It happened twice last week. Clients in two different groups asked how the holes were made in the rocks at Sunset Bay and Shore Acres.

Holes are created in our local shore-side rocks several different ways.  The size, shape, and, to some degree, the location of the holes are clues.

Occasionally spherical holes are found in rocks or on the cliff above the high tide line.  Those holes are likely left by concretions that were popped out of the base rock.  The formation of concretions begin in sediment: organic material, such as a bit of decaying meat, oozes out into the wet mud or sand that surrounds it.  Later, when the sediment becomes rock, that organic material cements the particles more firmly there than in the surrounding rock.

The result is a (usually) round or cylindrical form that's harder than the rock around it.  Some concretions have fossil shells or, rarely, bones hidden inside.  Most of the concretions found in the gravel and on the beach at Sunset Bay are golf-ball-sized and smaller; larger concretions—and some of the holes they leave behind—can be seen along some of the rock ridges off the cliff-side lookout at Shore Acres.

While concretions, necessary for concretion-removal holes, are made by dead animals or plants, most of the other holes in the rocks at sea's edge are made by live animals.

Purple sea urchins (Strongylocentrotus purpuratus) thrive at the lower levels of our rocky intertidal.  Although globular and armed with a mass of deep purple spines, sea urchins are relatives of seastars/starfish.  The urchins hold the rock with their tube feet and use their hard spines to grind into the stone, forming shallow, bowl-shaped pits.

The long, narrow holes are drilled by another intertidal animal: piddocks.  (Here, they're most likely created by the common piddock, Penitella penita.)  Bivalves, like more familiar clams, pear-shaped piddocks have heavy ribs on the broad bottom end of their two shells.  The animals apparently grip the rock with their foot and twist, grinding the ribbed shell against the stone.  The joined siphons ("neck") extend to the rock surface, drawing water with oxygen and food through one siphon and expelling water with waste through the other.

Like most intertidal inhabitants, piddocks start off life drifting as tiny, free-floating members of the plankton.  As they mature, the larvae undergo a change in the shape of their bodies and settle onto a suitable rock site.  The piddock grows as it drills, creating a long hole that characteristically becomes wider as it goes deeper in the rock.

Another rock-boring clam lives in some the Oregon coast habitats:  date mussels (Adula californiensis, sometimes called California pea-pod mussel).  Dark brown, small, slender, and rather delicate, date mussels dissolve limestone-rich rock with an acid to drill burrows.  Apparently, the dark film on the shell protects it from the acid.  Different from piddock burrows, date mussel burrows don't widen much as the hole deepens.

Urchin pits and piddock and date mussel holes are valuable habitat for other animals.  Such holes can furnish protection from heavy surf and large carnivores, as well as provide mini-tidepools and shade.

Holes in rock weaken it, making it more easily broken by crashing surf and logs.  The broken chunks of sandstone may wash onto the beach to be picked up by people intrigued by the borings.

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Visit our pages on related topics:  

tideflat denizens

tidepooling

beach in winter

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