Holey Roller

A rolling rock gathers... dimples?

I can't figure out where all these dimpled rocks come from. This photo is from the Pacific coast on the shores of Washington (state). Look up Cape Alava, WA in Google Maps. These rocks are pounded and rolled by the tides of the Pacific. Cape Alava is exposed to storms throughout the year, so the rocks become round. But many of them have these large dimples. The dimples are golf ball-sized and a few inches deep. What causes them?????