It's a mask, but I think it should be possible with some blue functions to get some sort of stairs, or a series of simple shapes added up or something. Then you indeed raise it with a displacement shader. But the trick is to get the computed terrain to think it's a flat area, hence you set compute terrain to smooth terrain. You can mask this area out if you need unsmoothed terrain afterwards, for instance for displacement intersection+smoothing. You may also have to set the patch size to something very small, like 0.1-1. Then add the fake stones in a surface layer, masked out by a slightly larger mask, or by height.
It's very tricky, and I'm not gonna give away more than this
You can figure it out, no doubt.