The Mystery of the Fossilized Oyster
October 23, 2016
On October 23rd I decided to take advantage of the extreme low tides, that the King Tides had also created, to dig some surf calms. The end of Plum Island had grown so far south that it had displaced the Parker River so the river’s currents were now cutting into the north end of Crane’s Beach.
Ever since last February these currents had kicked surf clams out of the sand stranding so many of the 5-inch long bivalves in windrows that the Ipswich shellfish warden declared that no permit would be needed to collect the clams before they died.
The week before, stormy weather had also licked thousands of immature surf clams out of the sand where they had sat in large piles, thrashing their feet in and out vainly trying to dig back into the sand before they being eaten by seagulls. They reminded me of inch-long coquina clams that you see in Florida.
So I expected to see large numbers of dead surf clams. But I didn’t expect to see hundreds of northern quahog shells interspersed with oyster shells. But they were not like modern oyster shells they were long and thin and had the patina of old age.
But oysters live in protected waters. How did they get on an ocean-facing beach? There were no beds of modern oysters near enough to account for such high concentrations, and the shells were too dispersed to be from middens. Besides the shells looked like they were thousands of years old not hundreds of years old, so what was going on here thousands of years ago?
We know that Plum Island formed about half a mile offshore and been pushed to its present location by the rising seas. But about 2,000 years ago the island ran into drumlins that had anchored it in place. Since then the beach has been remarkably stable, except for the waves that continue to erode the center of the island and cause its ends to grow.
That growth has gradually pushed the mouth of the Parker River south. But 2,000 years ago it was still about half a mile north of its present location as was a headland of sand and glacial that extended north from the Ipswich side of the river. The Crane estate sits on this drumlin today.
But 2,000 years ago the headland stretched about a quarter of a mile north along with spit of sand, which stretched south off the headland. Today we call that spit of sand Crane’s Beach. But Crane’s Beach was also about a quarter of a mile north of its present location and it probably protected a marsh and estuary with extensive oyster beds along its backside.
During the intervening years, Crane’s Beach had gradually rolled over these areas burying the ancient oyster beds and it is only today that the king tide and rising seas are unearthing their fossilized shells.
When I returned home I e-mailed Chris Hein whose graduate students from the Virginia Institute of Marine Science had been studying the south end of Plum Island for the past few years. He corroborated my theory and urged me to collect a few of the oyster shells so they could be carbon dated to determine their age.
That was fine with me. It would necessitate another expedition to Crane’s Beach. Not bad for a day’s work.