Below is a photo of a leaf. I don't know what kind of oak it is, but it is
an oak.
http://www.frontiernet.net/~toller/Leaf.jpg

On the leaf is a pile of debris that was on the ground below the tree. It
usually dries up in the summer, but this year is rainy and cold, so it must
think it is still Spring. Anyhow, it is like extremely coarse sawdust.
Could a fungus do that?

Because it is growing out of a shale cliff, fertilizing or water are both
problematic. It must however be a very stressful place to be growing!