I have several oak trees in my forested back yard that are in various stages of loosing a thick outer layer of bark, revealing thinner whitish bark underneath. Once this bark is lost, it doesn't appear to grow back. The the tree in the photo below is in the middle stages of this process: most of the other oak trees I have have nearly totally lost the thick bark (except for patches here or there), and a one or two haven't lost any.
I had the city arborist over a few years ago for a different reason, and he mentioned offhand that this didn't look normal. I believe he said my trees were Bur Oaks.
Is this condition normal, or is it the result of a disease or pest?