The physiography of the river Nile and is basin . of section contin-ous solid granite occurred but the geological examination of the cataractaffords no support to this contention and there is no doubt but that thewater channels are everywhere determined by less resistant rock whichhas usually been crushed along lines of faulting. The effect on the river of these ridges of crystalline rocks, whichform rapids and cativracts at numerous points batween Khartoum andAswan, is to delay the formation of a regular slops or grade since th:vofter a much greater resistance to the erosive action of the str
The physiography of the river Nile and is basin . of section contin-ous solid granite occurred but the geological examination of the cataractaffords no support to this contention and there is no doubt but that thewater channels are everywhere determined by less resistant rock whichhas usually been crushed along lines of faulting. The effect on the river of these ridges of crystalline rocks, whichform rapids and cativracts at numerous points batween Khartoum andAswan, is to delay the formation of a regular slops or grade since th:vofter a much greater resistance to the erosive action of the stream than dothe alluvium and sandstone which cccur between them. Thus we havea series of reaches with a low slo])e separated by short lengths of rapidswhere the slope is greater and where the erosive action of the river, I Dr. W. V. * Ei;yi)tian Irrigation, p. 31. 3 Dr. J. Ball. Tlie Geology of the Aswmii Cataract, D v)t. (in tli! |)), < riic Nile UfServoir Dam at Aswan, p. 7. VmXX 3T*J9 .fl EL ^. - 2C,1 — especially in Hood, is wearing away the ol)struction. No detiulcd exam-ination of the Shai)Uika gorge or of the 4th and )th cataracts has yetheen undertaken to sliow if these have been appreciably eroded in recenttimes, but at Hannck Gottl>erg states that river alluvium occurs 35metres above the ]>resent Hoods thus showing that this amount of erosionhas tiiken place comparatixely recently. At Semna Ball has maintainedthat the position of flood marks of the XII Dynasty (1875 )8 metres above present flood level is due to the erosion of this amountfrom the barrier of gneiss which here crosses the valley ; the height ofthe cultivated flood plains, which are well al)0ve the present floods atplaces such as Derr, also bears witness to the slow grading of the riverbed wliich lias heen nuich retarded by tliesc hard rocky bars occurringin it. Without tiiem the Nile from Khartoum to Aswan falling about275 metres in l<S7o kil
Size: 1806px × 1384px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1900, booksubjectwatersupply, bookyear
