It's not karst - it's beachrock

I have KAPped these rock formations along our coast for two-and-a-half years. They are fascinating and beautiful, especially from the air.

For two-and-a-half years I have (incorrectly) called these rock formations at the edge of the beach karst ('cause somebody told me that's what they are). They are not karst. Rather this stuff is called (appropriately enough) "beachrock." According to our friend Kathy Hall (who kindly and gently corrected my error), "This is beachrock, which consists of sand grains cemented together by calcium carbonate. It forms under the sand, but is exposed on shorelines where the sand has eroded (often due to sand mining)."

Beachrock forms quickly, usually in a few years, of "unconsolidated" sand in the "intertidal zone" (the area of beach exposed to air at low tide and covered with water at high tide). Different layers of beachrock represent different periods of cementation. (from Wikipedia)

Beachrock often "floats" on top of underlying sand, but is also sometimes covered by sand as well. How much beachrock is exposed at any given place at any given time depends on how the wind and waves move the overlying sand around.

Here at Shacks and Bajura, big waves will push sand up into the dune or suck sand off the beachrock, leaving more exposed. Other times smaller waves will deposit sand on the beachrock, covering it up.

This is really fascinating stuff! (Well, to me at least) Thank you, Kathy, for setting me straight. If you're interested you can find more information about Puerto Rico's coastline here and here. And there are more photos from this KAP set on Flickr.