Page 394 - Laboratory Manual in Physical Geology

A

Source area
Sand dunes
B Loess

EROSION TRANSPORT SILT TRANSPORT SILT
SAND SAND AND SAND SILT DEPOSITION
AND SILT AND SILT DEPOSITION
Followed by
re-erosion of silt

Erosional surface
B

Slip face
Land depositional
surface surface

Stoss side: wind erosion of inclined, previously deposited, Lee side: sand
slip face sands. Transportation of eroded sands deposition on inclined
downwind. slip face.

Net direction of dune migration through time

FIGURE 14.4 Eolian (wind-related) erosion, transportation, and deposition. A. Strong winds erode sand and silt from a source
area and transport them to new areas. As the wind velocity decreases, the sand accumulates first (closest to the source) and the silt (loess)
is carried further downwind. B. Hypothetical cross section through a sand dune. Wind erodes and transports sand up the stoss side (upwind
side) of the dune. Sand bounces onto the slip face and accumulates, forming the lee side (downwind side) of the dune. This continuing
process of wind erosion and transportation of sand on the stoss side of the dune, and simultaneous deposition of sand on the slip face of
the dune, results in net downwind migration of the dune.

resemble sinkholes (depressions formed where caves have ■ Transverse dunes occur where sand supply is greater.
collapsed), or kettles (depressions formed where sediment- They form as long ridges perpendicular to the
covered blocks of glacial ice have melted), but you can prevailing wind direction. The crests of transverse
distinguish these different types of depressions in the context dunes generally are linear to sinuous.
of other features observable on the map. Unlike sinkholes ■ Bachanoid ridge dunes form when barchan dunes are
and kettles, blowouts usually have an adjacent sand dune or numerous and the horns of adjacent barchan dunes merge
dunes that formed where sand-sized grains were deposited into transverse ridges. The crests of barchanoid ridge dunes
after being removed from the blowout. Blowouts also range are chains of the short crescent-shaped segments that are
in size from a few meters to a few kilometers in diameter. the crests of individual barchan dunes. They can easily be
distinguished from true transverse dunes that have long
Sand Dunes sinuous to very s inuous (like a snake) crests.
When wind-blown (eolian) sediment accumulates, it ■ Parabolic dunes somewhat resemble barchans.
forms sand dunes and silty loess deposits (see FIGURE 14.5 ). However, their horns point in the opposite direction—
The process of dune and loess formation is shown in upwind. Parabolic dunes always form adjacent to
FIGURE 14.4 . Some common types of dunes are illustrated blowouts, oval depressions from which come the sandy

in FIGURE 14.6 and described below:

sediments that form the parabolic dunes.
■ Barchan dunes are crescent shaped. They occur where ■ Longitudinal (linear) dunes occur in some modern des-
sand supply is limited and wind direction is fairly erts where sand is abundant and crosswinds merge to form
constant. Barchans generally form around shrubs or these high, elongated dunes. They can be quite large, up to
large rocks, which serve as minor barriers to sand trans- 200 km long and up to 100 m high. The crests of longitu-
portation. The horns (tips) of barchans point downwind.
dinal dunes generally are straight to slightly sinuous.
362 ■ L ABOR ATORY 14