Science, Tech, Math › Science Tombolo Gallery: Gorgeous Golden Sand Island Roads Share Flipboard Email Print Science Geology Landforms and Geologic Features Types Of Rocks Geologic Processes Plate Tectonics Chemistry Biology Physics Astronomy Weather & Climate By Andrew Alden Geology Expert B.A., Earth Sciences, University of New Hampshire Andrew Alden is a geologist based in Oakland, California. He works as a research guide for the U.S. Geological Survey. our editorial process Andrew Alden Updated February 23, 2019 01 of 09 Mudjin Harbor Tombolo, Middle Caicos Matt Anderson Photography / Getty Images A tombolo is a special kind of sandbar that forms in the shelter of an offshore rock, connecting it to the mainland. It is a depositional landfill, a term derived from the Italian language. There is something tantalizing about a tombolo. It is a road of golden sand leading to an island that is revealed only at low tide. In addition to a single tombolo, there are also double tombolos. A double tombolo can enclose a lagoon that then fills with sediment, as is the case off the coast of Italy. Mostly, tombolos come about by wave refraction and diffraction. Waves slow down due to the shallow water around the island when they come close. The wave pattern creates a convergence of a longshore drift on the island's opposite side. Essentially, the waves push sediment together from both sides; then when enough has built up, it will connect with an island. 02 of 09 Saguenay Fjord, Petit-Saguenay area, Québec, Canada Louis-Michel DESERT / Getty Images Tombolos are built as waves from two opposite directions. The water is what pushes the sand together. 03 of 09 Tombolo at Castle Tioram, Scotland Heartland-Arts / Getty Images Castle Tioram sits on a rock in Loch Moidart's south channel on the west coast of Scotland. 04 of 09 Tombolo at Goat Rock, California MARELBU [CC BY 3.0] This tombolo has been fortified to serve as a parking lot for Goat Rock State Park, at the mouth of the Russian River. 05 of 09 Tombolo at St. Michael's Mount, Cornwall, England TravelPics / Getty Images For centuries, this island that connected to the mainland by a tombolo was a holy site dedicated to Saint Michael. 06 of 09 Tombolo at Mont St. Michel, Normandy, France Maria Gorbatova / Getty Images Across the English Channel from St. Michael's Mount is the exactly analogous Mont St. Michel, sitting at the end of its own (now fortified) tombolo. 07 of 09 Oronsay Island in Loch Bracadale, as seen from Ullinish Point, Scotland Spike [CC BY-SA 4.0] Oronsay is a common place name in Scotland that means "ebb island," or tombolo. 08 of 09 Tombolo at Elafonissos, Greece fabdrone / Getty Images Cape Elena, in the foreground, is connected to the island of Elafonissos in the Peleponnese near Crete, by this lovely tombolo dividing Sarakiniko Bay and Fragos Bay. 09 of 09 Tombolo at St. Catherine's Island, Wales Aeronian at English Wikipedia [CC BY-SA 3.0] St. Catherine's Island is an island only at high tide. Castle Tenby sits on it just outside the harbor at Tenby, on the Bristol Channel. The nearby Dinosaur Park adds to the geologic attractions here.