What has cause this slate to fail?
I don’t see slate roofing in this area so my answer will be a guess. Looks like spalling caused by moisture infiltration and freeze thaw cycle.
We don’t see slate around here either but I wold guess rusting iron content, in the slate, expanding and staining downward.
Kent,
I see tree branches.
I’m willing to bet that there was something hanging over this roof that was rusting away.
Larry was close… it’s called a “mineral inclusion” and it’s a mineral impurity of some kind. Since this one is bleeding redish, iron is probably right.
Yep.