
Archeologists say the wall of limestone might have been left in place to keep the sea out of the quarry.
Karl-James Langford of Barry says the pottery remains show that the beach man-made walls might date back to 1,900 years ago, the state-funded BBC reported.
The quarry was used until the 19th century, but its origins were unknown.
"It's not in the records - it may have been completely ignored because it's too obvious," Langford said, adding that the quarry was the limestone source for the Roman fort whose ruins can be seen in the walls around Cardiff Castle.
"I've had this belief that there was a Roman quarry there all my life," he said.
According to Langford, a wall of unquarried high-quality lias limestone left on the beach was evidence of a quarry, similar to larger examples along the coast at Porthkerry, Rhoose and Aberthaw.