Desert processes

Why do deserts form where they do?

What characterizes the geology of deserts?

Dunes

Dunes at White Sands, New Mexico. The sand grains making the dunes up here are not typical quartz sand, but gypsum, and this is why they are so white. Gypsum is a common evaporite mineral formed in many desert playas - however, gypsum dunes are fairly rare. These dunes have the typical steep face (in the shadow here) and a shallow face that reflect the direction the wind blows and the dune moves (see diagram below). Note also the flat area, the interdune depression, between this dune the photographer was standing on and the next dune (on the very left side), with its shallow face.

A view of the White Sands dunes from down in the interdune depression. The curved lines on the interdune depression reflect the cross bed layers from a previous dune that had migrated through this area. Water at the base of the dune cemented some of the material together to leave this trace - the rest has blown on.

How do dunes move?

Simplified cross section diagram of a dune with its steep face on one side and the shallow on the other. The dune migrates with the wind, creating internal cross bed layering as it does so.

Underneath the masking grassland cover of the Nebraska Sandhills lies an abundance of dunes with their form still fairly well preserved. Nebraska used to be desert.

What is the character of playas and salt flats?

Looking west across the salt flats of Death Valley, with an alluvial fan off in the distance. The primary mineral here is gypsum, but halite can also be found growing locally. These salt flats were recently flooded, and so they are not quite as white as usual.

Close up of salt pans in Death Valley, the edges of which form a honeycomb, polygonal pattern. Note that here the rims buckle up, and the mechanics of how these form are more complicated than simple mudcracks and involve the expansive growth of salt crystals in the sediment and cracks.

Close up of salt crystals (gypsum) growing on the playa mud as evaporation takes place.

View to the East of the marginal part of the Death Valley playa where mud flats occur. This photo was taken in January 2011, after an unusually wet period, and the layer in front was newly deposited (we may have been the first to ever walk on it). A small channel can be seen just to the right. Note incipient mud cracks forming. The vegetation here is very salt tolerant.

Old mud cracks exposed in a small channel on the Death Valley playa mudflats, with a new layer of mud sitting on top. New mudcracks are forming with a geometry inherited from those underneath. This mud is very salt rich, and locally salt crusts were forming on top also.

This photo was taken in January of 2011 at Badwater, Death Valley, after an unusual amount of rain. What is typically salt and mud flats all the way across Death Valley is an ephemeral lake. The waters that flowed in here scavenged salt from the mountainsides and desert rocks and transported to this playa basin.

 

Badlands

Badlands on N side of Scotts Bluff. Source: http://www.nps.gov/history/history/online_books/geology/publications/pp/17/sec2.htm

Part of the Badlands National Park, South Dakota with classic badlands morphology.

A small geologic oddity from a stream bed in the South Dakota badlands - an armored mudball. These form as a current moves a ball of very sticky mud down the channel, and sand and gravel gets embedded in the growing mudball. They reguire just the right conditions to form. This one is 2-3 inches across.

 

Harmon D. Maher Jr. reserves copyrights to the materials in this site. Material may be used for non-profit educational purposes as long as proper attribution is given. For permission for any other use please contact author. Thank you.