1.
Chapter 18: The Topography
of Arid Lands
McKnight’s Physical Geography:
A Landscape Appreciation,
Tenth Edition, Hess

2.
The Topography of Arid Lands
• A Specialized Environment
• Running Water in Waterless Regions
• Characteristic Desert Surfaces—Ergs, Regs,
and Hamadas
• The Work of Wind
• Two Characteristics of Desert Landform
Assemblages
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3.
A Specialized Environment
• Desert terrain stark and
abrupt
• Desert special conditions
– Weathering: mechanical
weathering dominant,
slower weathering and
angular particle formation
– Soil and regolith: soil is thin
or absent, exposing
bedrock
– Soil creep: minor due to
lack of soil and lubricating
effects of water
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Figure 18-1

4.
A Specialized Environment
• Desert special conditions
(cont.)
– Impermeable surfaces:
caprocks and hardpans, high
water runoff
– Sand: some deserts have sand
abundance, allows for water
input into the ground, easily
moved by rain and wind
– Rainfall: limited, most streams
are ephemeral, effective agents
of erosion, alluvium unusually
common in deserts
4© 2011 Pearson Education, Inc.
Figure 18-2

5.
A Specialized Environment
• Desert special conditions
(cont.)
– Wind: wind action shifts
particles
– Basins of interior drainage:
most watersheds do not drain
into any ocean, water
transferred to basin or valley
with no external outlet
– Vegetation: lack of continuous
vegetative cover
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Figure 18-4

6.
Running Water in Waterless
Regions
• Running water most important external landform
agent
• Erosion tremendously effective with little plant cover
• Intensity of rain combined with impermeable land
surfaces create intense runoff
• Unpredictable imbalance between erosion and
deposition
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7.
Running Water in Waterless
Regions
• Surface water in the desert
– Exotic streams: permanent
streams that originate outside
of the arid land (i.e., Nile
River)
– Ephemeral streams:
periodically flow, result in
intense erosion,
transportation, and deposition
– Desert lakes: playas and
salinas (dry salt lake beds),
saline lakes
7© 2011 Pearson Education, Inc.
Figure 18-5

8.
Running Water in Waterless
Regions
• Fluvial erosion in arid lands
– Occurs during small portion of
the year, flash floods
– Differential erosion: variations
in slope and shape of landform
from rock type variations
– Residual erosional surfaces:
inselbergs (i.e., bornhardts),
pediments
– Desert stream channels:
ephemeral stream beds
8© 2011 Pearson Education, Inc.
Figure 18-6

9.
Running Water in Waterless
Regions
• Fluvial deposition in arid lands
– Talus accumulations at the
foot of steep slopes
– Piedmont: zone at the foot of
a mountain range
– Piedmont angle
– Basins of interior drainage
covered with fine particles
since flow volume and speed
are low
9© 2011 Pearson Education, Inc.
Figure 18-13

10.
Characteristic Desert Surfaces:
Ergs, Regs, and Hamadas
• Ergs—seas of sand
– Large area covered with sand
in dune formation from wind
– Hypothesized to have
originated in a more humid
climate
– Drying of climate combined
with wind created formations
seen today
– Sahara and Arabian deserts
10© 2011 Pearson Education, Inc.
Figure 18-14

11.
Characteristic Desert Surfaces:
Ergs, Regs, and Hamadas
• Regs—stony deserts
– Tight covering of coarse
gravel, pebbles, and/or
boulders
– Desert pavement or desert
armor
– Desert varnish: dark, shiny
coating consisting of iron and
manganese oxides
– Desert varnish is a useful
dating tool
11© 2011 Pearson Education, Inc.
Figure 18-15

12.
Characteristic Desert Surfaces:
Ergs, Regs, and Hamadas
• Hamada—barren bedrock
– Barren surface of consolidated material
– Exposed bedrock or cemented sedimentary material
– Regs and hamadas extremely flat
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13.
The Work of Wind
• Wind as a sculptor is a
relatively limited effect
• Air right near surface has
zero wind
• Wind speed increases
with distance above
ground
• Effects of wind shear
• Aeolian processes
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Figure 18-16

14.
The Work of Wind
• Aeolian erosion
– Two effects, deflation
and abrasion
– Deflation: shifting of
loose particles via the
wind, blowouts
– Abrasion: requires tools
such as airborne sand
and dust, sculpts
landforms already in
existence, ventifacts
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Figure 18-17

15.
The Work of Wind
• Aeolian transportation
– Only finest particles are
carried in suspension as
dust
– Dust storms
– Larger particles moved
by saltation (curved
trajectory) and traction
(rolled or pushed)
– Creep by saltation
15© 2011 Pearson Education, Inc.
Figure 18-18

