The photo caption mentions shale rock. Shale is a sedimentary rock type. The rock on Chinidere Mountain, however, is volcanic, specifically "platy" andesite, similar to the basalt so ubiquitous in the Columbia River Gorge, but containing a higher percentage of silica than basalt, lending a lighter color to the rock, in a fresh hand specimen.
Thanks for correcting and clarifying, dlcole.
I've noticed outcrops of even more "platier" rocks at or near the summits of Larch, Thielsen, and others. Is it the silica content of the andesite that determines the degree of this "shale-like" form? (I've heard this called "shell rock" too - probably a better term than "shale like"?