For 10 points, name this henge that shares its name with an island group, the capital of which is Kirkwall on Pomona Island, and which is separated by Pentland Firth from Caithness County, Scotland.
Like Stonehenge, it is an ancient ruin of a Neolithic culture.  The Ring of Brodgar is the main circle, lying between the lakes of Stenness and Harray.  Also at this site is the Maes Howe, considered one of the most impressive chambered cairns in Europe.  For 10 points, name this henge that shares its name with an island group, the capital of which is Kirkwall on Pomona Island, and which is separated by Pentland Firth from Caithness County, Scotland.
Like Stonehenge, it is an ancient ruin of a Neolithic culture.  
Orkney