 At 670 hectares ( 1 @,@ 700 acres ) in extent , Hirta is the largest island in the group and comprises more than 78 % of the land area of the archipelago . Next in size are Soay ( English : " sheep island " ) at 99 hectares ( 240 acres ) and Boreray ( ' the fortified isle ' ) , which measures 86 hectares ( 210 acres ) . Soay is 0 @.@ 5 kilometres ( 0 @.@ 31 mi ) north @-@ west of Hirta , Boreray 6 kilometres ( 4 mi ) to the northeast . Smaller islets and stacks in the group include Stac an Armin ( ' warrior 's stack ' ) , Stac Lee ( ' grey stack ' ) and Stac Levenish ( ' stream ' or ' torrent ' ) . The island of Dùn ( ' fort ' ) , which protects Village Bay from the prevailing southwesterly winds , was at one time joined to Hirta by a natural arch . MacLean ( 1972 ) suggests that the arch was broken when struck by a galleon fleeing the defeat of the Spanish Armada , but other sources , such as Mitchell ( 1992 ) and Fleming ( 2005 ) , provide the more credible ( if less romantic ) explanation that the arch was simply swept away by one of the many fierce storms that batter the islands every winter . 
