Firth, a bay in the mainland of Orkney. Opening on a line westward from the String, or the sound between the mainland and Shapinshay, it measures 2¼ miles from N to S across the entrance, penetrates 3¾ miles west-south-westward, and contracts to a width of 11 furlongs, but re-expands presently to a width of 15. It is noted for its oyster beds; contains, in its upper part, the islets of Damsay and Grimbister Holm; sends off, from its NW corner, the little bay of Isbister; and is bounded on the lower reach of its northern side by Rendall parish, of its southern side by Kirkwall or St Ola parish.

(F.H. Groome, Ordnance Gazetteer of Scotland (1882-4); © 2004 Gazetteer for Scotland)