La Dune is the name of the 12 kilometre-long tombolo

Tombolo

A tombolo, from the Italian tombolo, derived from the Latin tumulus, meaning 'mound,' and sometimes translated as ayre , is a deposition landform in which an island is attached to the mainland by a narrow piece of land such as a spit or bar. Once attached, the island is then known as a tied island...

 sandspit

Spit (landform)

A spit or sandspit is a deposition landform found off coasts. At one end, spits connect to land, and extend into the sea. A spit is a type of bar or beach that develops where a re-entrant occurs, such as at cove's headlands, by the process of longshore drift...

 which connects Miquelon Island

Miquelon Island

Miquelon Island is one of the three main islands of Saint Pierre and Miquelon. It is connected to the similarly-sized Langlade Island by a thin tombolo sandspit known as La Dune, which in turn forms a part of the boundary of the Grand Barachois.Miquelon and Langlade form the commune of...

 and Langlade Island

Langlade Island

Langlade Island, also referred to by foreigners as "Little Miquelon", is part of the commune of Miquelon-Langlade, in the French overseas collectivity of Saint-Pierre and Miquelon...

 in the French North American territory of Saint Pierre and Miquelon. A large natural lagoon, Grand Barachois

Grand Barachois

Grand Barachois is a large natural lagoon in Saint Pierre and Miquelon. It lies immediately south of Miquelon Island, and is formed largely by the 12 kilometre-long tombolo of La Dune....

 lies at the northern end of the sandspit, where it joins Miquelon. In the 18th century it was still possible to sail a boat between Miquelon and Langlade, but by the end of that century La Dune had closed in to form an isthmus

Isthmus

An isthmus is a narrow strip of land connecting two larger land areas usually with waterforms on either side.Canals are often built through isthmuses where they may be particularly advantageous to create a shortcut for marine transportation...

between the islands.