UNIVERSITY OF SOUTH ALABAMA
GY 301: Geomorphology
Last Time
A) Cenozoic temperature shifts (GY 112)
B) Causes of Plio-Pleistocene Glaciations
C) Consequences of Plio-Pleistocene Glaciations
Lecture 25:
Aeolian Processes &
Sandy Deserts
Plio-Pleistocene
Glacial/Interglacial Stages
Major world-wide cooling first
began in the early Eocene.
Glacial/
interglacial
stage
Milankovitch Cycles
Three major changes in
Earth’s orbit are linked to
glacial oscillations
•Eccentricity
•Obliquity
•Precession
Name of stage
Time frame
interglacial
Holocene
0 - 10 KA
glacial
Wisconsinian
10 – 75 KA
interglacial
Sangamon
75 – 125 KA
glacial
Illinoisan
125 – 265 KA
interglacial
Yarmouth
265 – 300 KA
glacial
Kansan
300 – 435 KA
interglacial
Aftonian
435 – 500 KA
glacial
Nebraskan
> 500 KA
Consequences of Continental Glaciations
• Great lakes
– Last glacial maximum
• 35,000-10,000 years
ago
• Wisconsin Stage
– Remained when ice
sheets melted back
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Consequences of Continental Glaciations
Consequences of Continental Glaciations
• The Great Flood (catastrophic emptying
• The Scablands: the area affected
by the flooding
of Lake Missoula)
http://www.nps.gov/iceagefloods/J-ext1.JPG
http://hugefloods.com/Lake_Missoula_Map.gif
Consequences of Continental Glaciations
• Lowering of sea level
– Exposed continental
shelves
Consequences of
Continental Glaciations
• Climate impacts were
felt globally
– Steepened temperature
gradients
– Increased aridity
– Exception: Great Basin
• Lakes
• Great Salt Lake
End of the Ice Age
• Glaciers began to retreat
around 15,000 years ago
–
–
–
–
Waters drained to lakes
Sea level rose
Tundra shifted northward
Deciduous trees migrated
northward
Today’s Lecture
A) Recap of arid lands/deserts & water erosion
B) Aeolian (wind) processes
C) Dunes and other desert features
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Arid and Semi-Arid Regions
Deserts with an annual precipitation sum <50 mm/year and
devoid of vegetation (Hot arid regions occur at 30 degrees latitude or in rain
Arid and Semi-Arid Regions
Important geomorphic processes in the dry regions of the
world differ from those in more humid environments:
shadows)
•streams are intermittent or ephemeral (and have very irregular discharges),
•mass-wasting processes and unconfined sheet floods are prominent,
•many rivers do not flow into the sea but end in inland depressions without
outlet,
•salt lakes are a common landscape feature,
•aeolian processes play an important role, particularly in areas receiving less
than 150 mm/year of precipitation
•physical weathering processes are prominent whereas hydrolysis of minerals is
subdued.
Water Processes
• Dry wash (Wadi):
stream channel of
intermittent stream
Water Processes
• Playa Lakes: due to rapid erosion and elevated base
levels
Water Processes
• Hoodoos: erosional
remnants that are spireshaped
Arid Vegetation
• Sparse concentration
• Shallow roots
• Not effective for erosion
control
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Arid and Semi-Arid Regions
Important geomorphic processes in the dry regions of the
world differ from those in more humid environments:
•streams are intermittent or ephemeral (and have very irregular discharges),
•mass-wasting processes and unconfined sheet floods are prominent,
•many rivers do not flow into the sea but end in inland depressions without
outlet,
•salt lakes are a common landscape feature,
•aeolian processes play an important role, particularly in areas receiving less
than 150 mm/year of precipitation
•physical weathering processes are prominent whereas hydrolysis of minerals is
subdued.
Global Wind Patterns
• Coriolis effect deflects wind direction
• Driven by convection
Effects of Aeolian Processes
• Ventifacts: faceted pebbles and outcrops….
Wind (Aeolian processes)
• A velocity of 50
kph is required to
produce significant
movement of sand
by wind
Effects of Aeolian Processes
• Rounding of grains
• Adds a “frosted” appearance to sand grains
Effects of Aeolian Processes
• Ventifacts: faceted pebbles and outcrops are found in cold
and hot deserts…..
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Effects of Aeolian Processes
• Ventifacts: faceted pebbles and outcrops are found in cold
and hot deserts and…
…on other planets
Wind Erosion Landforms
• Deflation: removal of topsoil by intense wind erosion
lowers elevation of region.
Effects of Aeolian Processes
• Blowouts: depressed areas affected by severe deflation
Effects of Aeolian Processes
• Desert varnish:
concentration of insoluble
products on land surface
Effects of Aeolian Processes
• Desert pavement:
result of deflation
Aeolian Landforms (Deposition)
• Produced solely by wind
action
• Transported material:
– Bed Load: Sand
– Suspended Load: Silt/Clay
• No dissolved load
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Aeolian Landforms (Deposition)
• Produced solely by
wind action
• Transported
material:
– Bed Load: Sand
– Suspended Load:
Silt/Clay
• No dissolved load
Sand Dune Formation
• Asymmetric cross section:
– Windward slope: low angle slope
facing wind direction
– Lee slope: high angle slope facing
opposite the wind direction
• Cross-bedding forms when lee
slope fails
Dune Types
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Barchan
Transverse
Linear
Draas
Ergs
Dune Types
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Barchan
Transverse
Linear
Draas
Ergs
Dune Types
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Barchan
Transverse
Linear
Draas
Ergs
Dune Types
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Barchan
Transverse
Linear
Draas
Ergs
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Dune Types
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Barchan
Transverse
Linear
Draas
Ergs
Dune Types
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Barchan
Transverse
Linear
Draas
Ergs
Dune Types
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Barchan
Transverse
Linear
Draas
Ergs
Dune Types
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Barchan
Transverse
Linear/longitudinal
Draas
Ergs
Dune Types
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Barchan
Transverse
Linear
Draas
Ergs
Dune Types
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Barchan
Transverse
Linear
Draas
Ergs
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Dune Types
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Barchan
Transverse
Linear
Draas
Ergs
Dune on Maps
Only large
(high) dunes
and dune fields
are visable on
topographic
maps
Erg = Sand Sea
Upcoming Stuff
Homework
Continental Glaciation Lab (due Friday)
Next Lecture:
Geological Maps (Change)
Friday’s Lab:
Map interpretation (Geological structures) (Change)
Coming Up Soon: Final Lab Exam Friday Nov 20th
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