The longest-known pectinid-bearing deposit in the Antarctic, the "Pecten-conglomerate" of Cockburn Island in the James Ross Island group, northern Antarctic Peninsula, is herein formally named the Cockburn Island Formation. A detailed account of its lithology, palaeontology, age and depositional environment is given. Deposition is thought to have taken place during a late Pliocene interglacial episode. The Cockburn Island Formation is younger than 2.8 Ma and is a possible correlative of the Scallop Hill Formation in the McMurdo Sound region, East Antarctica.