I like it a lot, but perhaps a low-opacity screen layer of actual limestone or a limestone texture would look good on the rocks. I can grab you a few photos when I go walk my dogge.

Unfortunately I have trouble reading the text at bottom left, it just doesn't contrast enough with the black on my monitor.

Finally, a good idea might be to think about adding a bit of a soil layer at the top. Sort of like in this image: http://igs.indiana.edu/images/bedroc...onFeatures.jpg

And I came a pretty good article on karst here.

I'll be keeping my eye on this!