inlet of the North Channel that connects the Irish Sea with the Atlantic, 12 mi (20 km) long and 3 to 5 mi (4.8 to 8 km) wide, indenting the northeastern coast of Ireland. Its sheltered harbour facilitated the growth of Belfast as a city and port, and its shores were sites of early settlements, including those at Whiteabbey, Carrickfergus, and Bangor. To the north is the elevated basaltic Antrim Mountains, and to the south are the older Ordovician and Silurian rocks.