Cliff

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A cliff is a steep rock face, usually almost vertical. Cliffs form when rocks are worn away by weathering and erosion and then pulled down by gravity. They are common on coastlines, in mountains, along escarpments, and beside rivers. Most cliffs are made of rocks that resist weathering, such as sandstone, limestone, chalk, dolomite, and sometimes granite or basalt.

An escarpment, or scarp, is a type of cliff created by movement in the Earth's crust (faults) or by landslides that change how rock layers erode. At the base of most cliffs you’ll find a scree or talus slope—loose broken rock. In dry places the base can be a pile of fallen rock, while in wetter places a soil slope may cover the talus. Some cliffs have waterfalls, or shelters in rock, and some end in rock columns or mushroom shapes along a ridge.

Coastal erosion can form sea cliffs where the land meets the sea. The British Ordnance Survey distinguishes cliffs, which run along the top edge with a face down the slope, from outcrops, which run along the lower edge.

The word cliff comes from Old English clif and is related to words in Dutch, German, and Norse. It probably ultimately comes from Latin clivus or clevus, meaning a slope or hillside. Because a cliff does not have to be perfectly vertical, experts sometimes disagree about whether a given slope is a cliff or not, or how much of the slope to count as the cliff.

Some of the largest cliffs are underwater. For example, deep under the sea there are very long drops at the Kermadec Trench. In the southern hemisphere, Tasmania has high sea cliffs made of hard dolerite rock. On land, many lists claim Mount Thor in northern Canada’s Baffin Island as having the world’s highest true vertical drop, about 1,370 meters, though the exact figures depend on how the measurement is made.

The highest sea cliffs are also debated, because it depends on how you define a cliff and where you measure. Kalaupapa in Hawaii is often cited as having the highest sea cliffs at around 1,010 meters, while Mitre Peak in New Zealand and Maujit Qaqarssuasia in Greenland are also contenders with even steeper or longer drops.

One of the tallest vertical drops in the Solar System is Verona Rupes on Miranda, a moon of Uranus, estimated around 20 kilometers high.

Cliffs aren’t just about rock and height. They create special habitats for plants and animals, and many birds nest on cliff faces for protection from predators. People have also lived in cliff dwellings. Some cliff plants, such as the rare Borderea chouardii, survive in just a few cliff habitats in western Europe.