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From the latin vallis, a valley is a kind of plain located between two mountains or hills. It is in fact a depression of the Earth's surface between two slopes, with a tilted and enlarged form. On the side of a Valley, the waters of a river can move (in the case of river valleys) or to accommodate the freezing of a glacier (glacier valleys).
A Valley can occur for many reasons, such as the erosion produced by a watercourse or tectonic movements. The Valley may take different forms depending on its origin and antiquity.
The younger valleys have a V-shaped, because the slopes are little shaped by erosion. When the erosion progresses, we speak of alluvial valleys, which have a wide, flat bottom. U-shaped valleys, generally of glacial origin, have a concave Fund and steep walls.
In addition, there is necessary to distinguish between the longitudinal valleys (they orient themselves parallel to the folds of the Cordillera/mountain) and transverse valleys (they are perpendicular to the Cordillera).
There are many important valleys to the world stage. The great rift valley, for example, has an extension of 4.830 kilometres and is home to the highest mountain in Africa, namely, the Kilimanjaro mountain, which rises to 5,895 metres above the sea level.
The Valley of the Nile, on the other hand, although it is much smaller (with approximately 19 kilometres wide), it has great historical significance of the fact of having made possible the development of civilization in Egypt.