§ Of the Rings of Power and the Third Age The Silmarillion

In the Great Battle and the tumults of the fall of Thangorodrim there were mighty convulsions in the earth, and Beleriand was broken and laid waste; and northward and westward many lands sank beneath the waters of the Great Sea. In the east, in Ossiriand, the walls of Ered Luin were broken, and a great gap was made in them towards the south, and a gulf of the sea flowed in. Into that gulf the River Lhûn fell by a new course, and it was called therefore the Gulf of Lhûn. That country had of old been named Lindon by the Noldor, and this name it bore thereafter; and many of the Eldar still dwelt there, lingering, unwilling yet to forsake Beleriand where they had fought and laboured long. Gil-galad son of Fingon was their king, and with him was Elrond Half-elven, son of Eärendil the Mariner and brother of Elros first king of Númenor.