This is a paid asset, but now you can download Rugged Rocks – StampIT! for FREE, but please remember this package is provided only for learning purposes or to be able to test before buying the product, NOT FOR COMMERCIAL PURPOSES.
Detail this asset from Unity Store: Original Link
Rugged Rocks – StampIT v1.0.0
Rugged Rocks as part of the StampIT! Collection consists of 20 high quality textures in stunning 4K resolution. These heightmap textures can be used as brushes with the Unity terrain tools in order to paint features on your Unity terrain. Or you could use these heightmap textures in other assets like Atlas, Map Magic 2, Gaia, etc.
The asset consists of the heightmaps and the brush presets, the terrain textures are not included.
You can see a preview of the Stamps in the Package Content tab.
Stamping
Stamping features on the terrain is a process which is supported by Unity out of the box. Simply put stamping is blending of various textures with each other. This process has been existing for a very long time and has been used in various applications, most notably image processing applications like Photoshop, Krita, Gimp, you name it.
Unity provides a very convenient means to blend those images together with their built-in terrain tools. The Unity terrain tools provide an editor with a brush preview which allows you to adjust a stamp in detail before you apply it.
For an even better experience the free Unity Terrain Tools package which can be installed in addition to the built-in tools via Package Manager is an excellent choice. It extends the terrain tools palette with even more tools which allow you to fine tune your stamped terrain to achieve the look of your personal artistic preference.
World Creator
In case you need complete terrains imported into Unity including textures, then the awesome World Creator is a preferred application. Highly recommended!