The San Gabriel Mountains, part 1
Once past Chilao, you're deep into the San Gabriel Mountains.
The San Gabriels are not as impressive as, say, the Sierras -- mostly less than 10,000 feet high. But they are the fastest growing and also the fastest eroding mountain range in the world. They grew a foot higher in the earthquake of 1994.
The granite that's exposed here is not rotten -- it was shattered by thousands of quakes over the millenia. The unstable matrix makes climbing in Angeles National Forest dangerous and often spills rocks and boulders onto Angeles Crest Highway.
Avalanches and mud and debris flows are common, as described in John McPhee's The Control of Nature. The Los Angeles basin was created by the rubble of the eroding San Gabriels, around 100,000 years ago. Rock slides sometimes close the road, especially after an earthquake.