|Pediments are desert plains – gently sloping areas of rock (usually covered in a thin layer of debris). They’re formed by the erosion of rock by sediment carried in sheet floods or small streams.
|Playas are shallow, extremely flat depressions at the low point of a pediment.
Temporary lakes periodically form in them when water drains into them after rain. The water quickly evaporates because of the high temperatures, and leaves behind layers of salt, silt or clay.
|Inselbergs are steep-sided hills that rise up from pediments.
They’re made of hard rocl that’s more resistant to erosion than the surrounding rock.
The surrounding rock is eroded by water, leaving the harder rock standing out. Inselbergs can also be formed by wind erosion.
|A wadi is a gully or ravine that’s been eroded by seasonal rivers, or by rivers in the past, when the climate was wetter.
Depending on the strength of the river, a wadi can have shallow or very steep valley sides. It can fill up with rainwater very quickly.
|Wadi Howar is 1100km long
|Alluvial Fans & Bahadas
|When there’s a flat desert plain at the mouth of a wadi, the water in a channel flash flood spreads out on to the plain.
This leads to sediment being deposited as energy is dissipated, forming an alluvial fan.
If there are several wadis whose mouths are close to one another, a bahada might form – this is where several alluvial fans have spread out and joined together, creating a uniform slope of sediment.
|Depressions in the ground.
Deflation removes a lot of material from one place – they often where material particularly fine
If hollow is deep enough, it will reach the water table – this prevents deflation from deepening it any further.
|Qattara Depression in Egypt covers 18,100 km2.
|Surfaces of interlocking stones
Wind blows away the silt and sand from the desert surface, leaving behind the gravel, rocks and pebbles that are too heavy (reg) to be removed by deflation.
|Gibber Plains, Australia are 100km
|Narrow, streamlined ridges that are usually three to four times longer than they are wide.
Strong unidirectional wind carry sand in suspension which erodes rocks by abrasion.
Softer rock (Sandstone) is eroded faster than harder rock, so ridges of hard rock are created (yardangs) due to differential erosion. The ridges(strata) aren’t always continuous. Overtime will look like upturned boat hulls as weakened heavily jointed rock erodes.
Have notch at base – Meso size: 10m high or mega: 100m high
|Tibestic Mountains, Central Sahara
|Long, block-shaped ridges of rock (1 ridge = zeuge).
Layer of hard rock sits above a layer of softer rock.
Cracks form in hard rock, weathering = jointed
Wind can erode through the cracks and into the softer rock beneath by abrasion
Softer rock eroded more than hard rock, and ridges (zeugen) are formed- top eventually collapses
|Wadi Rum, Jordan
|Individual stones with one or more smooth sides that have been abraded.
Side of rock that faces prevailing wind is abraded most, leaving a flattened face.
Rock may move, or the direction of the prevailing wind may change = different part is abraded
= a combination of smooth faces and sharp edges.
|White Desert National Park, Egypt
|SAND DUNES- sand grains carried by suspension are deposited as the wind slows down.
|Barchan dunes (crescent-shaped)
|Isolated dunes that develop from mounds of sand.
1. Form in direction of prevailing wind as sand is deposited when it hits sand catcher. Sand builds up n leeward side to form small dune.
|The Great Sand Sea covers a 72000km2
|Seif dunes (long wiggly lines)
|1. Seif dunes are long wiggly ridges of sand.
2. They form from barchan dunes if a change of wind direction occurs.
3. When wind blows from alternate sides the ‘arms’ of barchan dunes are elongated and from a wiggly line.
|The Great Sand Sea covers a 72000km2
- Individual landforms combine to form landscapes
- Hot desert landscapes can be dominated by erosional landforms, such as pediments and inselbergs, or depositional landforms, such as sand dunes and bahadas.
- They can also be dominated by aeolian landforms (those formed by wind) or fluvial landforms (those formed by water). This may depend on how frequent rainfall is in the area.
Process of alteration:
- Processes operating in desert systems can create new landforms or change existing landforms. This means that desert landscapes change over time. E.g.
- A change in inputs can change the landscape, e.g. an increase in sediment input may increase deposition. Over time, a landscape may change from one dominated erosional landforms to one dominated depositional land forms.
- Landforms that were made by past processes can still experiecne change, e.g. a wadi formed by a river flowing thousands of years ago might be eroded further every time it’s flooded during a rainfall event.
- Hot desert landscapes are therefore often made up of a mixture of landforms that reflect different periods of change. E.g. there may be landforms that were formed when the region had a more humid climate and landforms formed during current climatic conditions.
- Changes occur over a range of spatial and temporal (time) scales. For example, changes can vary from short and episodic (e.g. short, intense rainstorms transporting material in channels) to long and gradual (e.g. wind eroding rock over thousands of years to form zeugen).
- Landscapes in hot deserts often change much more slowly than landscapes in other climatic regions, because many weathering and erosional processes require water. Some deserts can go for several years without rain, so these processes operate slowly and episodically.