The tallest cliff in the Solar System may be 20 kilometres high
On Miranda, a small moon of Uranus, a fault scarp called Verona Rupes may drop about 20 kilometres. On Earth, rankings are messier: a cliff need not be perfectly vertical, so whether you count the steep slope below a rock wall can change which cliff wins by hundreds of metres.
A cliff is rock standing at or near vertical, shaped by weathering, erosion and gravity. Cliffs appear along coasts, rivers, escarpments and in mountains, and they tend to be built from tough rock: sandstone, limestone, chalk and dolomite among sedimentary types, granite and basalt among igneous ones. An escarpment forms when a fault moves or a landslide shifts how rock layers erode. Most cliffs have a scree slope of fallen debris at their foot, bare in dry places, often hidden under soil in wetter ones. The English word goes back to Old English clif, and possibly to Latin clivus, a slope.
Definitions decide records. By a strict standard, the east face of Great Trango in Pakistan's Karakoram, about 1,340 metres of near-vertical stacked pillars, is sometimes called the world's highest; include its steep approach and the drop to the glacier nears 2,000 metres. For sheer verticality, Mount Thor on Baffin Island is often cited, 1,370 metres tall with an overhanging top section and a 1,250-metre vertical drop, though other Baffin or Greenland walls may beat it.
Sea cliffs are just as contested. Guinness lists Kalaupapa in Hawaii at 1,010 metres, and Mitre Peak falls 1,683 metres into New Zealand's Milford Sound, but both average only about 60 degrees. A steeper plunge is the Thumbnail in southern Greenland, which drops 1,560 metres almost vertically. Some of the biggest cliffs are hidden underwater: a ridge in the Kermadec Trench drops 8,000 metres over a horizontal span of 4,250.
South Africa's Drakensberg has an unusually high proportion of cliff faces. Along its highest section, near-continuous walls of 600 to 1,200 metres run for almost 250 kilometres, the Amphitheatre being the best known. Together with Ethiopia's Simien Mountains, the range is rated among the two finest erosional mountain ranges anywhere.
Source: Cliff
