Scarp retreat is a geological process through which the location of an

escarpment An escarpment is a steep slope or long cliff that forms as a result of faulting or erosion and separates two relatively level areas having different elevations. The terms ''scarp'' and ''scarp face'' are often used interchangeably with ''esca ...

changes over time. Typically the cliff is undermined, rocks fall and form a talus slope, the talus is chemically or mechanically

weathered ''Weathered'' is the third studio album by American rock band Creed, released on November 20, 2001. It was the last Creed album to be released until '' Full Circle'' came out in October 2009, with Creed disbanding in June 2004. It is the only Cr ...

and then removed through water or wind

erosion Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is d ...

, and the process of undermining resumes. Scarps may retreat for tens of kilometers in this way over relatively short geological time spans, even in arid locations.

Scarp profiles

A scarp is a line of cliffs that has usually been formed by faulting or erosion. If it is protected by a strong

caprock Caprock or cap rock is a more resistant rock type overlying a less resistant rock type,Kearey, Philip (2001). ''Dictionary of Geology'', 2nd ed., Penguin Reference, London, New York, etc., p. 41.. . analogous to an upper crust on a cake that is ha ...

, or if it contains vertical fractures, it may retain its steep profile as it retreats. Scarps in dry climates typically have a near-vertical upper face, that may account for 10% - 75% of the total height, with a talus-covered sloping rampart forming the lower section. The caprock is undermined as the rampart and face are eroded, and eventually a section collapses. A strong caprock will typically create a relatively high cliff, since more undermining is needed to cause it to fail. Other factors determining how easily a cliff will fail are the bedding and jointing, direction of dip and thickness of the caprock. A thin caprock will result in low cliffs that retreat quickly. However, a caprock is not essential for scarp retreat to occur as higher humidity and weathering at the foot ensures erosion at the foot drives (or keeps up with) the erosion of the free face.

Mechanism

The most common way in which a scarp retreats is through rockfall, where individual blocks calve from the cliff or large sections of the cliff face collapse at once. In some high-energy situations, much of the rock may be powdered in a rockfall and easily eroded. Generally, though, the fallen debris must be weathered and the rampart eroded before scarp retreat can continue. Mechanical and chemical weathering followed by wind erosion may operate in arid regions, where cliffs may retreat for long distances. In such regions, large areas of shale badlands may be left behind as the scarp retreats. Erosion may be caused by the sea where the scarp runs along a coast, or by streams in humid areas.

Rate of retreat

The rate of retreat depends on the types of rock and the factors causing erosion. A study published in 2006 determined that the rate of scarp retreat in the Colorado Plateau today varies from per million years depending on the thickness and resistance to erosion of the

. Retreat of the

Great Escarpment The Great Escarpment is a major topographical feature in Africa that consists of steep slopes from the high central Southern African plateauAtlas of Southern Africa. (1984). p. 13. Reader's Digest Association, Cape Town downward in the directio ...

in Australia along the river valleys in the

New England New England is a region comprising six states in the Northeastern United States: Connecticut, Maine, Massachusetts, New Hampshire, Rhode Island, and Vermont. It is bordered by the state of New York to the west and by the Canadian provinces ...

region appears to be progressing at about per million years. A study of cuesta scarp retreat in southern Morocco showed an average rate of per million years in areas with thin conglomerate caprocks. Where there were thicker, more resistant limestone caprocks the rate of retreat was slower, about per million years.

Examples

The

Colorado Plateau The Colorado Plateau, also known as the Colorado Plateau Province, is a physiographic and desert region of the Intermontane Plateaus, roughly centered on the Four Corners region of the southwestern United States. This province covers an area o ...

has a cuesta scarp topography, consisting of slightly deformed strata of alternating harder and softer rocks. The climate has been mostly dry throughout the

Cenozoic The Cenozoic ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterised by the dominance of mammals, birds and flowering plants, a cooling and drying climate, and the current configu ...

. The conspicuous scarps on the plateau have massive sandstone caps over easily weathered rock such as shale. Freeze-thaw and groundwater sapping contribute to scarp retreat in this region. The Drakensberg mountains in South Africa are capped by a layer of Karoo basalts about thick, which overlay Clarens formation sandstones. They have long been considered a classic example of a landform created by scarp retreat following continental break-up, with the retreat controlled by an inland drainage divide. However, they have inland-facing scarps as well as seaward-facing scarps, so factors other than continental break-up have contributed to their formation. A 2006 paper argued that surface process models may be inadequate in explaining rates of scarp retreat, which can also be greatly affected by the types of rock encountered as the scarp retreats, and by other factors such as climate, tectonic processes and possibly plant cover.

References

Citations Sources * * * * * * {{refend Escarpments Weathering Erosion

Scarp profiles

Mechanism

Rate of retreat

Examples

See also

References