I was nicely surprised as quickly can be done quite complex rock valley just in a few steps.
Fracture cube (if planner or voronoi depend on your needs). Join shards in one mesh.
Add remesh modifier for several reasons
- "uniform" grid for displace modifier
- a bit roughness on edges
- creates connections between shards
Add sub surf (simple) and displace modifier
Add bump texture for details
And thanks to the modifiers you can quite easy edit the base mesh.
Download .blend file