Deserts Geologic processes in arid climates Role of water in arid climates Streams are dry most of the time Desert streams are said to be ephemeral Flow only during periods of rainfall Different names are used for desert streams including wash, arroyo, wadi, donga, and nullah

Deserts Geologic processes in arid climates Role of water in arid climates Desert rainfall Rain often occurs as heavy showers Causes flash floods Poorly integrated drainage Most erosional work in a desert is done by running water

A dry stream channel in the desert Figure 6.28 A

The same stream channel following heavy rainfall Figure 6.28 B

Deserts Basin and Range: the evolution of a desert landscape Uplifted crustal blocks Interior drainage into basins produces Alluvial fans and bajadas Playas and playa lakes

Deserts Basin and Range: the evolution of a desert landscape Erosion of mountain mass causes local relief to continually diminish Eventually mountains are reduced to a few large bedrock knobs called inselbergs projecting above a sediment-filled basin

Landscape evolution in a mountainous desert – early Figure 6.29 A

Landscape evolution in a mountainous desert – middle Figure 6.29 B

Landscape evolution in a mountainous desert – late Figure 6.29 C

Deserts Wind erosion Deflation Abrasion Lifting of loose material Produces Blowouts Desert pavement Abrasion

Formation of desert pavement Figure 6.34

Deserts Types of wind deposits Loess Deposits of windblown silt Extensive blanket deposits Primary sources are deserts and glacial stratified drift

Deserts Types of wind deposits Sand dunes Mounds and ridges of sand formed from the wind’s bed load Characteristic features Slip face – the leeward slope of the dune Cross beds – sloping layers of sand in the dune

Formation of sand dunes Figure 6.37

Sand dune types Figure 6.38

Sand dunes in the western United States