Shelfstone

Shelfstone is a ledge reaching from the edge of a cave pool. They are almost always formed from
calcite, when materials precipitated on top of a cave pool attaches to the side. The deposition continues to grow laterally, underneath. The shelfstone above the water level is an indicator of past levels of the pool. Shelfstone can be very thick when the water level has stayed constant over a long period of time. If the water level is constantly changing, the shelfstone will be thin and delicate.

Sources

*The Virtual Cave [http://www.goodearthgraphics.com/virtcave/shelfst/shelf.html]