It reminds me of a cave formation known as "cave pearls." They form when pebbles are lodged in a depression formed by millions of years of dripping water. Those pebbles in the depression get rolled around and agitated with every drop, which eventually forms smooth round stones, much like a rock tumbler.
A friend of mine is a world-famous professional cave photographer and he maintains this web site:
http://www.goodearthgraphics.com/under_earth/

Photo credit: Dave Bunnell and The Virtual Cave