16.
The Work of Wind
• Aeolian deposition
– Fine sand laid as thin coating,
no landform significance
– Coarser sand deposited
locally, sand plains or sand
dunes
• Desert sand dunes
– Some dune fields composed
of unanchored sand, moved
by local winds, slip face
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Figure 18-19

17.
The Work of Wind
• Desert sand dunes (cont.)
– Three most common dunes
• Barchan—individual dunes
migrating across landscape,
crescent shaped
• Transverse—supply of sand
greater than for barchans,
crescent shaped, but entire
landscape made of these dunes
• Seifs—long, narrow dunes that
are parallel, orientation seems
to represent an intermediate
direction between two dominant
wind directions
17© 2011 Pearson Education, Inc.
Figure 18-20

18.
The Work of Wind
• Coastal dunes
– Ocean waves deposit sand
on beaches
– Prominent onshore winds
move sands inland
• Loess
– Wind deposited silt
– Lacks horizontal stratification
– Great vertical durability
– Formation not well understood
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Figure 18-24

19.
Two Characteristic Desert
Landform Assemblages
• Basin and range terrain
– Largely without external
drainage
– Numerous fault-block
mountain ranges
– Three principle features
• Ranges
– Surface features shaped by
weathering, mass wasting,
and fluvial processes
– Long, narrow ranges of
different elevations
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Figure 18-27

20.
Two Characteristic Desert
Landform Assemblages
• The piedmont zone
– Sharp break in slope that
marks change from range
to piedmont
– Underlain by erosional
pediment
– Alluvial fan: channels on
piedmont break into
distributaries, deposit new
material on old material
– Coalescing alluvial fans
– Piedmont alluvial plain, bajada
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Figure 18-28

21.
Two Characteristic Desert
Landform Assemblages
• The basin
– Flattish floor, very gentle slope on all sides towards a low
point
– Shallow, ill-defined drainage channels
– Salt accumulations commonplace on playa due to
evaporation of water
– Playa lakes
– Basin floor covered in fine grain material
– Death Valley is a prime example of a basin and range
terrain
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22.
Two Characteristic Desert
Landform Assemblages
• Death Valley
– Excellent example of
basin and range terrain
– Graben, large portion of
valley is below sea level
– Surrounding mountain
ranges
– Piedmont at foot of the
mountains is alluviated
into a complex fan structure
– Basin filled with alluvium
– Salt pans and mobile dunes
in the basin 22© 2011 Pearson Education, Inc.
Figure 18-29

23.
Two Characteristic Desert
Landform Assemblages
• Mesa-and-scarp terrain
• Mesa—Spanish for “table,” flat
topped surface
• Scarp—short for “escarpment,”
pertains to steep cliffs
• Associated with horizontal
strata
• Variable resistance to erosion
in strata
• Plateaus and stripped plains
23© 2011 Pearson Education, Inc.
Figure 18-32

24.
Two Characteristic Desert
Landform Assemblages
• Sapping—groundwater seeps
out of the scarp face and
erodes soluble material
• Buttes—small surface areas
and cliffs that rise above
surroundings
• Pinnacles
• Buttes, mesas, and pinnacles
typically found near a retreating
escarpment face
24© 2011 Pearson Education, Inc.
Figure 18-34

25.
Two Characteristic Desert
Landform Assemblages
• Badlands
– Overland flows from occasional
rains develop tiny rills that
expand into ravines or gullies
– Characterized by maze of
ravines and gullies, lifeless and
nearly impassable
• Arches and natural bridges
– Arch formation
– Natural bridge formation
– Pillar formation
25© 2011 Pearson Education, Inc.
Figure 18-35

26.
Summary
• Desert topography is abrupt and stark
• Desert terrain has ten primary characteristics that
separate it from other terrain types
• Running water is the most important land formation
mechanism is deserts
• Surface water is relatively uncommon in desert regions
but does exist in isolated lakes and streams
• Fluvial erosion and deposition result in most of the land
formations that exist in arid regions
• Ergs are vast expanses of sand in desert regions
26© 2011 Pearson Education, Inc.

27.
Summary
• Regs are stony deserts, consisting of rocks, boulders,
and pebbles
• Hamadas are regions of barren, exposed bedrock that
is subject to mechanical weathering
• Wind acts as a sculptor of arid rock formations,
although it plays a minor role in the formation of arid
topography
• There are two primary effects of aeolian erosion
• Different sized particles interact with desert winds in
different ways
27© 2011 Pearson Education, Inc.

28.
Summary
• There are two primary desert land formations
• The basin-and-range formation consists of a basin
surrounded by mountain ranges
• Death Valley is a classic example of a basin-and-range
desert land formation
• The mesa-and-scarp land formation is made up of a
flat-topped terrain surrounded by steep slope
• Different compositions of the rock material result in
different orientations of the land form structure
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