This is a paid asset, but now you can download Cliffs & Stones Free.
Detail this asset from Unity Store: Original Link
Cliffs & Stones v2.0 (Latest version)
Cliffs & Stones is a package you can use for any environment and will allow you to create high quality rocking levels!
These assets include optimized geometry for optimum performance for any playform and high quality PBR textures that work for any realistic game or experience.
The contents of this package were built to be as modular as possible which include the following.
6 Cliff variations
3 Large Stones
4 Medium Stones
5 Small Stones