Geologist Patricia Corcoran of the University of Western Ontario in London, Canada, and Charles Moore, captain of the oceanographic research vessel Alguita, stumbled upon the new rocks on a beach on the Big Island of Hawaii. These stones, which they’ve dubbed “plastiglomerates,” most likely formed from melting plastic in fires lit by humans who were camping or fishing, the team reports this month in GSA Today. Although anywhere there is a heat source, such as forest fires or lava flows, and “abundant plastic debris,” Corcoran says, “there is the potential for the formation of plastiglomerate.” When the plastic melts, it cements rock fragments, sand, and shell debris together, or the plastic can flow into larger rocks and fill in cracks and bubbles to form a kind of junkyard Frankenstein.

Corcoran says some of the plastic is still recognizable as toothbrushes, forks, ropes, and just “anything you can think of.” Once the plastic has fused to denser materials, like rock and coral, it sinks to the sea floor, and the chances it will become buried and preserved in the geologic record increase.

The discovery adds to the debate about whether humanity’s heavy hand in natural processes warrants the formal declaration of a new epoch of Earth history, the Anthropocene, says paleontologist Jan Zalasiewicz of the University of Leicester in the United Kingdom, who was not involved in the study. Plastics in general are so pervasive that they’ve been documented in a number of surprising places, including ingested in wildlife and on the sea floor. The mass of plastic produced since 1950 is close to 6 billion metric tons, enough to bundle the entire planet in plastic wrap. Combine plastic’s abundance with its persistence in the environment, and there’s a good chance it’ll get into the fossil record, Zalasiewicz says. “Plastics, including plastiglomerates, would be one of the key markers by which people could recognize the beginning of the Anthropocene.”

How long the plastic will endure remains a matter of debate, however. Jerolmack says he doubts the material will stick around in the fossil record. After all, plastic melts, and rocks often pass through hellish depths and temperatures through tectonic processes and burial. Geologist Philip Gibbard of the University of Cambridge in the United Kingdom says he imagines that plastics might “revert back to a source of oil from whence they came, given the right conditions of burial.” But Zalasiewicz and Corcoran say that isn’t true for all the plastic. Some of the material can be preserved as a thin carbon film, much like the way fossil leaves are preserved. Zalasiewicz says that in some rare cases, in that etch of carbon “you may well be left the shape for a flattened plastic bottle.”