million years, which is quite recent as compared to the monzogranite at 85-million years of age and the gneiss at 1.7-billion years old.

Marker 8: We descend a pediment surface which is defined as a gently sloping rock surface at the foot of a steep slope, as of a mountain, usually thinly covered with a deposit of sand or mud formed by flowing water. (

According to

About seven to nine million years ago, the Mojave Desert was semi-arid, covered in soft rolling hills and vegetation, a vastly different scene than the dry, cactus-filled ground today. The granite bedrock was covered by soil which formed from water moving down through the rock, assisted by joints in the granite. Slowly, the jointed granite became more rounded,
resulting in tall, elongated boulders encased by soil. As the landscape became drier, less water was available to chemically weather the rock and remove debris away, and soil stopped forming so rapidly. Rounded boulders and small rocks were left behind, no longer having enough water to carry away eroded material. Inselbergs…. look like islands of rock.

The “ocean” that these islands rise from is made up of pediments. Pediments are rock surfaces which appear flat but have slopes from half a degree to six degrees. There is some debate as to how pediments form. Some believe that they formed at the same time as inselbergs, by the same erosional processes. A more common belief is that they form when mountains retreat. When we talk about retreating mountains, it doesn’t mean the mountains are picking themselves up and moving. Instead, the front of the mountain has been eroded by physical and chemical weathering, and a pediment is left on the land that mountain front used to occupy. Pediments are subject to winds and sheet floods, that for the most part, relatively clear them of debris. In order to be considered a pediment, any debris on the plane must be less than ten feet thick.

Joshua Tree National Park Geology
Joshua Tree “Geology Tour Road” Guide

Marker 9: We now approach Squaw Tank. This is our turn-around point as we are not driving a 4-wheel-drive vehicle.

The brightness and clarity of the next set of photos let’s you know we have arrived at the parking area and are taking pictures outside of the car:

By now you are an expert in correctly identifying the make-up of these boulders… monzogranite, you got it!

The pits and hollows on rock surfaces in this area are a product of cavernous weathering. This process begins with irregularities on the rock surface that trap water. The water promotes a chemical breakdown of the rock into clay, which in turn holds more moisture and promotes more breakdown of the rock. (Geology Road Tour Guide)

It’s too hot to explore this area and hike to the “tank” of Squaw Tank. I never even questioned the term, to be honest, until I started doing some “homework” for this post and became curious about a White Tank and Squaw Tank. I thought tank was a geologic term…🥴😳🥴

According to, tanks were small dams built by local ranchers some hundred plus years ago to collect rain runoff for watering cattle. This concrete dam, known as Squaw Tank, was built by cattlemen in the late 1880s. And according to, a short walk from this ample parking area leads to a sandy, rock-enclosed wash leading to Squaw Tank, a watering hole found behind a human-made wall that spans the width of the wash.

In 2008 Joe Orman posted this picture of Squaw Tank on his website

So, unfortunately, we have to turn back.

But there are 7 more points of interest that I can only read about and describe and wish I could experience.

Marker 10: This section of Geology Tour Road becomes a one-way loop in Pleasant Valley.

Pleasant Valley lies between the Hexie Mountains and the Little San Bernardino Mountains. (According to, there are 6 mountain ranges in Joshua Tree NP. Who knew? Right?) The Little San Bernardino Mountains are in the southwest. The Cottonwood, Hexie, and Pinto Mountains are in the center of the Park. The Eagle and Coxcomb Mountains are in the east.

Between the mountain ranges there are valleys identified according to their method of formation. As previously discussed, Queen and Lost Horse Valleys were formed by the difference in the rate of erosion between the rock underlying these valleys and the rock composing the surrounding mountains. The mountainous rock is more resistant to erosion and therefore rises above the valleys. Pleasant Valley was formed by a collapsed block of rock along faults that formed basins called grabens, meaning ditch or grave in German. (

Joshua Tree is also crisscrossed with a number of earthquake faults, the most familiar being the San Andreas Fault which borders the south end of the park and can be seen from Keys View.

The Blue Cut Fault is in the center of the park extending roughly 50 miles through the Little San Bernardino Mountains, under Pleasant Valley, and into the Pinto Basin. Blue Cut is named for the igneous rock, blue granodiorite, that is exposed along the main branch of the fault. (, courtesy of Walter Feller

Activity along this fault zone uplifted the Hexie Mountains while dropping Pleasant Valley.

Fault zones create localized natural springs. Movement by faults result in impenetrable zones of shattered rock fragments that form an underground dam forcing ground water to rise. Fault-caused oases support the native palm tree, Washingtonia filifera (California fan palm), and supply food and water to a variety of wildlife. (

Marker 12: Extensive mining activity took place throughout the late 1800s and early 1900s. Gold, silver, copper, lead, and other metals of economic importance are believed to be deposited when intruding magma cools and crystallizes, and various gasses and liquid solutions rise from the magma. The Hexie Mountains are riddled with tunnels and shafts dug by miners in search of gold and other precious metals. Very few mines, however, were profitable.

Here are some pictures of abandoned mines in the Hexie Mountains. No, they are not visible from Geology Tour Road…

Silver Bell Mine, courtesy of

Marker 13 crosses a dry lake or playa, evidence of a wetter climate during which a periodic lake existed in Pleasant Valley.

Sediments carrying sand and clay from higher elevations are deposited in depths of several hundred feet. As these lake beds rise, the slopes from canyons lessen and the desert heat turns the sand and clay into silt and evaporating salt deposits that level off the bottom of the valley creating a dry lake, or playa. (

Marker 15: Pinyon Well Junction is an alluvial fan at the upper end of a bajada., courtesy of Walter Feller

The website explains alluvial fans and bajadas:

Joshua Tree National Park has many mountain ranges with flat-lying areas between them. Alluvial fans form at the base of a mountain as eroded sediment carried within a stream is dumped out when the slope becomes flat. When it hits the flat-lying area at the foot of the mountain, it slows. The faster a stream flows, the more rocks and sediments it can carry, so when the stream slows down, it drops the debris that it is carrying. This debris spreads out in a shape that often resembles a wedge, or a fan. Alluvial fans are often poorly sorted, meaning that they have rocks ranging from small sediments to larger rocks. When multiple alluvial fans meet and overlap, they are referred to as a bajada.

Joshua Tree National Park Geology

Further up this canyon is the site of Pinyon Well, a source of water for processing ore and watering cattle. (

Marker 16: Apparently this place affords a great view of many of the geological processes along this tour of the Queen and Pleasant Valleys. Below is a panoramic view captured by Walter Feller…, courtesy of Walter Feller

Geology studies the history of the constantly changing earth; the processes of mountain building, erosion, and rebuilding. Rock records of nearly 2 billion years present evidence of mountain building along the Blue Cut Fault, the igneous rock intrusions in Malapai Hill, and the monzogranite rock piles. Alluvial fans, bajadas, and dry lakes/playas are evidence of how erosion slowly destroys mountains. And even more amazingly, this tour shows the evidence of how indigenous peoples and their descendants survived in the desert utilizing the products of geology. (1975 copy of “Geology and Man: An 18-Mile Self-Guided Motor Nature Trail” by Elden K. Wanrow, Park Naturalist)

All I have to share are my pictures as we return from Squaw Tank and head back to Park Boulevard:

I highly recommend this geology tour, even if you have no idea what you are seeing. There are plenty of resources to help you decipher the views and learn so much about the dynamic history of this scenery. I hope I helped you understand and appreciate these ever changing processes as well.

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