3.1.2.3 arid landscape development

• Deflation

  The removal of fine material by wind
• Deflation hollows

  • Depressions in the ground formed when deflation removes a lot of material from one place, usually when material is very fine
  • If the hollow is deep enough it will reach the water table, preventing deflation from deepening
  • An enclosed depression produced by wind erosion
• Deflation hollows examples

  • Qattara depression
• Desert pavement

  • The wind blows away the finest material (fines) through deflation, leaving the bigger particles behind as a lag deposit
  • Desert pavement serves as a mulch for plants
• Ventifacts

  • Individual stones/rocks that have been abraded by wind-blown sediment/sand, resulting in smooth/flattened sides, sharp edges, and ridges
  • The side of the rock facing the prevailing wind is eroded the most
  • Several flattened faces may be due to the rock changing position or the direction of the prevailing wind changing (e.g. seasonal reversals)
• Yardangs


  • Elongated, narrow streamlined ridges that are usually 3-4 times longer than they are wide
  • Formed by the prevailing wind/strong wind carrying sand in suspension which erodes rocks by abrasion, scouring the rock in between the yardangs
  • Softer vertical rock is eroded faster than harder vertical rock, creating ridges of vertical hard rock and deep troughs
• Zeugens

  • Long, block-shaped ridges formed when more resistant rock overlies a layer of less resistant geology
  • The more resistant rock joint is weathered, allowing the wind to erode through the cracks and into the softer rocks beneath by abrasion
  • Undercutting of the furrow leads to a pedestal-like shape with a flat cap rock protecting the underlying less resistant rock
• Pedestal rocks

  • Also called 'mushroom rocks', they are the final remains of a zeugen and are primarily formed as a result of aeolian abrasion
  • Made of alternating, horizontal bands of sedimentary rock, with the softer, least resistant rock eroded faster than the case-hardened upper cap
• Dunes


  • Landforms of deposition that form around a sandcatcher (obstacle) that slows the wind and encourages deposition on the leeward (sheltered) side
  • Their shape and layout are determined by wind speed, direction, and consistency, as well as the amount of sand transported and the nature of the ground surface
• Barchan dunes

  • Crescent-shaped dunes that form at right angles to the prevailing wind in one direction, with horns that curve towards the slip face and are pushed forward by the wind
  • They have a smooth, flat surface over which the wind can blow sand, but subtle changes in the shape of the land allow sediments to collect
• Seif dunes


  • Elongated, linear sand dunes formed from two dominant prevailing winds in two different directions, with one blowing in one direction for part of the year and the other blowing from the opposite direction for the remainder of the year
• Wadi


  • A gulley or ravine that has been eroded by seasonal rivers or rivers in the past, now receiving runoff from torrential storms
  • They can fill up with rainwater very quickly and have thick sediment layers, loose debris from flash floods, and may contain vegetation
• Pediment


  • A gently sloping, nearly flat surface formed by erosion at the base of a mountain or escarpment in arid or semi-arid regions, primarily through sheetflood erosion and the gradual weathering and erosion of the base of the mountain
• Inselbergs


  • Rounded, isolated hills found in deserts and humid tropics, resulting from deep chemical weathering of granite during wetter periods and the subsequent removal of surrounding surface deposits through mass wasting and fluvial erosion
• Alluvial fans


  • Cones of sediment deposited by rivers owing to an abrupt loss of energy as they leave their mountain channels and enter a plain, with the sediment sorted in size from largest to smallest
• Bajadas


  • The convergence or blending of many alluvial fans, common in dry climates and can be narrow or wide depending on the number of streams contributing to them
• Playa lakes


  Shallow, often saline, and short-lived lakes that form in low areas of deserts following intense precipitation, with the dry lake bed referred to as a playa or salt pan when the water evaporates
• Salt pans


  Shallow containers or depressions in the ground in which salt water evaporates to leave a deposit of salt
• Landscapes change over time, with changes in inputs like sediment leading to a shift from an erosional landscape to one dominated by landforms of deposition
• Landforms of past processes can undergo further change, with a wadi formed by past river flow being further eroded by episodic sheet and channel flooding
• Changes to desert landscapes occur more slowly than in other regions because many weathering and erosional processes need water, and some deserts like the Atacama can go years without rain
• Many current desert landforms began to develop during pluvial periods several thousands of years ago


  Pleistocene geological epoch, ended around 12,000 years ago
• During this interglacial period, conditions were warmer and more humid, with forests thriving and many present-day deserts being grasslands


  About 8,000 years ago
• During this time, aridity was widespread, with cold deserts in the north and more extensive deserts in the south in similar locations to the present day


  Last glacial maximum, 20,000 years ago
• Over Asia and Africa, the monsoon rains began to lessen, and arid conditions similar to today began to develop


  5,000 years ago to the present day