What's a cheap/good rock or material to make caves with?

go to a place like loes or the home depo, and buy a bag of rocks, they ussually call them river rocks. you can ussually use these to make caves. they probably have realistic looking stepping stones, these will work well for caves as well. what you do is make 2 piles with some river stones, with a gap in between, then put the stepping stone on top and you now have a cave. that is what my convict tank is like.