Green Mountains, part of the Appalachian Mountain system, U.S., extending for 250 miles (402 km) from north to south through the centre of Vermont and having a maximum width of 36 miles (58 km). Many peaks rise above 3,000 feet (900 metres), with the loftiest being Mount Mansfield (4,393 feet [1,339 metres]; highest point in Vermont) and Killington Peak (4,235 feet [1,291 metres]).