Weathered wooden beams possibly from a 19th-century shipwreck have emerged from the sand along Nantucket’s southern shore after a powerful nor’easter battered the island. The Egan Maritime Institute says an expert planned to survey the wreckage Monday and report the findings to Massachusetts underwater archaeologists, who will work to identify the vessel. The remains surfaced near an area where pieces believed to belong to the Warren Sawyer, a schooner wrecked in 1884 while carrying cotton and scrap iron from New Orleans to Boston, have previously emerged and been reburied by shifting sands.