Daya do not occur in the middle of the Sahara but on its edges. We have seen what was a reg. However, a reg is never completely flat. There are always places that are a little lower (may be only a few centimeters!) When there is some rain it will be collected in theses low lands, stay a while and infiltrates in the soil. Thus, it is possible to try to grow wheat in a daya, though it will not grow each year and when driving on the road, you will notice these strange round places 50,100 or 200 m wide, where there is some vegetation, different from everything else around. Moreover, a few trees will often grow in the daya, mostly pistachio trees, betoum in Tamachek, Pistacia lentiscus for the scientist.
This photo shows only the betoum in a daya.
An oued is a riverbed and there are plenty of oued (sorry ouadi is the Arab plural for oued!) in the Sahara. However, these riverbeds are dry most of the time. They might carry water one or two days every 5-10 years but there is water staying underneath the soil and there is always more vegetation around an oued than elsewhere. There are also more animals (snakes)! Do not camp in an oued. Moreover, there is a hazard (low, I know!) of a flash flood! Do not camp in an oued
We have explained what was an oasis, though, most if not all oasis have a problem. They, use water to fertilize either the palm grove or the gardens. A part of the water gets into the soil, another evaporates and the remaining gathers in the lowest part of the oasis. However, in the beginning, this water had a given salt content, low or high. In the end, it has a very high salt content and gives a sebkha. A sebkha is a salted pond or even an area with dry salt. In Tunisia, Chott El-Djerid is a large sebkha but there are small sebkha standing along most oasis.
It is hardly necessary to explain what is an oasis. An oasis is place where there is enough water to allow the growth either of palm trees and this is a palm grove oasis (El-Oued, In-Salah for example) or the growth of vegetables and this is a garden oasis (Tamanrasset for example). A sedentary population has lived in the oasis forever.
Tassili means calcareous plateau in Tamachek. The Tassili N'Ajjer is the one that is the best known because of its famous rock art. I have built a page on the Tassili N'Ajjer National Park. There are several Tassili around the crystalline massif of the Hoggar : the Tassili N'Immidir stands in the North and I will build a page about two trecks in two different part of this little known Tassili.
Djebel means mountain in Arab. There are several massifs in the Sahara. The two that are best known are the Hoggar (Algeria) and the Tibesti (Chad). I will build a page on the Hoggar and will show the magnificent landscapes of the Tefedest and the Assekrem, the very heart of the Hoggar. Here is a sample.
A typical sight in many deserts is a “butte témoin”. In Northern America, they are called “mesa”. In the Sahara the nomads name them “gour”. They are parts of sedimentary layers that have been protected from the erosion because they were somehow harder that the neighboring rocks.
When the wind has blown off all the grains and that there does not remain any soil, you can reach the rock. This gives a Hamada. A Hamada is a place where the soil is pure rock, more or less flat. True hamada are not many. You will find more often a Hamada with a thin layer of gravel or of sand and that will look almost like a reg.
Most of the time people think that a desert is a place entirely covered with sand, with dunes, sand winds that bury entire caravans if not cities, etc.
This is not true and actually, erg (places covered with sand dunes) account for only 15% of the Sahara. When you drive across the Sahara, you will see few : roads avoid as much as possible the ergs as sand can cover a road by a thick layer of sand in a matter of a few month.
From where does the sand come? If you have read the precious tip, you have certainly guessed that it was the finest grains, dug out by the wind. As soon as the wind gets weaker or as soon as some obstacle stops the grains, sand will gather and make a crescent shaped dunes, called a barkhane: sand is stopped in the middles and escapes on the sides, hence the crescent shape (third photo) .When there are many barkhanes and that they mix, that gives an erg. The dunes have not any more the shape of a pure crescent but nevertheless; you can view it, more or less.
The fourth photo shows a reg in the foreground with an erg in the background.
Deserts are not places without any vegetation but places where vegetation is scarce. Thus the soil, when dry, is not protected by a coat of vegetation and the wind will easily carry out the finest grains, leaving the coarsest, gravel and stones. After a while, that gives a reg: small or medium stones cover the whole landscape. However, if cars pass always at the same place, they will reveal that underneath, there are also fine grains of sand. It is easy to drive on a reg, even with a regular car. However, there are stony reg and sandy reg, depending of the size of the stones remaining on the surface. In the Sahara, reg account for around 80% of the area. The Tademaït plateau is a fine example of reg with small stones.