The individual slides for this presentation (and many “spares”) can be found on Flickr at http://www.flickr.com/photos/geojuice/sets/72157625747481392/ . Enjoy.
Namibia is an arid country that has a semi- arid desert (100-200mm per annum) on its south eastern edge (Kalahari) and a severe desert (less than 50mm per annum) on its western edge (Namib). A desert is an area which receives less than 250mm of precipitation. This definition is often contested. Why?This map is courtesy of the Digital Atlas of Namibia. Higher resolution download available from www.uni-koeln.de/stb389/e/e1/download/atlas_namibia
Wind, by itself, can remove only dry unconsolidated deposits. This process of lowering the land surface through the removal of clay, silt and sand particles is called deflation. Wind and fluvial (water) erosion, transportation and deposition have resulted in the formation of three main types of desert surface.
The rocky desert (the HAMADA of the Sahara) - a desolate surface of bedrock with patches of rubble and sand. An example of hamada is provided here by the Luderitz peninsula. Chemical weathering by salt appears to be contributing to this landscape. (see the notes on Death Valley - Ventifact Ridge)
An example of a stony desert (the REG of the Sahara) - with a surface covered by gravel, pebbles or even boulders.
An example of a sandy desert (the ERG of the Sahara).
Geology, relief and water determine the landscapes of Namibia. A fourth major desert landscape type has to be mentioned when studying Namibia. Numerous PANSor dry lake beds are to be found in Namibia. One of these pans, Etosha, is large enough to be seen from space as shown here.
These large dry lake beds occupy large basins or depressions which have been enlarged and deepened by wind deflation / ablation. The excavation of these basins is limited by the level of the water table. Once the desert floor has been lowered to the level of the ground water, it is difficult for the wind to pick up the moistened particles. The base level for wind action is that of the water table. So what happens when the water table drops? (see the Flickr set on Dead Vlei)
There are two kinds of pan surface in Namibia. Etosha is a SALT PAN. It does flood occasionally but the main cycle of water is that of groundwater being drawn upwards through the crust of the pan to be evaporated off on the surface leaving behind a variety of salts.
Sossusvlei is a CLAY PAN where the water table is a considerable distance below the surface for most of the year and the main movement of water is downwards leaving behind a clay residue on the pan surface. The infiltration capacity of the clay is low and surface water lasts days or even months, depending on the depth of the inundation.
The variety of desert landscapes is breath-taking. This map locates the main landscape components of this fascinating country and can be used as a key to the images in this section.This map is courtesy of the Digital Atlas of Namibia. Higher resolution download available from www.uni-koeln.de/stb389/e/e1/download/atlas_namibia
Inselbergs are common in most deserts. These are isolated residual hills which project above the weathered material which surrounds them.
A thin layer of closely packed gravel protects many of the plain surfaces in Namibia. This desert pavement protects the underlying sediment from deflation. It is also sometimes referred to as desert armour or desert mosaic. These lag deposits usually form a thin layer over predominantly finer material. Lag deposits usually form as a result of deflation from poorly sorted deposits such as alluvium.
Desert! Some transitional zones are difficult to categorise!