We used a few hours coming back from a weekend escape to stop by for a quick look at Blackhawk Landslide, one of the more famous examples of a rock avalanche deposit in the world. Somehow in all my time in southern California I had not got around to this out-of-the-way corner of the desert so finally visiting felt like a pilgrimage of sorts. I was hoping to use the opportunity to take a few drone photos and to compare some of the rock textures in the distal part of the landslide deposit to those in the source area.
The landslide broke free of the bowl-shaped region on the north side of Blackhawk Mountain at about 6600' of elevation, traveled down the varied terrain in its path at the northern rangefront of the San Bernardino Mountains, and then continued several miles further across the more gradual grade of the alluvial fan. The Blackhawk Landslide was inhibited from spreading further east to to the presence of an older, very similar landslide deposit impounding it. It flowed over outcrops of metamorphic rocks exposed in its path, despite being composed of dry rock. After what must have been only a few exciting minutes about 20,000 years ago, the landslide came to rest far into the desert. It's wall-like toe made it a distance of 6 miles away from the headscarp, dropping to a high desert elevation of 3100'. Most landslides are physically limited to traveling laterally about as far as they fall vertically (in this case that would be only 3500 feet), but rock avalanches have special, still largely unknown physics that allows them to have extremely long runouts. Ripple-like flow textures are still preserved on the distal surface of the 6 mi2 landslide deposit. It is really a spectacular and fascinating deposit. While these rock avalanche events are exceptionally rare globally, deposits indicate that similar events happened multiple times in the past from the same massif of marble.
The toe of the landslide is right off the highway and I have to say the landslide is underwhelming at ground level here, just a low 100 ft rounded ridge arcing away from the highway. At 400 ft height above ground my drone was a little better able to capture the scale, but still a higher view and lower light would have better illustrated the surface features.
Next we drove onto the deposit itself and checked out the outcrops in the walls of a wash partially excavating the deposit. The rock textures were more uniform than I was expecting, but I could still pick up the tell-tale textures of jigsaw breccia, monolithic flow bands, and occasional megaclasts. Interestingly the top few feet of the deposit were cemented like concrete with carbonate precipitate.
Next we drove around to the Mitsubishi Cement Corporation and headed up the rough 4WD road towards the landslide source. The road soon turned steep, narrow, and outward sloping, a little more hair-raising than I hoped. We made it to the far side of Monarch Flat (undoubtedly a feature of yet another landslide deposit) and parked after the next steep section looked even worse. I managed to convince Heather to go on foot and so we strolled up the road about 1.5 miles. The day was hot and sweaty but the occasional cloud cover and mountain-sourced air helped a lot, particularly as we gained elevation. At last we made it to the sloping clearing on the west flank of the Blackhawk Mountain scarp that was the glider launch. It was neat to finally be here and hard not to fantasize about swooping around the headscarp bowl and gliding over the landslide far out into the desert (interestingly my wing's glide ratio is very comparable to the fall-to-run ratio of the Blackhawk Landslide). Here too the scale and vastness of the landslide made it hard to appreciate, especially as the dappled shadows from scattered clouds obscured the furthest reaches of the deposit. I flew the drone again over some of the outcrops comprising the headwall area, which were more varied in texture than I was expecting and flew it up high for a photosphere (below). The drone misbehaved and completely lost connection, something that has never happened to me before, but did manage to return to home where I could fish it out of the sky.
We returned back down the ball-bearing covered road back to the car, out without incident, and then drove the back way through Big Bear. Thanks for Heather being willing to satisfy my curiosity for a couple hours. I hope to return someday with my wing!
