Dunes

As I have previously mentioned, desert landscapes are indeed unique and owe their characteristics to the weathering and erosion processes which occur.

One aspect which I did not talk about in detail is abrasion. The process of abrasion can be compared with the effect of sandblasting a building. Wind carries sediment in suspension and ‘blasts’ it against the rock usually wearing it smooth. In desert environments it wears away layers of soft rock close to the ground surface resulting in spectacular pillars of slightly more resistant rock. Monument Valley, which spans the border of Utah and Arizona in the USA, is an excellent example.

Of course we cannot discuss desert landforms without mentioning dunes. This would be the first image that many students will associate with deserts and as such, they cannot be ignored. Dunes typically form in sandy deserts where the wind is strong enough to transport material and the surface is subsequently moulded into dune shapes. There are various different types of dunes and the strength, direction and consistency of wind all play an important part in what type of dunes will be formed.

Wind does not flow in a uniform pattern. It meanders and eddies, moulding the surface into a shape which follows the path of least resistance. The most common dune studied is probably the barchan. As shown in the picture these are crescent shaped and can reach heights of around 30 metres. It is important to note that these dunes are not fixed but they migrate in the direction of the wind. The diagram below attempts to explain this.

Dunes move as sand on the windward slope is carried over the crest and deposited on the slip face where the wind slows and therefore cannot carry its load due to a lack of energy. There are other types of dune that need to be discussed such as parabolic, transverse and longitudinal there are even more but we are probably becoming too complicated if we go any further.

Once the basic concepts of dune formation have been taught they students could be given a group exercise and attempt to describe the formation of the above dunes (parabolic, transverse, and longitudinal) based upon the information already provided. Their answers could then be discussed with other groups within the class and either confirmed or rejected. Group exercises allow for ideas and information to be processed on a larger scale and allow for a contribution from all students in the class and not just those who volunteer to answer questions.