I have seen them at Altenbeken and it can REALLY fool you! It is literally a gray and white sheet of paper with a photo of very cragged rocks. Wrinkle it up (like into a ball or roll it loosely, then squeeze it around the middle or just the ends only, then unroll it and tack it into place !!!
You also could go find the desired rocks surface, take a picture and print it. It would really only work wit rock surfaces that a LOT of changes.
I have to build such a structure (even if rocks at the right would be one block, without the vertical slot) :
It is a version of Taft tunnel, Connecticut, mirrored to fit the shape of my layout, it's exactly what I was looking for. I am afraid that one sheet can't cover such a complex shape and that, if I make some junctures they will not fit, even if I hide them with "earth". So I wonder if it would not be better to carve the structure directly into the foam (as I did for the "wall" under the bridge), then harden the surface with my "miracle" paste, and paint the whole part (in fact I would assemble it to an "U" shape "outside" of the layout, spray it and only then put it at its location). What do You think ?