Yardang
Encyclopedia
Bedrock
In stratigraphy, bedrock is the native consolidated rock underlying the surface of a terrestrial planet, usually the Earth. Above the bedrock is usually an area of broken and weathered unconsolidated rock in the basal subsoil...
or any consolidated or semiconsolidated material by the dual action of wind abrasion, dust and sand, and deflation. Yardangs become elongated features typically three or more times longer than wide, and when viewed from above, resemble the hull
Hull (watercraft)
A hull is the watertight body of a ship or boat. Above the hull is the superstructure and/or deckhouse, where present. The line where the hull meets the water surface is called the waterline.The structure of the hull varies depending on the vessel type...
of a boat. Facing the wind is a steep, blunt face that gradually gets lower and narrower toward the lee end. Yardangs are formed by wind erosion, typically of an originally flat surface formed from areas of harder and softer material. The soft material is eroded and removed by the wind, and the harder material remains. The resulting pattern of yardangs is therefore a combination of the original rock distribution, and the fluid mechanics
Fluid mechanics
Fluid mechanics is the study of fluids and the forces on them. Fluid mechanics can be divided into fluid statics, the study of fluids at rest; fluid kinematics, the study of fluids in motion; and fluid dynamics, the study of the effect of forces on fluid motion...
of the air flow and resulting pattern of erosion.
Etymology
The word itself is of TurkicTurkic languages
The Turkic languages constitute a language family of at least thirty five languages, spoken by Turkic peoples across a vast area from Eastern Europe and the Mediterranean to Siberia and Western China, and are considered to be part of the proposed Altaic language family.Turkic languages are spoken...
origin, meaning ‘steep bank’, and was first introduced to the English-speaking world by the Swedish explorer Sven Anders Hedin
Sven Hedin
Sven Anders Hedin KNO1kl RVO was a Swedish geographer, topographer, explorer, photographer, and travel writer, as well as an illustrator of his own works...
in 1903.
Description
A yardang is formed in cohesive material. HedinSven Hedin
Sven Anders Hedin KNO1kl RVO was a Swedish geographer, topographer, explorer, photographer, and travel writer, as well as an illustrator of his own works...
first found the wind-sculptured "clay terraces" or yardangs in the dried up riverbed
Riverbed
Riverbed may refer to:*Stream bed, the channel bottom of a stream or river or creek*Wadi, a dry riverbed that contains water only during times of heavy rain*Riverbed Technology, an American technology company...
of the Kurruk-daria in Central Asia
Central Asia
Central Asia is a core region of the Asian continent from the Caspian Sea in the west, China in the east, Afghanistan in the south, and Russia in the north...
. However, yardangs can be found in most desert
Desert
A desert is a landscape or region that receives an extremely low amount of precipitation, less than enough to support growth of most plants. Most deserts have an average annual precipitation of less than...
s across the globe. Depending upon the winds and the composition of the weakly indurated deposits of silt and sand from which they are carved, yardangs may form very unusual shapes — some resemble various objects or even people.
Yardangs come in a large range of sizes, and are divided into three different categories: mega-yardangs, meso-yardangs, and micro-yardangs. Mega-yardangs can be several kilometers long and hundreds of meters high and are found in arid regions with strong winds; meso-yardangs are generally a few meters high and 10 to 15 meters long and are commonly found carved in semiconsolidated playa sediments and other soft granular materials; and micro-yardangs are only a few centimeters high.
A large concentration of mega-yardangs are found near the Tibesti Mountains
Tibesti Mountains
The Tibesti Mountains are a range of inactive volcanoes located on the northern edge of the Chad Basin in the Borkou- and Tibesti Region of northern Chad. The massif is one of the most prominent features of the Central-Sahara desert and covers an area of approximately 100,000 km². The northern...
in the central Sahara
Sahara
The Sahara is the world's second largest desert, after Antarctica. At over , it covers most of Northern Africa, making it almost as large as Europe or the United States. The Sahara stretches from the Red Sea, including parts of the Mediterranean coasts, to the outskirts of the Atlantic Ocean...
. There is a famous yardang at "Hole in the Rock" in Papago Park in Phoenix, Arizona
Phoenix, Arizona
Phoenix is the capital, and largest city, of the U.S. state of Arizona, as well as the sixth most populated city in the United States. Phoenix is home to 1,445,632 people according to the official 2010 U.S. Census Bureau data...
