This fire trail in the hills is beginning to disappear downslope. The biggest mover of sediment is not erosion, it’s landslides—or mass wasting, to use the geologist’s more general term. The land is like a building in that respect: neither of them wear out, the way an ice cube melts; instead they get more and more rickety, then collapse.
This headscarp will concentrate the infiltration of rainwater at the same time it admits air underground. The comfortable stasis of the underground is now broken. The alternating wetting and drying and the rapid loading and unloading give gravity more advantages until one day it will pull the road down. Then the city will help mass wasting along by cutting a fresh track into the hillside. This will never end until the hill or humanity is gone. For us it’s just part of the cost of living.
Once you know these crescent scarps, you’ll see them everywhere in the hills.