The main anorthosite exposure is located near the Monte Cristo campground off Angeles Forest Highway. It's somewhere between 1.2 and 1.5 billion years old, among the oldest rock in California, and intruded into the Mt. Josephine granodiorite (the dark stripes you see in some of the photos; the anorthosite itself is white, sometimes with a subtle violet cast).
I expected the anorthosite to be hard, like quartz, and like the samples of plagioclase I'd handled in geology lab. But plagioclase weathers to clay minerals, and this very old plagioclase has become crumbly and soft. You can break it apart easily with your fingers. The granodiorite below it is equally crumbly and soft, because the plagioclase crystals in it are also weathering to clay. Higher up in the San Gabriels are other (younger?) exposures of granodiorite which are much harder and more cohesive.
Other exposures of the San Gabriel anorthosite are west of I-5 near Castaic and east of highway 14 near Soledad Canyon.