Cramond Island is a small tidal island in the Firth of Forth it is about a third of a mile long, and less than that wide. It is reached at low tide by a causeway which extends for just over ¾ of a mile into the river from the village of Cramond.
In World War 1 the northern end of the island was taken over by the War Department as part of the defences of the Firth of Forth. At the outbreak of World War II the entire island was requisitioned. Many gun emplacements were built and remain today.
The most interesting structures are not obvious, the large concrete teeth alongside the length of the causeway are also military in origin, intended to ensure that small surface craft such as E-Boats or U-Boats could not pass south of the island at high tide.
Many people get cut off every year due to not understanding the tides see link