I was finding that painting rock faces was really boring, repetitive and good results were difficult to achieve without reference. Naturally I made a brush
Includes the brushes, dynamics, and tool presets to get you set up immediately in Gimp 2.7
Also has the source .xcf file for modification and a small version suitable for Gimp 2.6.12 and before.
As with all of my brushes:
* Attribution not required except for published modifications
* You may use these for commercial purposes
You have to download the .zip, should be up the top right
Have a look at some of my others: [link]