=Wadi Nilateib=, another small wadi further north, drains the south end of Gebel Um Rasein and the north-east faces of the hills close south-west of it. Coursing at first south-east for about three kilometres, it then curves round the foot of Gebel Um Rasein to join Wadi Hasium by two channels. There is a clear passage from the heads of both Wadi Odruk and Wadi Nilateib into Wadi Merdiyeb. All the wadis hereabouts contain plenty of trees and scrub, especially in their upper parts; as Wadi Hasium is approached they become more sandy and barren. It is, of course, the presence of vegetation available for grazing which determines the possession of a name by such small wadis as these two. The drainage from the east face of Gebel Um Rasein joins Wadi Hasium directly by many small channels, which appear not to bear any special name. =Wadi Medarai=, which forms the Wadi Ibib by its junction with Hasium, ten kilometres due east of Gebel Um Rasein, has its main head in Gebel Medarai, its length along its main channel being about twenty-five kilometres. By numerous tributaries, many of which contain abundance of trees and scrub, it drains the whole of the high hill country between Gebel Medarai and Gebel Um Rasein, and it also takes a part of the drainage from Kwat Hewah. One of its heads is said to lead to a rather difficult pass, south of Gebel Medarai, into the =Wadi Kirwau=, a tributary of Feqoh. For the first ten kilometres or so of its length, Wadi Medarai courses a little north of east; it then receives the feeder from Kwat Hewah above referred to, and changes its direction to a little south of east, passing close north of Gebel Um Rasein on its way to join Wadi Hasium on the sandy plain of the numerous feeders of Wadi Medarai from the hills west and north of Um Rasein, the principal is =Wadi Merdiyeb=, which originates ten kilometres south-west of Gebel Um Rasein by the splitting of Wadi Abu Hireiq. From this point of origin, it runs in a north-easterly direction in a rather sinuous course among the hills for about twelve or thirteen kilometres, joining Wadi Medarai close north of Gebel Um Rasein. Another little feeder of Wadi Medarai, though only about two kilometres long, is of some importance from its leading to =Bir Um Rasein=, a small well among the north foot-hills of Gebel Um Rasein. Vegetation ceases to be abundant both in Wadi Medarai and in Wadi Hasium before their junction to form Wadi Ibib, and Wadi Ibib itself is for the most part a barren sandy ill-defined channel coursing north-eastward across a dreary plain for some fifty-seven kilometres to Mersa Shab. In this fifty-seven kilometres its fall is very nearly 300 metres, giving an average slope of only a little over five metres per kilometre. The slope of the lower part of Wadi Hasium is rather steeper than this, while that of Wadi Medarai is about twice as great.