sentences:  Flint stones are to be found round the shores of the Pool and as flint does not occur naturally in Shetland, it is assumed these are discarded ballast stones from the continental boats.At the end of the 16th century the Pool silted up and became much as it is today, with the only anchorage available towards the east end or in the Virkie Marina which was formed in the 1980s.Because of the large area exposed by the ebb tide, the Pool of Virkie has become an important site for bird watching in the South Mainland.