18 the topography of arid lands

“Probably the most basic fact of desert Probably the most basic fact of desert geomorphology is that running water is by far the geomorphology is that running water is by far the

most important external agent of landform most important external agent of landform development .”development .”

— — McKnight and Hess, p. 530.McKnight and Hess, p. 530.

v A Specialized Environmentv Running Water in Waterless Regionsv Characteristic Desert Surfaces—Ergs, Regs,

and Hamadasv The Work of the Windv Two Characteristic Desert Landform

Assemblages in U.S. Deserts

The Topography of Arid Lands

A Specialized Environment• Special Conditions in Deserts

– Mechanical weathering dominates, e.g. salt wedging– Soil and regolith are thin, leading to rocky terrain (below)

-Fig. 18-1

Special Conditions in Deserts (continued)

– Impermeable surfaces predominate e.g. caprocks (bedrock) and hardpans (hardened soil). So the little rain that falls, ends up as run-off. Never gets into groundwater, were plants can draw it up later.

– Sand & Wind – but not all deserts have sand or have landforms shaped mainly by wind. See pg 529-530.

– Rainfall – occurs as intense convective thunderstorms which leads to brief flooding, and fluvial deposition.

– Interior drainage that does not lead to the sea, Fig. 18-3. Also, see pg 530, #9.

– Lack of continuous vegetation cover, Fig 18-4, pg 531.

- Fig. 18-2

Running Water in Waterless Regions• Significance of Running Water

– Aeolian (wind) processes is less significant

– Sparse vegetation leads to overland flow erosion, and hence large volumes of sediment moved within a short time.

• Surface Water in Deserts– Exotic Streams: fed by sources outside

the desert. E.g. the Nile travels 2,000 miles thru the Sahara, without additional tributaries! Pg 531-532. (But its entire length with tributaries, is actually 4,000!).

– Fig. 18-5

– Ephemeral Streams– Desert Lakes

• Playas (dry lake beds – see next slide), or Salinas, pg 546-548• Playa lakes tend to be temporary. Permanent playa lakes are

mostly salty e.g. Great Salt Lake, UT.

– Fig. 18-6 and 18-7

– Fig. 18-8. Dry lake bed (playa) in central Nevada.

• Fluvial Erosion in Arid Lands– Differential Erosion (common in sedimentary layers)

• Differential hardness of rock layers (see next slide)

– Fig. 18-1

Hard rock

Soft rock

– Fig. 18-9. The effects of differential erosion on the Red Cliffs near Gateway in Western Colorado.

– Residual Erosional Surfaces• They form Inselbergs (“island mountains”)

– Particularly a type called Bornhardts (resistant rocks that are round). This is how Uluru (Ayers Rock), in Australia was formed.

Fig. 18-11. The development of a bornhardt.

– Fig. 18-10. Kata Tjuta (the Oglas) in the desert of central Australia.

– Desert Stream Channels or Wadi– They’re called “Wadi” in Arabic. E.g. the Wadi of Egypt.

– Fig. 18-13. Mojave Desert, near Baker, CA.

• Fluvial Deposition in Arid Lands– These depositional features are more obvious than

erosional features, and they occur in ephemeral stream channels

– Piedmont zone. This is any zone at the foot of a mountain range where you find:

• Talus accumulations and alluvial fans

– Fig. 18-14. Idealized cross section of a desert piedmont zone.

Characteristic Desert Surfaces—Ergs, Regs, and Hamadas

• Erg—A Sea of Sand

– ‘Erg’ – Arabic for sand

– Found in the deserts of Arabia, the Sahara and Namibia.

– The Arabian desert is the sandiest of all deserts; and even then, only a third of it is covered in sand. Moreover, most of that sand is not ‘true erg’.

– “Relict ergs” (covered with vegetation) are found in Western Nebraska. The “Sand Hills” on Hwy 2.

– Fig. 18-15

• Reg—Stony Desert‘Reg’ – Arabic for “stone”– Desert pavement– Called ‘gibber plains’ in

Australia

• Hamada—Barren Bedrock‘Hamada’ – Arabic for “rock”– Comprises of exposed

bedrock or cemented sedimentary material (these are large expanses of bedrock, as opposed to stones scattered all over in a “Reg”). Is the picture a Reg or a Hamada?

– Fig. 18-16

The Work of Wind• Aeolian Erosion

– Abrasion – wearing away of rock by airborne sand and dust particles

– Deflation – shifting of loose particles

– Fig. 18-17 and 18-18

Sand-blasted rock, or ventifact

Wind deflation

• Aeolian Transportation

Animation (Wind Transportation of Sediment)

– Suspension, saltation, traction and Creep (just like water movement).

– Fig. 18-19

• Aeolian Deposition

Animation (Desert Sand Dunes)

– Desert Sand Dunes• Vegetation and Dune Stabilization• Dune migration

– Fig. 18-20

– Types of Sand Dunes (see next slides)• Barchan• Transverse• Seif

– Fig. 18-21

– Barchan in Namibia (also found in the Desert Dome of the Henry Doorly Zoo)

- Fig. 18-22

– Seif dunes in Simpson Desert of central Australia

- Fig. 18-23

– Loess• Another aeolian feature that can be outside of deserts• It is wind-blown silt; fine texture, no horizontal layering

– Fig. 18-25

• Major loess deposits of the world.

– Fig. 18-26

Two Characteristic Desert Landform Assemblages in U.S. Deserts

– Fig. 18-27

• Basin-and-Range Terrain

• Mesa-and-Scarp Terrain

• Basin-and-Range Terrain– Ranges

– Fig. 18-28

– Piedmont Zone• Alluvial fan

– Fig. 18-29

– Basin• Playa (dry lake bed)

– Fig. 18-30

– Basin• Salt pan

– Fig. 18-33. Death Valley, CA

• Mesa-and-Scarp Terrain– Horizontal layers of

differential rock• ‘Mesa’ is Spanish for

‘table’

• Slopes have multiple scarps. ‘Scarp’ is short for ‘escarpment’

• Due to differential weathering: the resistant rocks become ‘scarps’ and softer rock become gentler inclined slopes.

– Fig. 18-35

– Residual landforms• Flat-topped summits in horizontal sedimentary strata• Hard cap rock; if they are bounded on one or more sides

by a prominent escarpment, they’re called plateaus. • Plateaus are “tablelands” (bigger than Mesas)• Buttes are just eroded remnants of mesas• Pinnacles are the smallest remnants

– Fig. 18-36

– Fig. 18-37. Mitten Buttes, Monument Valley, AZ.

– Badlands• Innumerable ravines and gullies (see photos).• Steep slopes.

– Fig. 18-38.

– Arches and Natural Bridges• Combination of weathering and fluvial erosion.• See video link, below

Arches, Figure 18-39

http://www.nps.gov/arch/photosmultimedia/index.htm