escarpment An escarpment is a long, relatively continuous line of cliffs or steep slopes that divides the landscape into two level or gently sloping surfaces. Of French origin, the term ‘escarpment’ is often used synonymously with ‘scarp’, although the latter is more often used to refer to smaller-scale features and/or to features of tectonic origin.
Escarpments can originate through faulting, differential erosion, or by a combination of the two. A ‘fault scarp’ is formed by vertical motion along a fault, where one block of land is displaced upward relative to an adjacent block (see
fault scarp). Fault scarps are common along the edges of fault-block mountains, such as those in the western United States and the eastern African rift valleys, where valley floors are sometimes downfaulted by as much 2.5 km. A ‘fault-line scarp’ is an escarpment formed where ancient faulting has juxtaposed rocks of contrasting resistance to erosion.
Escarpments can also form by differential erosion, where erosion or faulting has exposed an erodible rock layer lying beneath a more resistant one. The resistant rock layer (typically sandstone, limestone, or basalt) will often act as a ‘caprock’, supporting the top of a cliff face. An escarpment thus formed can wear back over time, in a process known as ‘escarpment retreat’, as the less-resistant materials (typically shale or clay) are weathered and eroded from beneath the caprock, causing its collapse. Such caprock-supported escarpments are common in arid regions containing layered sedimentary rocks. In the Colorado Plateau region of the western United States, for example, escarpment retreat is responsible for forming the dramatic ‘butte-and-mesa’ topography.
Long, continuous escarpments formed by undercutting of a resistant limestone layer can be found in the deserts of Arabia and the Gulf of Suez region in Egypt. Low rates of weathering in these arid settings promote escarpment retreat by enhancing the contrast between different rock types. However, caprock-supported escarpments can also form in humid regions; the Niagara escarpment, over which the eponymous Falls spill, is one example. In some cases, escarpments occur where there is no obvious lithologic or tectonic origin. One such example is the Blue Ridge escarpment of western Virginia and North Carolina in the eastern United States, which is sculpted from apparently homogeneous metamorphic rocks.
Escarpment retreat has long been an important problem in geomorphology. The geologist L. C. King believed that escarpment retreat was an ubiquitous erosional process on Earth. As an alternative to William Morris Davis's ‘geographical cycle’, King proposed a model of landscape evolution based on the concept of cyclic episodes of escarpment backwearing. King's ‘pediplanation’ model retained Davis's view that landscape evolution is characterized by brief periods of rapid tectonic uplift separated by long intervals of erosion. But where Davis considered landscape erosion to occur by a process of more or less uniform relief reduction, leading ultimately to a nearly level surface (or ‘peneplain’), King instead proposed that erosion occurs primarily through escarpment retreat, which would leave behind ‘pediplains’ formed at the foot of a retreating escarpment. Each episode of uplift would lead to the creation of new escarpments along the shoreline or fault lines, which would then wear back to form a new set of pediplains sloping gently toward the coast. Multiple episodes of uplift and erosion over millions of years would create a staircase-like series of escarpments and pediplains, each of a different age.
King's theory was certainly influenced by the landscape of his homeland, South Africa. One of the largest escarpments on Earth is the Great Escarpment of southern Africa, which rings the sub-continent 100 or more kilometres from the coastline, separating the interior Kalahari Plateau from the coastal plains. The highest elevations in southern Africa are found along or near the rim of the Great Escarpment, which rises to over 3 km in elevation in the Natal region. Escarpments similar to the Great Escarpment occur along the edges of many continents, including eastern Brazil, western India (the Western Ghats), and eastern Australia. Many geomorphologists now believe that these ‘great escarpments’ originated along newly formed continental margins during the early break-up of the supercontinent Pangaea, and have since been eroded back to their present inland positions (sometimes hundreds of kilometres from the original rift zone near the coast). It has been speculated that the long-term survival of these ‘great escarpments’ may be due in part to coastal isostatic rebound as material is eroded from the escarpment face.
Even more impressive escarpments can be found on the planet Mars, where erosional processes have presumably been much less vigorous than those on Earth. The Valles Marineris, a vast, steep-walled, equatorial canyon that dwarfs Earth's Grand Canyon, is bounded by escarpments up to 6 km high.
Gregory E. Tucker
