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An oasis is an isolated area where water appears to support vegetation surromded by desert. Oases are formed by underground rivers or aquifers where water can reach the surface even in vast desert areas. This is done by natural pressure or by man made wells. Substrata of impermeable rock can trap large volumes of water and retain it in vast pockets. Other structures like long faulting subsurface ridges or volcanic dikes water can collect water which percolate to the surface. Before the arrival of man, desert oasis were found by migrating birds who through their dropping deposit seeds allowing natural vegetation to develop. Sometimes there is only enough water to to sustain a small well. In other cases a small area of land can be irrigated to support a limited population. The location of these oases has been of vital importance for trade across the Sahara and other desert areas. Before the appearance of the camel, oasis were not common enough or close enough to allow trade to flow. As a result, the primary conduit for Sub-Saharan trade to reach Europe and the Middle East was the Nile. There are oases in the Sahara, but before the cammel caravans could not reach the scattered oasis, caravans had to travel for days to reach the scattered oasis.
This changed with the introduction of the camel. The camel gave traders the capability of crossing vast tracts of desert by connecting the widely dispersed Sahran oasis. It is no accident that the great sub-Saharan African empires began to form after the camel made possible the development of trans-Sharan trade routes centerd around the desert oasis. Caravans must transit the Sahara via the oases so that neededsupplies of water and food can be replenished. Camels can travel long distances without water, but they need water like any animal. And of course humnan exhaust the awter carried by camels. Thus control of a desert oasis meant control of trade routes which was of vast econimic and political importance.
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