Ventifact
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Ventifacts are rocks that have been abraded, grooved, or polished by wind driven sand. These geomorphic features are most typically found in arid environments where there is little vegetation to interfere with eolian particle transport, where there are frequently strong winds, and where there is a steady but not overwhelming supply of sand.
Ventifacts can be abraded to eye-catching natural sculptures. In moderately tall, isolated rock outcrops, mushroom shaped pillars of rock may form as the outcrop is eroded by saltating sand grains. This occurs because, even in strong winds, sand grains can't be continuously held in the air. Instead, the particles bounce along the ground, rarely reaching higher than a few feet above the earth. Over time, the bouncing sand grains can erode the lower portions of a ventifact, while leaving a larger less eroded cap. The results can be fantastic stone mushrooms.
Individual stones, such as those forming desert pavement, are often found with grooved, etched, surfaces where these same wind driven processes have slowly worn away the rock.
When ancient ventifacts are preserved without being moved or disturbed, they may serve as a paleo-wind indicators. The wind direction at the time the ventifact formed will be parallel to grooves or striations cut in the rock.
See also: yardang
