re: Caves?
Hi, Nick!
"Don't expect miracles from these hills, BTW. They are simply more of the same Gusev fill that has been uplifted a few hundred metres by ridge-forming processes (global compression). They are still over 1.5 billion years old and only marginally eroded, so we may still not be seeing below the layer of basalt."
Ahh, well here I thought that the arc look of The Columbia Hills are remnants of a very old crater 
"You never know, though, with Mars... "
I hope we'll know soon. :}