Wind erosion This photo, taken in the White Desert of Egypt, shows a structure called a “mushroom ro

Wind erosion This photo, taken in the White Desert of Egypt, shows a structure called a “mushroom rock” or a “pedestal rock”. The rock is a fine-grained limestone, deposited on the shorelines of the ancient Tethys Ocean which once sat between Europe and Africa. The structure is a result of erosion. The land surface throughout this entire area was once higher, above the the top of this stone today. That surface was gradually eroded by processes like water and salt, which opened cracks and channels through more resistant layers. As the Sahara dried up and became a desert, wind erosion became a dominant force. Wind has peculiar properties when it erodes – it is most powerful about a meter above the surface. Wind on its own doesn’t do much erosion, but wind can pick up sand and sandblast objects. Very near the ground is where the maximum amount of sand is moved, but close to the ground the wind speed is reduced due to friction with the ground. Higher up, the winds are faster, but the farther away from the ground the wind is the less sand it will carry. The maximum erosion therefore happens at a height a little less than a meter above the ground, where the wind is strong and still carries some sand. The rock layers are sandblasted the most at this level, leaving the pedestal above it somewhat untouched until it is undermined so much that it falls over. -JBB Image credit: Wikimedia commonshttp://en.wikipedia.org/wiki/File:Weisse_Wüste.jpg -- source link
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