Fort Island (St Mary's)

Small tidal island in Derbyhaven bay with a medieval fort and earlier remains. The fort was built to defend Derbyhaven harbour. Only accessible at low tide via a causeway. WHY: Tidal island sites are rare and fascinating. The causeway area (covered at high tide) accumulates items washed from both the island and the mainland shore. Low-tide detecting on the causeway and the island foreshore can be very productive. The fort's military history adds potential for military items.