Fidra, archaically Fetheray, is north-west of North Berwick. Like the nearby islands it is the result of volcanic activity and has a substantial seabird population. It can be accessed via a primitive jetty on the eastern shore. It is said that R. L. Stevenson based his map of "Treasure Island" on the shape of Fidra. The island is also attended by companion islets called North Dog and South Dog and Castle Tarbet is another islet to the south separated from the main bulk at high tides.