G. W. Ombham— Wells of North- Eastern Sudan. 269 



Many of these wells appear to be of very ancient date. There are 

 numbers of stone implements to be found on the surface around some 

 of them, and at Geili there is a stone inscribed with hieroglyphics. 

 Many of the old wells may have been lost or forgotten daring the 

 troubles of this country, and it appears that stone implements in 

 a locality may be taken as a clue to the previous existence of water 

 supplies, but it must be borne in mind that these might be found in 

 a place where the supply took the form of rain pools and existed for 

 part of the year only. 



The wells of the Gedaref district may be mentioned here. They 

 derive their water from an outlier of the Abyssinian Plateau Basalts 

 resting on the Nubian (?) Sandstone. The natives have dug their 

 wells wherever the rock is soft enough, and the water is obtained at 

 small depths in rotten rock. 



Wells in Arid Rkgions of Crystalline Rock. 



The Red Sea is flanked by an area of crystalline rocks where deeply 

 carved valleys are partly filled with gravels and detritus, which form 

 a deposit that we may speak of as Valley Fill. This is the only 

 permeable deposit of the district, as the rocks themselves are not 

 sufficiently decomposed, at any rate on the surface, though they may 

 be so under the Valley Fill. There are exceptions to this, for in some 

 favoured places, where high hills rise close' to the Maritime Plain, the 

 rainfall amounts to much more than is usual, supports a rich flora, 

 and under these conditions the rocks are decomposed and become 

 permeable. I mention this as such places, though they cannot be 

 called arid, occur within the region, and are marked by very sharply 

 defined boundaries. 



There are no rainfall statistics that apply to this area, for the 

 existing stations are on the seaward edge of the Maritime Plain, and 

 another is at Erkowit, an oasis among the hills. There is evidence 

 that it is extremely small, and perhaps does not exceed 4 inches 

 per annum. The state of preservation of the fort, built of sun- 

 dried mud, at Sinkat points to a figure even less than this. The rain 

 occurs in violent storms that seldom affect an area of more than 

 a hundred square miles in extent, and the resulting stream generally 

 flows beyond it. When a shower occurs the water is soon coursing 

 down the hill-sides, and forms a surface stream on the broad valley 

 bottom. The gradient is enough to allow of a rapidly flowing stream, 

 chocolate- coloured, with fine sand and mud carried in suspension. 

 The Valley Fill is very porous, and the level of the water-table near 

 the new stream rises rapidly. The water is absorbed most rapidly 

 when the stream is flowing fastest. As soon as the flow begins to 

 abate, and the size of the particles that can be carried in suspension 

 begins to diminish, the pores of the stream-bed become clogged and 

 infiltration is checked. The final result of this is that, after the flow 

 has ceased, pools remain in the stream-bed and are maintained by 

 a thin film of mud which prevents percolation into the dry sand 

 beneath. It has been proved by holes in the sand near the edge of 

 these pools that the level of the water-table was below that of the 

 pool. It appears, then, that the rate of percolation from a stream into 



