Visually indistinguished from rocks piled up along the sandstone coast where they were found, identifiable as vintage surfboard foam only when touched. Left alone, they disintegrate into smaller and smaller rocks, just as sandstone becomes sand, eventually reduced to microscopic floating plastic bits resembling plankton, consumed and killing. Eroded by wave action, weathering, and UV exposure, they are a foreign substance indistinguishable from the environment – predatory rocks, a camouflage of irony.
Notes: these were collected over years by NPS volunteers and Rangers from a marine preserve adjacent to an area that's known to surfers as Tanks. The preserve includes the Cabrillo geological formation. Ordinarily, surfers would retrieve boards that break: lesser breaks in expensive boards can be repaired, and completely trashed boards are trophies, memory-markers of a particularly grueling surfing experience. But at Tanks, it's too dangerous to even paddle out from shore; surfers have to boat in to catch waves, and the shoreline is guarded by the Coast Guard, the Navy, and National Park Service law enforcement.