The Great Gulf was an immense gulf that divided Beleriand and Eriador from southern regions of Middle-earth during the First Age.
Geography
South of the river Gelion, "the land narrowed suddenly" and gave way to the Great Gulf. The Great Gulf almost touched the Blue Mountains, but the land around the southern end of the mountain chain was wide enough to allow travellers to pass between Beleriand and Eriador.
After the War of Wrath, most of Beleriand was drowned, and much of the Great Gulf would have vanished with it. Although not explicitly stated, the eastern end of the Great Gulf in Eriador likely became the Bay of Belfalas.
The Great Gulf was created in the Battle of the Powers, when the Valar waged war against Melkor after they discovered the first Elves. As a result of the war, Belegaer broke into Middle-earth, forming the Great Gulf and several lesser bays to its north, including the Bay of Balar.