Mushroom rocks
From The Art and Popular Culture Encyclopedia
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A mushroom rock, also called rock pedestal or a pedestal rock, is a naturally occurring rock whose shape, as its name implies, strikingly resembles a mushroom. The rocks are deformed in a number of different ways: the processing of erosion and weathering, glacial action or from a sudden disturbance. Mushroom rocks are related, but different to yardang and zeugen.
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Erosion
Usually found in desert areas, these rocks are formed over thousands of years when wind erosion of an isolated rocky outcrop progresses at a different rate at its bottom to that at its top. Abrasion by wind-borne grains of sand is most prevalent within the first 3 feet of the ground, causing the bases of outcrops to erode more rapidly than their tops. Running water can have the same effect. An example of this type of mushroom rock is the one in Timna Park, Israel (see photograph opposite).
Occasionally the chemical composition of the rocks can be an important factor; if the upper part of the rock is more resistant to erosion and weathering, it will erode more slowly than the base. Its formation has also been attributed to chemical weathering at the base of the rock due to the collection of dew near the surface.
A mushroom rock may ultimately form from a zeugen, an originally flat area of hard rock overlying soft rock, similar to the pattern of rocks which cause the formation of a waterfall. Weathering of the exposed hard rock layer eventually exposes the lower rock to erosion from wind, water, salt intrusion, etc., depending on local conditions. The layer of softer rock is more readily eroded, leading to the formation of a depression or blowout. The overlying harder rocks are resistant to this process, and may ultimately end up as isolated 'mushroom rocks' standing above the new, lower plain.
The nature of wind erosion is that it concentrates a few feet over the ground - wind speeds increase with height, but sediment load reduces. This means that the combination of highest sediment loads and fastest wind speed exist a few feet over the ground - leading to the characteristic narrowing of zeugen at this height.
Glacial action
In contrast to a mushroom rock formed by the process of erosion of a single rock, these mushrooms are a type of balancing rock formed from two separate rocks, one of which has come to rest on the top of the other. Typically the uppermost rock has been transported and deposited by the very slow action of a glacier; the lower part of the rock formation might or might not have also undergone a degree of erosion to accentuate the mushroom shape. Examples of this type of rock are 'Mushroom Rock' in Signal Point National Park, Tennessee,
Rock disturbance
This type of mushroom rock is formed from two rocks, one of which has come to rest on the top of the other, caused by a very sudden disturbance such as a rock fall or earthquakes.
Examples
- Ciudad Encantada, Cuenca, Spain
- Mushroom Rock State Park, Kansas
- Phae Mueang Phi, Thailand
- The stone mushrooms, Bulgaria
See also
- Aeolian processes
- Hoodoo (geology)
- Precarious boulder
- Tea table
- Mantegna and the picturesque
- Limestone rock formation in the White Desert, Egypt
