Hi,
Here is the way I would do it:
First you would want a surface to start on, rather than just having a big lump of putty. This could be I piece of cork, or anything hard to \"bulk out\" the area. Make it into a rough rocky shape.
Put a layer of putty over this, keeping the top relatively flat. Using a sculpting tool (The GW one will work fine), make long indentations along the side, similar to the shaggoth image.
Now the rock should look close to what you want, but it still will be pretty smooth.
You could use a needle and make some very delicate cracks and small holes/pits, and you could take a real rock and lightly press it into the putty in some spots to get a realistic texture.
Hope this helps!
-Matt