Past & Present Desert Landscapes (AQA A Level Geography)

• Each desert landscape is unique due to past and present interactions of rocks and processes operating on them
• Wind and water action are the most important processes, although weathering, mass movement and vegetation also play a role
• Processes operating in desert systems can create new landforms or change existing landforms
• This means that desert landscapes change over time
  
  • Changes in inputs change the landscape, e.g. an increase in sediment input may increase deposition
  • Over time the landscape would then change from an erosional landscape to one dominated by landforms of deposition
• Landforms of past processes can undergo further change
  • A wadi formed by past river flow will be further eroded by the episodic sheet and channel flooding
• Hot desert landscapes are, therefore, made of a mixture of characteristics and landforms that reflect different periods of time
  • Canyons formed when the climate was wetter (interglacial period ≈8000 years ago) and present-day landforms such as pedestals
• Changes occur over a range of spatial and temporal (time) scales
  • Changes can be short and episodic such as flash floods transporting material in wadis
  • Too long and gradual, such as wind eroding rock over thousands of years to form a pedestal rock
• Changes to desert landscapes occur slower than in other regions because many weathering and erosional processes need water
  • The Atacama Desert can go years without rain, therefore, processes operate slowly and episodically there

Basin and range

• This topography develops in landscapes where past tectonic faulting has occurred through extension, deformation and sedimentation
• The horizontal extension of crustal plates stretches the area and faults appear
• Vertical deformation (displacement) takes place along these fault lines
• Creating a rising 'range' of mountains and a dropped valley 'basin' 
• Mountains continue to erode, with streams carrying sediment down to the valley floor

Image showing the simplified formation of a basin and range landscape 

Case study

• The Western US Basin and Range extends to the Rocky Mountains, South Central and Southwestern states
• It includes all of Nevada, south-eastern California and Oregon, with high mountain ranges running mostly north-south with alternating low valleys
• 20 million years ago, the crust along the Basin and Range stretched, thinned, and faulted into approximately 400 mountain blocks
• Upward pressure from the mantle, lifted some blocks, forming stretched peaks, whilst leaving the lower blocks to form down-dropped valleys
• Boundaries between the basins and ranges are acute (sharp) because the faults are straight between them
• Also, many of the faults are still active

Image showing the formation of the basin and range of the Sierra Mountains and Cascades, USA 

Pleistocene pluvials

• The Pleistocene is the geological epoch that lasted from about 2,588,000 to 11,700 years ago, spanning the world's most recent period of repeated glaciation
• The end of the Pleistocene corresponds with the end of the last glacial period and also with the end of the Palaeolithic age used in archaeology
• A pluvial refers to the characteristic of rainfall
• The evolution of arid and semi-arid landforms is often affected by events that occurred long ago
• Many current desert landforms, began to develop during pluvial periods several thousand years ago
• The sporadic activity takes place with sudden downpours/deluges where a lot of erosional activity takes place in limited locations such as wadis, but this would not be enough to create the range of water-eroded features
• Therefore, pluvial periods in the Pleistocene and beyond are seen as the likely period for most landform formation of deserts
• Some do argue that wind erosion and deposition are still actively giving rise to depressions, yardangs, zeugens and dune systems
• But, the scale suggests that water action and past weathering have been a significant factor
  • Wadis and integrated valley systems are unlikely to have formed under present conditions
  • Piedmont zone landforms show evidence of having been formed by intense water action
  • Dunes are the reworking of vast sand seas, a result of alluvial deposition in a wetter climate

The Changes to Hot Desert Climate Over Time