The Harbor Hill Moraine, in the geography of Long Island, forms the northern of two ridges along the "backbone" of Long Island. The Harbor Hill Moraine, skirting the North Shore, represents the terminal moraine of the most recent advance of the Wisconsinian glaciation, which reached its most southward advance about 18,000 years ago; the earlier Ronkonkoma Moraine, much cut through by outwash streams from the Harbor Hill Moraine, lies to the southeast. The Harbor Hill moraine is represented by the North Fork of eastern Long Island and in three disjunct sections farther east, Plum Island, Great Gull Island, and Fisher's Island.