most difficult and dangerous in the country, especially in the summer when temperatures can soar to 115 degrees. There's no water or shade for most of the climb.
Well, I had my hat. I had half a bottle of water. It was 4 PM and only 103 degrees and no one knew where I was. So off I went up the trail. At this point you may be thinking that I am about to describe a real disaster of bad judgment and pointless rescue, but no, I'm not quite that stupid (Mrs. Geotripper may have an alternate opinion on this). I went up maybe 200 yards, high enough to get some pictures of the city over the tops of the palm trees on the adjacent golf course, and headed back down to the museum to cool off.
But just the thought of the trail had reawakened a great many memories of my days as a teen in Southern California. I was practically indestructible in those days and was recognized as the fastest long-distance hiker in my high school. I climbed Mt. Baldy via the "old" trail that climbed 6,000 feet to the top, and summited at least fifty other peaks. But I never walked to the top of San Jacinto starting from the desert floor. At the time, I hadn't heard of the Skyline Trail, but had learned of a climber's route up Snow Canyon, but never found the time. All I knew was that San Jacinto Peak was a very high mountain that rose from desert plains only a short distance above sea level. The escarpment of San Jacinto is one of the highest and steepest in North America.
There is a mountain ridge across from San Jacinto that is even higher. San Gorgonio Peak is the highest mountain in Southern California at 11,499 feet. The pass between the two mountain ranges tops out at just 2,600 feet, meaning that San Gorgonio Pass is probably the deepest pass in North America, at nearly 9,000 feet. It's nearly twice as deep as the Grand Canyon, and deeper than the deepest canyons on the continent, Hells Canyon of the Snake River, and Kings Canyon in the Sierra Nevada.
For those of you who are not familiar with the tramway, it is quite an engineering feat. The tram begins at 2,643 feet in the Lower Sonoran life zone and climbs to 8,516 feet in the Canadian or Hudsonian zone. It is two and a half miles long and is supported by five towers, the tallest of which is more than 200 feet. It was constructed in 1963 and with no roads or trails in the rugged terrain, much of the construction was accomplished with helicopters.
We were lucky last weekend, because the atmosphere was reasonably clear and the view was spectacular. Joshua Tree National Park stretched across the valley beyond Palm Springs. We could see the deep canyon of the Whitewater River, the main canyon draining the east side of San Gorgonio, now part of Sand to Snow National Monument. At our feet we could see some of the 4,000 turbines of the San Gorgonio Wind Farm in the pass (below). The pass is one of the windiest spots in all of California, owing to the height of the surrounding mountains and relative narrowness of the pass.
There was also a panorama above, as we were standing along the edge of a high-altitude plateau of the San Jacinto massif. The relatively gentle slopes and valleys of the highland are a relict surface of the landscape that existed before the mountains rose thousands of feet along the bounding fault lines.
First, some regional context. The San Jacinto Mountains are part of the Sierra Nevada, sort of. They are officially described as the northern end of the Peninsular Ranges, which extend all the way down the Baja Peninsula for another 700 miles south of the border. But the rocks themselves originated along the huge Mesozoic subduction zone that extended from Mexico to Canada. There was a continuous mass of granite that included the Sierra Nevada, the Salinian Block, and the Peninsular Ranges batholith. Later on the Salinian Block was carried northwest along the San Andreas fault, and the Peninsular Ranges more or less ended up in the location from where the Salinian Block had been removed.
The rocks themselves are what most normal people would call "granite", light-colored (feldspar and quartz-bearing) plutonic rocks overall with black specks of biotite mica or hornblende. Geologists will concede to calling the rocks "granitic", but look at the proportions of feldspar varieties and quartz to come up with some more imposing names like quartz diorite, quartz monzonite, and hornblende diorite. These rocks formed in the subduction zone where oceanic crust and continental sediments were pushed so deep into the mantle that some of the material melted and rose through the crust as magma plutons. These masses rose until they reached some form of buoyancy and cooled slowly over tens of thousands or hundreds of thousands of years, forming the coarser-grained granitic rock. Later on erosion removed the miles of overlying crust to expose the sparkling plutonic rocks.
