What are those weird rocks I found on the beach?
Editor’s note: This story was first published in the August 2025 issue of the Ocracoke Observer.
Text and photos by Connie Leinbach
Ocracoke beach shelling is different from elsewhere along the East Coast.
When you walk along the beach you will see myriad bivalves and gastropods, sometimes in large clusters of whole shells and pieces called “shell hash.”
Among the shell detritus, you might see a hunk of coal (most likely from a sunken ship in the Graveyard of the Atlantic), or what looks like a river or landscaping stone, or what look like a hunk of petrified sand.
Someone once suggested that “petrified sand” was made from lightning striking the sand and melting it together.
Those items are called fulgurites, but the “petrified sand” specimens that can be found on the Ocracoke beach are not fulgurites.
They are locally called “marl” and are pieces of prehistory, according to coastal geologist Stanley Riggs, East Carolina Distinguished Research professor emeritus and author of “The Battle for the North Carolina Coast.”
Riggs visited Ocracoke with the Coastal Land Trust in the spring and graciously took time to answer questions about certain beach finds.
Some of them – the smooth ones — are “mud stones” and were created over thousands of years under pressure in the ocean.
“They probably go back 75,000 to 100,000 years,” he said while examining several specimens.
These “rocks” broke off from the “fine mud” off the edges of the capes.
If you look at the Carolina coast, you have several capes, he said, which differ from shorelines in the north.
“Every one of these capes has this kind of rock,” he said. “The reason these capes stick out in the ocean is that most of them have something underneath them and that something is this rock.”
These smoother specimens are probably calcareous mud.
In other specimens that look like coarse sand, he noted the presence of little holes made by clams boring into them and the imprints of sponges that lived on the top, which, over the many years, became bound together.
These were not in the mud flats.
“These are coquinas,” he said, which differs from coquina clams one sees drilling down into the surf, and which the dictionary says is “a soft, whitish limestone formed of broken shells and corals cemented together.”
If you went out into the surf beyond the breakers and drill down, you’d get multiple layers of sand and muddy sand all packed together for tens if not hundreds of thousands of years, he said.
Gretchen Miller, a senior professor of geology at Wake Tech Community College, Raleigh, also looked at the specimens while visiting Ocracoke in the spring.
Upon examining the coquinas, she noted that the ocean, which is mostly salt, contains myriad other minerals and chemicals.
“So, the critters take these chemicals out of the water to make their hard parts,” she said.
And when they die those hard parts fall to the ocean floor and become sediment, which then, with the help of minerals acting as glue, cement it together turning it into rock.
Riggs solved the mystery of a black, cylindrical piece this reporter found on the beach and which she thought might have been a fulgurite.
“This is a crab burrow that was cemented in the tide zone,” he said, noting that it got its black appearance by having been in a muddy, peat environment that was without oxygen, or an “inoxide” environment.
Black shells, many of which are oyster shells, are plentiful on Ocracoke’s beach, and Riggs said these are fossil shells.
“Those oyster shells will be anywhere from 200 to 1,000 years old,” he said. “They lived on the back side of the barrier, in the marsh. They’re intertidal. The island came over the top of them and washed them into the ocean.”
