Yardang
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A yardang is a rock ridge feature caused by wind and water erosion. Some are found in a dried-up riverbed. Yardangs may also be found in deserts and may form very unusual shapes and some resemble various objects or even people.
These features come in three different scales: Mega-yardangs, meso-yardangs, and micro-yardangs. Mega-yardangs can be several kilometers long and hundreds of meters high. A large concentration of mega-yardangs are found near the Tibesti Mountains in the central Sahara. Meso-yardangs are generally a few meters high and 10 to 15 meters long. They are more common, and can be found throughout the Sahara. Micro-yardangs are only a few centimeters high.
Yardangs form as the result of preferential erosion of surrounding media. In desert environments, mildly cemented "cores" of sediment form the basis for the structure. Loose sediment surrounding the cemented section erodes faster, leaving the core behind. Yardangs are elongate, with their long axis parallel to the prevailing wind direction. One side is almost always steeper than the other, similar to the shape of a dune. However, in yardangs the blunt, steeper side is the windward side while the shallower slope is on the leeward side.
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. 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.
[edit] Reference
- Yardangs on Mars. NASA SSE Multimedia Gallery. Retrieved on March 21, 2005.
