ch18

Chapter 18: The Topography of Arid Lands

McKnight’s Physical Geography: A Landscape Appreciation,

Tenth Edition, Hess

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

2© 2011 Pearson Education, Inc.

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


Figure 18-1


• 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


Figure 18-2


• 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


Figure 18-4

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



• 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


• 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


• 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


Figure 18-13

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


Figure 18-14


• 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


Figure 18-15


• Hamada—barren bedrock– Barren surface of consolidated material– Exposed bedrock or cemented sedimentary material– Regs and hamadas extremely flat


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


Figure 18-16

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


Figure 18-17

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


Figure 18-18

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


Figure 18-19

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


Figure 18-20

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


Figure 18-24

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


Figure 18-27


• 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


Figure 18-28


• 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



• 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


• 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


Figure 18-32


• 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


Figure 18-34


• 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


Figure 18-35

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


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


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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Technology

characteristic

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soil

ergs

basin

regs

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