This is the Alpine Lakes, in the Cascades region of Washington State. I spent a week here with friends in 2007. There was a particular stretch of trails that we were on that keep coming to mind when I imagine this scene from the Hobbit.
I am certain that trolls have met here. North American trolls at that.
Tolkien's story ends with the Trolls being turned to stone. It made sense to use rocks for the main reference for the trolls.
"And there they stand to this day, for trolls as you probably know, must be underground before dawn, or they go back to the stuff of the mountains they are made of and never move again."
I had considered making the trolls more organic, and more inspired by moss covered rocks. Moss-covered rocks are wonderful. They conjure the feelings of places that are either remote, and undisturbed by man or secluded and well-tended by man. In the end, adding the moss to the trolls stony skin seemed too tranquil a covering for a set of characters who are about to cook our hero and a dozen dwarves.
At some point I think I may go back and do a piece of the 3 trolls as the giant stones, from when Frodo and Sam camp amongst them. Alan Lee has done what could be considered the definitive version of this scene, and it is marvelous. But I would still like to illustrate it, with these trolls, covered in moss and sticks and over-run by woodland creatures.