I don't think that is deer. Deer seem to specialize in eating the tips and new growth. There is a critter in Oregon called a mountain beaver (though it is not a beaver at all) that I have seen do damage like that. They live in holes in the ground and usually you can find the branches sticking out of their holes.Deer topped vegetation growing in my beds.
I saw the deer munching on it early this morning. I thought it was munching on my plants which they've never done and I've always been thankful...Its actually a tomato plant. The compost I have has all kinds of vegetable and fruit seeds and they sprout right alongside my plants quite often.
The boards are 2" x 12" redwood and just dark in color. They've gotten darker as they've been outside over the years. I haven't ever treated them or put anything on the outside.
Edit: Looked closer at what you were talking about. It does look much darker. Maybe it has to do with lots of moisture being in that area.