I've done a lot of spelunking, so everything I say is from personal experience.
Looks like limestone-
If there are large depressions/sinkholes(Karst features) in the top of the ground above and around this area
then it could be a space formed after a collapse(tumble down cave).
Some of them can have formations and some have been used by people and/or animals in the past.
Be very careful when digging or moving stones, make sure they are not structural to the integrity of the cave.
If you're coming in from the top you've already compromised that part of the ceiling.
Tumble downs with sinkholes are tricky. You don't know what has fallen away underneath and you don't know
when or if the ceiling will fall more.
From your other pics this looks like an old tumble down that got sealed off.
Some of the rocks appear to have formations on them.
Stalactites, stalagmites, bacon, sunny side eggs, soda straws, etc. are very cool.
Try not to break any.
Recon the whole area and see where or if any holes are blowing or sucking air.
Look windward and leeward.
If they are, it's a good indication that this part of the hill has a lot of cave.
It also indicates that the caves are breathing and there is probably good air inside.
Probably, not definitely.
If you don't have an O2 meter be very careful. Lots of caves have low O2, methane, CO2 etc.
We opened one that was in the top of a ridge between hills. The whole ridge was a sinkhole.
Found some interesting animal skeletons inside. The entrance was the size of two fists.
NEVER go alone!
One in with a rope attached, one out with a phone/radio at the ready.
Have you done much spelunking?
? Anyone any ideas ? I see old metal wood stone 
