"Things You Wouldn't Know If We Didn't Blog Intermittently."
I have heard two versions of how these holes are created. The first is that a clam or other bivalve rotates and drills the hole for protection. The other is that when a depression or crack is created in the stone, the waves might wash one or more pebbles into it (visible in the "mouth" of the picture). As more waves come in, these pebbles are violently rotated about the hole, enlarging it bit by bit.The second version is my favorite.
i have heard the second. eventually, those holes can get huge, like boulder size huge.I-)