In geography and geology, a cliff is a significant vertical, or near vertical, rock exposure. Cliffs are formed as erosion landforms due to the processes of erosion and weathering that produce them. Cliffs are common on coasts, in mountainous areas, escarpments and along rivers. Cliffs are usually formed by rock that is resistant to erosion and weathering. Sedimentary rocks most likely to form cliffs include sandstone, limestone, chalk, and dolomite. Igneous rocks such as granite and basalt also often form cliffs.
Most cliffs have some form of scree slope at their base. In arid areas or under high cliffs, these are generally exposed jumbles of fallen rock. In areas of higher moisture, a soil slope may obscure the talus. Many cliffs also feature tributary waterfalls or rock shelters. Sometimes a cliff peters out at the end of a ridge, with tea tables or other types of rock columns remaining. Coastal erosion may lead to the formation of sea cliffs along a receding coastline.
The Cliffs of Moher (Irish: Aillte an Mhothair) are located at the southwestern 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 receive almost one million visitors a year.
O'Brien's Tower is a round stone tower near the midpoint of the cliffs built in 1835 by Sir Cornelius O'Brien to impress female visitors. From the cliffs and from atop the watchtower, visitors can see the Aran Islands in Galway Bay, the Maumturks and Twelve Pins mountain ranges to the north in County Galway, and Loop Head to the south.