"Nature Guide Journal"
28 September 2002
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
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,
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
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.
Visit our pages on related topics:
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