Thousands of years ago there was an earthquake that spectacularly shook the San Gabriel Mountains, causing the southeast face of its highest peak Mt San Antonio to detach and jumble its way downslope in a brief but terrifying instance of time. This landslide traveled at great speed (100 mph perhaps) down San Antonio Canyon, splashed up the east wall of the canyon as it rounded a bend, and decreased momentum to deposit person- to house-sized boulders on a sizable flat (Manker Flat). In a minute or two 50 million cubic meters of rock had traveled up to 2.5 miles away from where they started. This landslide has had effects on the landslide that still persist today- Manker Flat forms a sizable area of flattish ground where nearly a hundred cabins have been built; erosion into the higher parts of the landslide deposit have driven downcutting into a steep bedrock gorge, San Antonio Falls. As part of my mapping efforts in the area I was very interested to check out this canyon and document the amount of incision that has occurred. That it was supposed to be one of the highest quality bang for buck canyons in the San Gabriels (5 nice rappels for about 2 hours round trip) that I had not been to before would be an added bonus.
We walked up the paved access road, marveling at the steep loose slopes and precarious positions of some of the cabins in the region. At the first big switchback with a viewpoint of the lowest falls we veered off the trail and up a gully, careful to avoid spiky yuccas and loose rocks. The route was not the most obvious and anastomosing social trails had clearly developed but we found our way to the top of the first rappel with minimal delay.
Being a warm day, neoprene booties, rain pants, and a rain shell sufficed to keep the cold splashy water from chilling us. We found good bolts at all the drops- where we had anchor choices we always chose the wetter rappeling option. The rock was surprisingly solid and the rappels were not awkward at all.
We made good time down the five rappels and couple short downclimbs. Being a weekday there were only two groups at the base of the final falls. The last rappel was probably the easiest to go smooth and fast and so left me wanting more. I noticed the hints of a social trail that looked to climb above this final falls and so could not resist taking one last lap for joy's sake before packing up and heading back to the car. The considerable incision to create this gorge certainly seemed to post-date the landslide. Once I get results of the age of the landslide back I will have a better handle on how rapidly a gorge like this can form. Thanks to Heather for field assistance and company.
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