The Cliffs of Moher are located at the south-western edge of the Burren region in County Clare, Ireland. They rise 120 metres (390 ft) above the Atlantic Ocean at Hag's Head, and reach their maximum height of 214 metres (702 ft) just north of O'Brien's Tower, eight kilometres to the north. The Cliffs of Moher were originally the site of a gigantic river delta and were formed about 320 million years ago during the Carboniferous period. The Napoleonic signal tower at Hag's Head was built in 1803, this is close to the site of a long gone 1st century BC ruined fort "Mother" from which the Cliffs get their name. Being almost vertical, their sheer drop into the heaving Atlantic ocean is a haven for sea birds.