Bill Burke, Cape Cod National Seashore historian, speaks on the disappearing shoreline. Ocean beaches from Eastham to Truro lose an average of 3 feet of shore each year. Nor'easters cause the greatest threat to erosion; for example, the breach at North Beach Chatham in 1987 and the break at Ballston Beach Truro in 191 temporarily causing north Truro and Provincetown to become an island.