Richard F. explains...
A cave is just a hole in the rock. It's pretty hard for holes to form in most rocks, and holes often get filled up with dirt, rocks or minerals. One place where they don't is where water keeps on eroding the hole, which is partly full of water. This kind of cave is found at the edge of rivers and oceans.
Another kind of cave is found in a kind of rock called limestone.
In limestone, holes can form more easily than in most rocks, because limestone dissolves in water. It dissolves pretty slowly, but faster than most other rocks. So if water runs through cracks in limestone, it can dissolve away a channel and make a hole, a cave, or even an underground river.
I found a couple of books about caves in the Carlingwood public library. One is called "Going Underground", by Anabel Dean.% % filename IO.DOT % % page 1
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