A nice article about the meghalaya expedition has come from the BBC. It's doesn't look too badly sensationalised having never been their.

http://www.bbc.co.uk/news/world-asia-india-43708758

Accurate piece of journalese although I do feel its a cliche referring to a cave as sinister. Some are quite friendly. Good to see Brian, Thomas, Simon and co getting some credit. I can see a documentary film crew heading out there soon. Previously Meghalaya has been more known for the Khasi matriarchal society, Cherrapunji's monsoon rains and it's root bridges.

So what's a the mode of action for formation in sandstone? The article says it's just the vast amount of water acting as it would in limestone, but while limestone is sparingly soluable, I think sandstone is completely insoluble so no matter how much water you add you won't get dissolution. Although maybe there's enough friction power to degrade it? Or some of Hazel Barton's funky microbes? Or something else?

caving_fox said:

So what's a the mode of action for formation in sandstone? The article says it's just the vast amount of water acting as it would in limestone, but while limestone is sparingly soluable, I think sandstone is completely insoluble so no matter how much water you add you won't get dissolution. Although maybe there's enough friction power to degrade it? Or some of Hazel Barton's funky microbes? Or something else?

Click to expand...

I read (a fair few years ago so the memory may be faulty) that sandstone caves are formed by the mechanical action of water washing out sand grains. An initial crack would have to appear through some other mechanism but if there is a path through the rock for the water to flow, then this is enough to cause the widening of the path and creation of larger conduits and I assume, given enough time, some pretty big ones would be possible.

The problem with sandstone caves is that although they form rather easily, as it is easily eroded by the water. The reason we don't normally see large sand stone caves is how fragile/unstable they often are when compared with limestone, being that sandstone is a much softer rock. We found and explored a few sand stone caves near the North Pennines a few years back and let me tell you those places were definitely not stable. Though one did get over 40m in length and even had quite a large chamber.

So it is quite amazing these systems have formed in such a size and from the photos and the fact you can explore them, appear to be generally of good quality rock. I would therefor guess that these caves in Megalaya are not pure sandstone, and are probably a sandstone/limestone composite, this seems likely due to the proximity of large limestone systems.

I thought a lot of the Pennine sandstone caves were basically collapse features.

That would explain the instability.