1. McKnight's Physical Geography The Topography of Arid Lands
Lectures
Chapter 18
The Topography of Arid Lands
Andrew Mercer
Mississippi State University
© 2014 Pearson Education, Inc.

2. A Specialized Environment
Top – thinner soil
Slope – very thin soil
Foot – thicker soil
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

3. 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 – is the most important shaper of landscapes in DESERT country
Wind – also matters, but less so than even occasional rain
Even in very dry country, water is still the most important shaper of topography

4. A Specialized Environment
Desert conditions
Utah is semi-desert
Basins of interior drainage – most dry-land watersheds do not drain into any ocean –
Water usually evaporates before reaching the ocean
or a large lake.

5. Running Water in Waterless Regions
Running water most important external landform agent in dry country and desert
Erosion tremendously effective with little plant cover
Intensity of rain combined with impermeable land surfaces create intense runoff
Unpredictable imbalance between erosion and deposition

6. Running Water in Waterless Regions
Surface water in the desert
Exotic streams – the Nile is an example of a river that originates in one region and flows through an alien region to some other destination.
Ephemeral streams – periodically flow, result in intense erosion, transportation, and deposition
Perennial – fulltime flow, year-round
Desert lakes – playas and Salinas (dry salt lake beds), saline lakes
C – more mild
B climate - desert
A climate

7. 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 (e.g., bornhardts), pediments
Desert stream channels – ephemeral stream beds

8. Consider Mr. Allred’s yard – the rocky top of a hill was once the ‘armor-plated’ stream bed at the bottom of the hill. Eventually, everything else washed away, leaving the river bed as the new top of the hill.

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

10. The Work of Wind Wind as a sculptor is a relatively limited effect
Air at the surface has nearly zero wind
Wind speed increases with distance above ground

11. 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

12. The Work of Wind Aeolian (wind) 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

13. The Work of Wind Aeolian (wind) deposition Desert sand dunes
Fine sand laid as thin coating;
Desert sand dunes
Some dune fields composed of unanchored sand, moved by local winds; slip face

14. 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

15. The Work of Wind
Common types of desert sand dunes

16. Loess
Wind deposited silt
Lacks horizontal stratification
Great vertical durability
Formation not well understood
Millions of people have lived in silt caves across thousands of years.
However, when such soils are WET they can collapse during an earthquake – thousands of people may get killed.

17. The Work of Wind
Major loess (SILT) locations of the world

18. Two Representative Desert Landform Assemblages
Notice the effect of water even in a desert – occasional, sharp rain can result in vast erosion. It is not a good idea to build or occupy ‘alluvial fans’ like the one below. River deltas are similar. Watch out.

19. Two Representative Desert Landform Assemblages
Death Valley
Excellent example of basin and range terrain
Surrounding mountain ranges
Basin filled with alluvium
Salt pans and mobile dunes in the basin
Notice that salt collects in basins after washing out of mountains.

20. Two Representative Desert Landform Assemblages
Mesa-and-scarp terrain
Mesa – flat topped surface
Scarp – steep cliffs
horizontal strata
Some layers resist erosion well -- others erode quickly
Harder rock lasts longer
Softer rock erodes, allowing the whole formation to fail

21. Two Representative Desert Landform Assemblages
Monument Valley – Utah/Arizona
These features are the scattered remains of vast areas that were eroded and washed away.
Only a few “hard spots” remain – the cap rock protected softer rock nearby or underneath.
Once water got past the cap rock, the whole region washed away.

22. Two Representative 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
A small amount of cap rock
is all that remains of the entire formation.
Cap rock protects the softer rock underneath.
All of it will be gone very quickly.