I am sorry for this delayed reply to your gardening question. The spring rush has brought a deluge of questions and we are working hard to catch up!
Based on that description alone I cannot determine the cause of the problem. Squirrels will often chew the bark on trees. There is a disease of oaks that causes bark to fall off leaving patches that can be olive or gray or black beneath. I suggest you take some photos to your County Extension Office along with some samples if possible to see if they can identify it and provide a recommended course of action.
Thanks for the question. Best wishes for a wonderful gardening season. Please stop in again soon!
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