Hiking Lake Elsinore

Yesterday, as a means of combating the sloth and gluttony inherent in Thanksgiving weekend, some friends and I went on a nine mile hike in the Santa Ana Mountains, above Lake Elsinore. It was a trip intended just for walking and being outside, rather than a specifically geological venture, but I was still pretty psyched about getting to hike around in a fault zone with which I didn't have much up close experience. To my disappointment, the trail we selected was far enough up into the mountains that the fault trace itself wasn't actually visible, and all of the outcrops we encountered were between granite and granodiorite, with rounded weathering, and undeformed/unmetamorphosed. (I was not the only one slightly bummed - the only bugs we saw were termites, which was a disappointment to the entomologist on the hike, and the two plant biologists were not thrilled that most of the vegetation was invasive.) That said, it was still a wonderful hike. The weather was great, the scenery was gorgeous despite its lack of faults, and it just plain felt good to get out and walk without the purpose of going somewhere. I wish I could afford the time to go and do this more often in the middle of the quarter. Backlogged homework would kill me dead if I tried, though.

On the drive down from the trailhead, there were some fantastic views:
You can see three of southern California's most prominent peaks from here: from left to right, Mt. Baldy (highest in the San Gabriels), Mt. San Gorgonio (highest in the San Bernardinos), and Mt. San Jacinto (highest in the San Jacintos). You also get a great look at three major fault zones: the San Andreas runs at the foot of the San Bernardinos, from just to the right of Mt. Baldy off to the end of the page; the San Jacinto runs in front of its like-named mountain, branching off the San Andreas to the right of Mt. Baldy and continuing in the other direction toward the end of the image; the Elsinore is in the foreground, with one strand on either side of the lake. Lake Elsinore is, in fact, there to begin with because of this extensional stepover in the fault (and I miiight even look at the possible dynamics of that particular stepover in my thesis, maybe).

This panorama also illustrates a particularly heinous case of Houses On Faults. The houses directly on the banks of Lake Elsinore are smack dab on the fault trace, but the neighborhoods that sit between the base of the Santa Ana mountains and the lower string of hills directly opposite the lake are sandwiched in that extensional zone. That does not bode well for ground motion in and of itself, but it becomes an even sticker situation when you consider the soft lake sediment around there. I also wouldn't be surprised if the liquefaction hazard there is much higher than in pretty much the rest of the Inland Empire (though I'd have to look it up - I haven't done that yet). The houses we drove past also look pretty new, likely newer than the Alquist-Priolo act. I guess most of them fall within the fifty-feet-away clause, but really, what's fifty feet compared to the speed of P-waves? The Elsinore may not have one of the higher seismic hazards in California according to this spring's figures, and it may not be as well known of a fault, but it definitely does still move (last September's 4.7 was on the Elsinore), and I definitely wouldn't want to live on it.

I would, however, be more than glad to hike there again.

Box Springs Mountains

The Box Springs is not a large mountain range, but its location, not its size, makes it a fairly prominent feature in inland southern California. It juts up in the middle of the Inland Empire, separating Riverside from Moreno Valley from Loma Linda and Redlands, about halfway between the San Bernardino Mountains, separating suburbia from the desert, to the Santa Ana Mountains, beyond which lies the alternate dimension that is Orange County. It is not a particularly unusual range compositionally, consisting mostly of granite and granodiorite, with some diorite, that was uplifted by faulting. It is a good place to introduce people to igneous rocks in the field (all of the Geo1 classes here go into the Box Springs for one lab), and it has some nice examples of granitic pegmatites and of weathering by lots of wind and not much rain.

The Box Springs also happens to be the backdrop to our campus, marked with a huge concrete yellow C that's a common destination for hikers when the weather is temperate. Considering that the street on which I live goes right up to the base of the mountain, there's really no excuse for why I've only gone up a couple of times. A couple of my friends from the Javanese gamelan ensemble in which I play felt like going hiking today, so we headed up there - only my third time ever.

Today's hike pointed out what I personally think might be the most geologically interesting thing about the Box Springs - the view. Because the range is fairly central, on even a moderately smoggy day (but not a day on which the whole region is on fire), one can see many of the large scale features of the area. The San Jacinto Fault runs through the Box Springs, so looking down the length of the range (and following that same line to the northwest, past the last peak) shows the location of that fault. Looking north from the big concrete C, there's a fantastic view of Mt. Baldy, the highest point in the San Gabriel Mountains. At the base of those mountains, one can see a little bit of alluvial fan patterning before suburbia encroaches on the slope. Looking in this direction, the notch of the Cajon Pass, where the San Andreas Fault pulls the darker San Gabriels past the fainter San Bernardinos, is also clearly visible. From here, looking to the west shows the line of the Santa Ana mountains, with the Elsinore Fault tracing along their base. On the loop of the trail we took that went between two of the peaks, we got an excellent look at the San Bernardinos and their San Andreas straight base from the Cajon Pass to where one of the Box Springs peaks blocked the view. There were some other trails heading down around that peak that we didn't take, but I'd like to go down that way some time, since I can at least hope that the whole straight section until the fault hits the San Gorgonio knot could be seen from up there.

(I'd post photos, except none of them came out terribly well due to distance and encroaching smog. Bah!)

I tried to refrain from going on and on about how awesome I found it to be able to see all these major features in panorama from the middle, since I figured not all ethnomusicologists or composers would find this as exciting as I do. But as soon as one of my friends' attention was pulled in by the shininess of k-spar in a pegmatite, and as soon as she asked me what it was, all bets were off, and I kept pointing things out. The comment that, "I had no idea this stuff could be so interesting," was all the reason to keep talking.