ARID AND EOLIAN LECTURE NOTES
VOCABULARY: wind; saltation; deflation; desert pavement; blowout; abrasion; ventifact; dune; sand supply; wind effectiveness; vegetation cover; barchan; slip face; parabolic; transverse; longitudinal; crossbedding; loess; early; middle; late; block faulted mountain; alluvial fan; bajada; playa; interior drainage; canyon; pediment; inselberg; exotic stream; arroyo; knick point; badlands; caprock; mesa; butte
agent--wind; process--eolian
wind is the least effective of all erosional agents, does considerably shape the land
most effective in areas that are 1) arid, 2) along seashores, 3) flat areas, and 4) areas of glacial deposition
sand moves by saltation--short jumps, usually below 3"
sand 1/100", silt and clay 1/400"
wind erodes the land surface by
- DEFLATION--picking up and removing particles--MOST IMPORTANT EFFECT OF WIND
removes the smallest particles and leaves larger ones as surface covering--desert pavement
also excavates hollows and depressions--blowouts
- ABRASION--(grinding)--particles carried by wind strike surfaces--ventifacts--polished and faceted
(gently curved, shiny surfaces) fragments of rocks, may have several facets
Deposits
form on deserts, plains, floodplains, beaches
when wind slows down, it drops material; size of particle and amount depends on velocity
Dunes--hills of wind-blown sand--need strong wind, sufficient sand
- 1) an obstruction slows down wind, makes deposition
- 2) windward side more gentle because of flattening action of wind
- 3) dunes migrate downwind as particles are blown across dune
If mean wind direction is nearly constant, sand dunes form as a function of three variables:
- sand supply
- wind effectiveness
- vegetation cover
Dunes
types of dunes
- 1) BARCHAN or crescent--form when wind blows around edge of dune, steep slip face is concave leeward
(downwind) face; dry vegetation-free environment, migrate across non sand surface (gravel, clay), several tens of meters per year, symmetrical = nearly constant wind direct, sand ripples on windward side
- 2) parabolic dunes--single, appear in groups, horns point upwind, hairpin like form, slip face is convex,
sand into moist vegetated area which retards motion of base
Indiana Dunes are coastal blowout dunes which are parabolic
- 3) transverse dunes (anklé)--linked barchans with some parabolic, right angle to wind, effective winds one
direction
- 4) longitudinal dunes--parallel to wind, especially abundant sand, very long, high, strong winds, different
directions, whale backs are smooth, seif are knife-edged, complex
Dune Illustration
crossbedding--internal structure of sand dunes, steeply inclined layers of sand from sand building upon slope, can
change
loess--wind-blown deposit of silt-sized particles, may be several hundred feet thick blanket, unweathered angular fragment, almost 90° angle of repose, easily eroded, good soil
sources--desert regions or deposits left by glaciers
erosional processes in arid region differ from humid
- low precipitation
- sparse vegetation
- poorly developed soil profile
- large amount of loose erosional debris
3 stages of erosion
- early--maximum relief due to folding or faulting
block faulted mountain
alluvial fans; bajada--coalescing alluvial fans
shallow temporary lakes in central part of basin--playas
interior drainage--does not reach sea
- middle--mountain dissected to canyons, peaks, divides
front has retreated, pediment begins to form--inclined bedrock erosion surface that fringes
mountains as they become smaller, concave upward profile; local relief diminishes
- late--filling of basins continues
pediment grows larger
inselbergs--island-like remnant of the mountain remains
lots of debris
exotic stream--flows through an arid region from elsewhere
streams erode--ARROYO--water only after a rain
valley widening, slope remains fairly constant
knick point--distinct angular break
horizontal sedimentary rocks
badlands--form from erosion of fine grained clastics into rough topography in an arid or semi-arid area
when alternating resistant and non-resistant rocks in arid region
CAPROCK--resistant top layer of bedrock which protects underlying rock
mesa--a large flat-topped plateau or upland formed by the slowed erosion of a caprock
butte--a small mesa; forms as a mesa becomes dissected
Arid, Eolian, and Coast Block
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