the north it is lined with homes. In the refuge it is paved for about half its distance and is a dusty dirt road for the remainder. Along it are numbered parking lots with boardwalks leading to the beach, overlooks and trails, and facilities for the maintenance of the refuge. Toward the south is the former site of Camp Sea Haven, a therapeutic camp for those stricken with polio. Visitors are restricted from the levelled site. At the southern end of the road, the tip of the island, is Sandy Point State Reservation, a state park. It is for day use only. At the northern end of the road, the northern tip of the island, is the Plum Island Lighthouse, the only lighthouse remaining on the island. It marks the narrow entrance to the mouth of the Merrimack River. The swift tidal currents through the outlet and through the channel between Ipswich Bay and Ipswich Harbor on the southern end make boating and swimming hazardous. Touring facilities Like most coastal communities, Plum Island has historically been a popular vacation destination. Several large hotels operated during the late-19th and early-20th centuries. Today, there are numerous lodging options for tourists, including bed and breakfasts, inns, and rental cottages. In addition, there is a population of year-round residents. From Memorial Day weekend through Labor Day weekend, the city of Newburyport offers a Friday-Sunday shuttle bus from Plum Island Point to downtown Newburyport and the Newburyport commuter rail station with a connection to Boston. The service is operated by the MVRTA, costs $1.25 for adults paying cash. Ecology Great Marsh Plum Island is a barrier beach sheltering the Plum Island River, Plum Island Sound, and the mouths of the Parker, Rowley, Eagle Hill and Ipswich rivers. The entire area between the islands and the mainland is grassland laced with tidal creeks. At high tide the grassland is entirely submerged, in some places by only a few inches of water. At all other times the extensive stretches of grass appear. The creeks either dry up completely or are small channels within mud flats where shellfish proliferate. The lower stretches of the rivers and Plum Island Sound are thus tidal estuaries receiving the fresh-water flows of the rivers. The grassland is historically termed "Great Marsh". However, the name also comprises the similar grasslands behind the barrier beaches at Castle Neck (Crane Beach) and Coffins Beach, which