270 G. W, Grahham— Wells of North-Eastern Sudan. 



its bed is a function of the size of particle it can carry in suspension, 

 and, consequently, of the speed of the current. I am not aware of 

 having seen this principle put forward before, though, once observed, 

 its truth is apparent. 



When the water is absorbed by the Valley Fill it is protected from 

 evaporation, and as it flows slowly down the rock valley it is readily 

 tapped by wells. In Khor Arbat, a valley north-west of Port Sudan, 

 there is a place where the valley narrows and the gravels become con- 

 stricted in section, with the result that the normal underflow cannot 

 pass and the water is forced out, forming a stream at the surface. 

 This is probably the only place in these arid regions where there is 

 a perennially flowing stream, and it is interesting as containing 

 a small freshwater fauna. There is no evidence of a rock bar across 

 the valley throwing up the water, and the explanation given is 

 sufficient, even supposing that the longitudinal section of the rock 

 valley is a continuous curve. 



The wells of this area are all situated in the Valley Fill, from which 

 they derive their water. Where the railway traverses it, a good deal 

 of experience has been gained in sinking wells. From Summit down- 

 wards the line follows valleys both towards the Nile and the sea. I can 

 only speak with experience of the seaward side, but Capt. S. F. 

 Newcombe, K.E., tells me that the conditions are similar towards the 

 Nile. The difference of slopes, however, makes the topography very 

 different. In one place on the eastern side he has proved the section 

 of the rock valley beneath the Valley Fill, showing conclusively that 

 there is no need for the line of the surface stream to coincide in plan 

 with the deepest part of the rock valley. This is a point to be borne 

 in mind, for the tendency has been to choose a site near the stream- 

 bed or actually in it. The native ones are probably in the stream-bed 

 as a result of deepening a water -hole, made immediately after the 

 rains, in pursuit of a falling water-table. It appears also that better 

 water is got from wells in this situation than from others at a distance 

 from the bed, and the reason may be that the underflow mainly 

 infiltrates from it and so traverses less Valley Fill in reaching the well. 

 The position of the stream-bed is always marked by trees or denser 

 vegetation, which covers narrow strips of ground alongside it. Seeds 

 have the best chance of germinating and growing up here, as it is the 

 moistest place in the valley, and, once the growth of vegetation begins, 

 the fine dust, whether carried by wind or water, is arrested and forms 

 a good soil among the roots. 



As examples of wells in this area the following may be mentioned : — 



The Okok wells at Sinkat find their water at a depth of about 

 30 feet, and are situated high up in the valley, within 7 miles of 

 the watershed. They are in the stream-bed on the north edge of the 

 Vallej' Fill, and Capt. Newcombe, who dug one of them, tells me 

 that he reached the solid rock only just below water-level. In former 

 times there were other wells at a distance from the bed, but the water 

 is said to have been much inferior to that of the existing ones. There 

 does not seem to have been any attempt to make wells in the middle 

 of the valley, where the Fill is probably deepest. 



At Gebeit, lower down the valley and about 14 miles from Summit, 



