A tombolo is a bar of sand or other sediment that connects the mainland to an island—indeed, the island often plays a part in its creation as incoming waves refracted around the island deposit sand behind it. As a result, tombolos are especially common on relatively youthful coastlines where the offshore islands have not yet eroded away. Perhaps the longest tombolo on Earth was the so-called Rama’s Bridge, which once connected Sri Lanka across eighteen miles of the Palk Strait to the coast of India. Built, according to Hindu legend, so that Rama could rescue his wife from the demon king Ravana, it was breached by a violent storm in the fifteenth century and now survives as a chain of shoals that menace mariners. Closer to home, a tombolo connects the tied island at Marblehead to the Massachusetts mainland. Another noted tombolo, perhaps the longest in the Great Lakes, connects the major part of Grand Island on Michigan’s Lake Superior coast to the portion of itself now known as The Thumb. On the West Coast, what rises up from a flat beach today as Point Sur was formerly a detached coastal island.

Bill McKibben