Another late entry for me. I am going to attempt a side view of a cavern system.

Starting with a real picture of a hillside cliff from cgTextures
Cavern-Side1st.jpg

Then I darkened all but the top of the ridge.
Cavern-Side 2nd.jpg

Next I will use this cloud picture to create the outline of the cavern,
clouds-afternoon.jpg

So lets see how far I can get in the next week.