How do you sweep the back of a fireplace above the ledge?

I can't get a brush through to the fireplace from the top. There is some kind of ledge, and I can't get my arm up through the fireplace either. DO I need to clean that part? Where does all the creosote build up? At the top, it looks almost clean, less than 1/4" of loose dry build-up. But its hard to see down very far, and I can't see anything of the ledge.
Thanks again!
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