During the tram ride up the mountain, the rider is treated with some awesome views of the granitic rocks. In many places the granite has been pierced by light-colored dikes. These were silica-rich masses of the last bit of molten material that filled cracks in the final stages of cooling. Some of the dikes are pegmatites, made up of unusually large (inches across, sometimes a foot or more) crystals of quartz, feldspar, and sometimes gemstones like tourmaline, garnet, topaz, or beryl. The gemstones are still mined in other parts of the Peninsular Ranges. I really wanted to call for the tram to stop and let me down a rope to check out the exposures!
The next picture below shows an especially prominent dike that has resisted erosion and now stands out in relief. It actually looks like some of the "cartoons" I use in lab to teach sequence of geologic events. Which came last, the grey rock, the horizontal white dikes, or the vertical dike that stands out in relief?
The final part of the story is in some ways the most difficult. How did the Peninsular Ranges become mountains, and why is there such a deep pass between them? I don't know that I have a great answer to that question, but one can assume that fault lies with the faults. Several major faults pass through the pass, including the most familiar of all, the San Andreas. But the San Andreas is a strike-slip, or lateral moving fault, not one that leads to vertical uplift. There are complications in the path of the fault though that can explain why there are mountains here.
If the trace of a strike-slip fault is absolutely straight, over time the fault won't cause much in the way of uplift as it moves. The trace of the San Andreas through the Carrizo Plains farther to the northwest demonstrates this (the fault has only produced low hills there). But if there are bends or turns in the path of the fault, local areas of extension or compression can develop that will cause vertical uplift (or basin subsidence). The Peninsular Ranges form the western margin of the Coachella Valley, which is the northern extension of the fault basin (and divergent boundary) of the Gulf of California. The continent here is ripping apart, and the valley has sunk thousands of feet. In at least one way, the San Jacinto massif can be thought of as forming from the sinking of the adjacent valley rather than being "pushed" upwards, although there may be some element of that going on as well (thrust faults are present on the north side of San Gorgonio Pass).
The San Andreas fault was visible from the tramway. The zoomed image above shows Highway 62 where it passes through the Little San Bernardino Mountains on the way to Morongo Valley. The fault passes along the base of the mountain range.
A small postscript: "That's AMAZING!" I miss Huell Howser and his unique explorations of California. Here is his 1997 episode that included the Palm Springs Aerial Tramway as well as the 13 mile long tunnel underneath the San Jacinto Mountains that was constructed to deliver Colorado River water to Southern California:
Geotripper in my (it was yours before mine) backyard. Thanks for the exciting post. Exciting because I've done the tram ride once in winter which went from comfortable desert weather to a snowstorm at the top. And the canyon views were "Amazing" with a dusting of fresh snow. And I have a fear of heights. Now you have to explain the weathering of the dikes. It was your tease...
Geotripper, Hi! This is the first time I've seen your site, but you can believe it will not be the last. I'm overwhelmed at all it's content, and extremely glad I found it. Like you, I am a teacher as well, teaching middle school Science here at the American School of Bangkok. I'm currently taking my 9th grade Earth Science class through a section (just finished) on tectonic plates, and we're now moving on to the changing surface of the Earth. I'm sitting at home this evening collecting pics of folded mountains (and some incredibly colored ones in China) plus some other odd landforms.
I don't have time tonight but this weekend I will get back to your site and check out some of the (what I'm sure will be amazing) archives. I think I will limit myself to the article on "Where the Sierras rise from the sea". (Great title, BTW.)
I will, if I may, take you up on your offer to answer questions on matters geologic. When folds form in mountains, why does the rock not crumble into minute particles over the (LONG) time the pressure is being applied? When I see the exposed folded areas of some mountains and hills, I wonder how the material within the folds remained so solidly joined together.
Thanks for creating and sharing such a great site.
American School of Bangkok
I'm sorry I missed this comment at the time. The virus panic was just beginning and I was overwhelmed with the changes at the college. If the rocks were folded near the surface under lower pressure, they probably would have crumbled (been "brecciated"). But they were instead buried under miles of overlying rock and at those depths and temperatures the rock behaves plastically, folding without breaking.
I'm going to guess as a non geologist, but a geographer instead, that the rules of what I believe is called superimposition are in play. Basically if something crosses a rock bed but nothing crosses ut in turn it is the last thing formed. So that verticle dike came after the base rock and after the white dike/sills since ut kies over the those two. And the white sills/dikes came before the veticle one and after the base rock.
This is my guess.
My friends were just up the mountain and stuck due to mechanical failure of the tram. They sent pictures and I recognized one of the outcrops from your post. Came back to reread.
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