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are shaded in black. A distinct feature of arid upland regions in crystal-line rocks is the very prominent breaks of slope on hill crests or hill sides which result from the relative resistance to weathering and erosion of dykes which run in parallel or intersecting lines. These features are accordingly represented though in other climatic zones they would be insignificant.

Hillslope features are distinctly different where the slope is undercut by river action from those areas where the hill profile has developed by retreat of the slope. In one case an oversteepened slope occurs with a sweeping debris slope while where slope retreat occurs weathering and mass wasting will have picked out the rock details. Distinction between the two is important and in the latter case a dotted linę marks the edge of the hillslope in the undercut slope a film linę is used.

Features due to wind action are prominent in arid and semi-arid regions and their representation presents special problems. Sheet sand is very common and may form a thin mantle over wide areas. In very arid regions it may be best left blank or indicated by a widely spread symbol. Sand drifts of various thickness mantle the windward sides of hills and can overtop the hill features without having any well defined limits. Sand shadows are leeside depositions with marked crests. In most parts of the Sudan sand accumulations of these types associated with topographic features are morę common than the free moving dunes. The mobile dunes are morę common only where plainland topography is found together with a plentiful sand supply. Two important smali features are sand drifts, smali lines of sand accumulation 1-2 metres high usually aligned parallel to the prevailing winds, and mounds of silt and sand either at present accumulating around vegetation or remaining from such an accumulation well after the vegetation has disappeared. Such mounds may reach 4-6 metres high and be important and lasting local features.

Included under features associated with wind action are the lag gra-vels common in the arid zonę. They may be madę up of large stoney gra-vels or smaller pea iron size gravel remnants. Both are often a surface accumulation due to the removal of the finer materiał by wind and they often do not represent the grade size rangę to be found one or two feet underground. In such areas there may be considerable thickness of sand or finer materiał but they are mainly protected from wind erosion by the ‘lag’ cover.

Rather special symbols are also needed to represent the distinctive type of water action associated with the arid zonę. Most streams are seasonal but even in the driest country water action makes its impression on a large part of the topography. Incised water courses are distinguished by their depth of incision and the relation between valley size and present flow rates are indicated by the flow channels represented in the stream bed. On gently sloping sandy surfaces and on some limited rock platforms sheet wash occurs, though often water moves in very smali 10 cm. deep channels over the surface. This type of wash is represented by unconfined arrows.

In stream beds or in depressions varying grades of sediment are found and these are represented by relevant symbols. Terraces obviously related to the present stream courses are designated with an additional symbol. These morę extensive older surfaces are classified with the plainlands, though if any reliable age dating and distinction is available

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