A long arm of the Great Sea in the northern lands of Middle-earth, more commonly known as the Gulf of Lhûn, into which the waters of the river Lune emptied. From the mouth of the Lune, where Círdan kept his Grey Havens, the Firth ran on westward for two hundred miles or more before reaching the open waters of the Sea at a narrow breach. The Firth broke the Blue Mountains into a northern and a southern range, and also divided the Elvish land of Lindon into the regions known as Forlindon ('North Lindon') and Harlindon ('South Lindon').