Second to the Joshua Tree itself, the fascinating sculptures and gravity-defying piles of rocks are one of most prominent features of the landscape in Joshua Tree National Park. The land looks as if a giant moved rocks into piles, leaving vast swaths of land boulder-free with towering stacks of stones scattered about, many sculpted into artistic forms.
To best learn about the geology of the park, one should take the 18 mile Geology Motor Tour. To complete the tour on the unpaved, soft sand road, one needs a four-wheel-drive high-clearance vehicle. While John loves the challenge of off-roading with the van, when we are at a campground for multiple nights we prefer not to drive the van in the middle of the stay. As we explained previously, packing everything up to drive is time-consuming, and it is wonderful when we can leave the van parked for the day. So we took our Bromptons to attempt the Geology Tour.
Sedans and two-wheel drive pickup trucks can make it to informational marker 9, 5.4 miles into the tour. The 6.1 mile one-way loop that follows is where the four-wheel-drive is necessary to complete the tour. On our Bromptons, which are somewhat better on soft ground than a road bike but not nearly as good as a mountain bike, we made it to marker 6, two or three miles in. The way back out, which was uphill, was substantially harder, but we successfully made it out and returned to our campsite at Jumbo Rocks campground.
Two rock types form a majority of the geology in the park – pinto gneiss and monzogranite. The gneiss is 1.6 billion years old, formed deep underground with high pressure and heat. A thick layer of gneiss formed the bedrock under this land 85 million years ago, when magma forced its way up from below into the gneiss, widening small cracks into huge cavities full of the magma in a process called “intrusion”. The magma cooled, crystallizing into granite. It crystallized slowly, which allows large crystals to form. These large crystals make for a course surface texture and lead to extra “grippyness”, which makes climbing these rocks extra fun and easy!
Over the intervening 84 million year, earthquakes cracked the granite, groundwater seeping down etched it, tectonic movements lifted it, and erosion removed the gneiss to expose it. The exposed granite is shaped by weather. A rectangular grid of “joints” formed on the rocks as small cracks. Over millions of years, water seeps into these joints and slowly widens them by dissolving some minerals, hence freeing insoluble crystals. Dissolution along these joints left behind large stacks of square boulders, which are further sculpted by weather. Cubes are turned into spheres as the surface is eroded, similar to sharp corners on an ice cube melting into rounded shapes. Depending on the level of erosion on each rock formation, you may see the rectangular grid, or the erosion may be so far advanced that the rounded boulders have collapsed into the eroded spaces and no clue is left of the process that formed these strange piles.
After the geology tour I cycled over to the White Tank campground to check out Arch Rock. While there, I also tried to find Grand Tank. There are several “tanks” in the park – natural rock water storage pools. I was disappointed that we did not make it to Squaw Tank on the Geology Tour, so I thought that this was a good consolation prize. The tank is one mile due east from Arch Rock, which seemed easy enough to find. Well, walking due east in a field of thirty-foot tall boulder piles is not exactly easy. I did find White Tank, the namesake of the campground. White Tank was dammed by cattle ranchers and is a bit south of where I was aiming. I had hoped to see the natural tank, but at this point the sun was low and I decided that off-trail hiking by myself so late in the day was maybe not such a good idea, and headed back.
I was glad I headed back – I had completely forgotten that there was an evening ranger program back at Jumbo Rocks. I returned just in time to eat a quick dinner, watch the sunset with John from atop Jumbo Rock’s namesake boulders and head over to the ranger program. A quick note on the campground: it is a large (124 site) first-come first-serve campground. A few sites are suitable for RVs or trailers, but it is really a tent campground with tent sites nestled in among the rocks. Despite the number and density of sites, this campground, like Indian Cove, feels more private due to these rocks. Also, the rocks are low and offer fun scrambling for all ages.
The evening program, given by Ranger Joshua, provided lots of interesting information about Big Horn sheep. For instance, the males sometimes ram each other with their horns so hard that their hoofs fly off. Ouch! Walking with no hoof must hurt a lot. These fights can last up to 24 hours. They do not shed their horns, and the horns have annual growth rings that help to age the males. It only helps up to about age 7; after this the horns get very narrow and the tips break off in fights. Females also have horns, but they are smaller and not used to fight. Male horns can weigh up to 30 pounds – imagine carrying 30 pounds around on your head! Females lead the pack and males roam around, joining for a little while before they are run off so another male can bring more genetic diversity the herd. Fragmentation of populations by roads endangers the species by preventing the spread of genetic diversity. The species is listed as endangered. Efforts to save it have allowed populations to rebound some, but as the desert becomes hotter and dryer survival is more difficult.
After spending a week at Joshua Tree National Park, it was time to move on. Yet another amazing national park added to our adventure list. The geology is fascinating and the Joshua trees are truly unique plants. Our timing was especially fortuitous; not just our luck at snagging the last campsite available on our first night there and again when we moved to Jumbo Rocks campground, but also at finding spring. The Joshua trees, yuccas and several cactuses were on full display sporting beautiful and fragrant flowers. We had a wonderful time in our last stop in the desert.
March 20-21, 2014