, a rock formation with a roughly circular hole in it. Another yardang in Arizona is Window Rock, near the town of Window Rock. It is a 60-meter sandstone hill with a very large circular hole in the middle of it. Some geologists have suggested that the Great Sphinx
Great Sphinx of Giza
The Great Sphinx of Giza , commonly referred to as the Sphinx, is a limestone statue of a reclining or couchant sphinx that stands on the Giza Plateau on the west bank of the Nile in Giza, Egypt....
of Egypt
Egypt
Egypt , officially the Arab Republic of Egypt, Arabic: , is a country mainly in North Africa, with the Sinai Peninsula forming a land bridge in Southwest Asia. Egypt is thus a transcontinental country, and a major power in Africa, the Mediterranean Basin, the Middle East and the Muslim world...
is an augmented yardang. Pictures from Mars show that the yardang ridges occur on a massive scale there; some individual ridges are tens of kilometers long with intervening valleys nearly 1 km wide. Yardangs on Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...
are typically found in the Amazonis region but the best ones are found in the equatorial region. Yardangs on Mars demonstrate that much of the eolian erosion is recent since they are sculpted in young geologic units.
Formation
Yardangs form in environments where water is scarce and the prevailing winds are strong, uni-directional, and carry an abrasive sediment load. The wind cuts down low lying areas into parallel ridges which gradually erode into separate hills that take on the unique shape of a yardang. This process yields a field of yardangs of roughly the same size, commonly referred to as a fleet due to their resemblance to the bottoms of ships. Alternatively, one can be formed by the migration of a duneDune
In physical geography, a dune is a hill of sand built by wind. Dunes occur in different forms and sizes, formed by interaction with the wind. Most kinds of dunes are longer on the windward side where the sand is pushed up the dune and have a shorter "slip face" in the lee of the wind...
that leaves behind a cemented core. As the process of formation continues, typically a trough will form around the base of the yardang. Most yardang fields are in sand-poor areas, but the associated troughs, espcially in grooved terrain, may be invaded by sand. Sometimes this sand will accumulate to build shallow moats around the bottom.
They are more commonly created from softer rock types like siltstone
Siltstone
Siltstone is a sedimentary rock which has a grain size in the silt range, finer than sandstone and coarser than claystones.- Description :As its name implies, it is primarily composed of silt sized particles, defined as grains 1/16 - 1/256 mm or 4 to 8 on the Krumbein phi scale...
, sandstone
Sandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...
, shale
Shale
Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...
and limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
, but have also been observed in crystalline rocks such as schist
Schist
The schists constitute a group of medium-grade metamorphic rocks, chiefly notable for the preponderance of lamellar minerals such as micas, chlorite, talc, hornblende, graphite, and others. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is...
and gneiss
Gneiss
Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks.-Etymology:...
.
Other common names for them include Mud-lions, sphinxlike hills, mushroom rocks, inselbergs, Koukour (common in Tunisia), kalut (Iran) and cretes et couloirs (Chad).
See also
- Aeolian processes
- VentifactVentifactVentifacts are rocks that have been abraded, pitted, etched, grooved, or polished by wind-driven sand or ice crystals. These geomorphic features are most typically found in arid environments where there is little vegetation to interfere with aeolian particle transport, where there are frequently...
- DuneDuneIn physical geography, a dune is a hill of sand built by wind. Dunes occur in different forms and sizes, formed by interaction with the wind. Most kinds of dunes are longer on the windward side where the sand is pushed up the dune and have a shorter "slip face" in the lee of the wind...
- BarchanBarchanA barchan dune, also barkhan is an arc-shaped sand ridge, comprising well-sorted sand. This type of dune possesses two "horns" that face downwind, with the slip face at the angle of repose of sand, or approximately 35 degrees . The upwind side is packed by the wind, and stands at about 15 degrees...
- SandhillSandhillA sandhill is a type of ecological community or xeric wildfire-maintained ecosystem. It is not the same as a sand dune. It features very short fire return intervals, one to five years. Without fire, sandhills undergo ecological succession and become more oak dominated.Entisols are the typical...
- BlowoutBlowout (geology)Blowouts are sandy depressions in a sand dune ecosystem caused by the removal of sediments by wind.Blowouts occur in partially vegetated dunefields or sandhills. A blowout forms when a patch of protective vegetation is lost, allowing strong winds to "blow out" sand and form a depression...
- SaltationSaltation (geology)In geology, saltation is a specific type of particle transport by fluids such as wind or water. It occurs when loose material is removed from a bed and carried by the fluid, before being transported back to the surface...
- Zeugen