ARTIKEL EOLIAN [yardang]

A yardang is a wind-abraded ridge found in a desert environment. Yardangs are elongate features typically three or more times longer than they are wide, and when viewed from above, resemble the hull of a boat. Facing the wind is a steep, blunt face that gradually gets lower and narrower toward the lee end

Description

A yardang is formed in cohesive material.^[5] Hedin first found the wind-sculptured "clay terraces" or yardangs in the dried up riverbed of the Kurruk-daria in Central Asia. However, yardangs can be found in most deserts across the globe.^[6] Depending upon the winds and the composition of the weakly indurate 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, meso-yardangs are generally a few meters high and 10 to 15 meters long, and micro-yardangs are only a few centimeters high.

A large concentration of mega-yardangs are found near the Tibesti Mountains in the central Sahara. There is a famous yardang at "Hole in the Rock" in Papago Park in Phoenix, Arizona, 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 of Egypt is an augmented yardang.^[7] Pictures from Mars show that the yardang ridges occur on a massive scale there, giving visual support to the theory that Mars has once had groundwater as most yardangs are composed of sedimentary rocks (originally deposited through fluvial or marine mechanics)

Yardangs form in environments where water is scarce and the prevailing winds are strong, unidirectional 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 dune that leaves behind a cemented core. As the process of formation continues, typically a trough will form around the base of the yardang.

They are more commonly created from softer rock types like siltstone, sandstone, shale and limestone, but have also been observed in crystalline rocks such as schist and gneiss.

Yardang

From Wikipedia, the free encyclopedia

A yardang near Meadow, Texas (USDA photo by J.E. Stout).

A yardang is a rock ridge feature caused by wind and water erosion. The word itself is of Turkish origin, meaning ‘steep bank’. Some are found in dried-up riverbeds. Yardangs may also be found in deserts and may form very unusual shapes and some resemble various objects or even people.

Yardangs are elongate features typically three or more times longer than they are wide, and when viewed from above, resemble the hull of a boat. Facing the wind is a steep, blunt face that gradually gets lower and narrower toward the lee end₁. They 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, meso-yardangs are generally a few meters high and 10 to 15 meters long, and micro-yardangs are only a few centimeters high.

Yardangs form in environments where water is scarce and the prevailing winds are strong, unidirectional 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 dune that leaves behind a cemented core. As the process of formation continues, typically a trough will form around the base of the yardang.

They are more commonly created from softer rock types like siltstone, sandstone, shale and limestone, but have also been observed in crystalline rocks such as schist and gneiss.

A large concentration of mega-yardangs are found near the Tibesti Mountains in the central Sahara. There is a famous yardang at "Hole in the Rock" in Papago Park in Phoenix, Arizona, 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 200-foot sandstone hill with a very large circular hole in the middle of it. Some geologists believe that that sphinx of Egypt is an augmented yardang. Pictures from Mars show that the yardang ridges occur on a massive scale there, giving visual support to the theory that Mars has once had groundwater.

 

A yardang is a topographical feature that has been carved out of a surface by the wind. The word is derived from the Turkic word yar, which means ridge or steep bank. On Earth they are most commonly found in deserts where there is a sand supply, which abrades the surface when moved by the wind, and soft sedimentary rocks that the sand easily erodes. Over time, the sand wears down the surface into beautiful streamlined shapes that are aligned with the prevailing sand-moving winds.

Image 1: Emmenides Dorsum on Mars. Image captured with the Thermal Emission Imaging System visual camera (THEMIS VIS V12350012), taken on Sept. 26, 2004. Yardangs reveal that this surface has been wind-sculpted and planed off by ~450 m.

Image 1 shows a region on Mars called Emmenides Dorsum, a high flat plain about 1000 km southwest of Olympus Planitia. The many long, needle-like features are yardangs that have been cut into an easily eroded (possibly sedimentary) unit. There are several small buttes and mesas that are erosion remnants, indicating the level of a former surface that has since been almost entirely stripped away by sand abrasion. A shadow measurement from one of the buttes indicates that ~450 m of sediment has been removed from this surface. Some of the mesas have a circular shape, suggestive of former craters. This indicates that the former high surface remained unrobed long enough for a few craters to impact it.

Image 2: The Lut Desert, Iran. The image is a Digital Globe image from Google Earth. To find this location in Google Maps type the following coordinates into the 'Find...' bar and display the 'satellite' image: 30° 35' 38.78" N, 58° 16' 0.25" E. You may need to zoom out to see the image.

The second image shows yardangs in the Lut Desert of Iran. These yardangs are of comparable size to the martian yardangs in Image. 1. The dark material in the lanes between the yardangs is sand that is traveling downwind to a sand sea south of this yardang field. In this case, the sand that has eroded the surface is still present and actively changing the landscape. In the martian case, the sand may be long gone, reflecting an ancient climate regime in which winds were strong and sand was plentiful.