No problem. Create your canyon before compute terrain and in the line of shaders after compute terrain add a mix of fake stone shaders as a child of your first surface layer. There are plenty rock, riverbed and canyon tgc's and tgd's to be found in the posts here. Blend this surface layer by height (lowest parts), or make a painted shader as a mask, or use a distribution shader to blend the stones to a certain height. Plenty of possibilities... also more sophisticated and complicated ones than just described.
