It is to the alluvial material in the wadis that the Eastern Desert owes most of its perennial water supplies and the vegetation it possesses. The alluvial deposits are the great conservers of water. Pools form, it is true, in the bare rocky beds of the higher drainage lines, and may last for many months where they are screened by the walls of a gorge from wind and sun, and are in consequence not subject to rapid evaporation. But such pools are difficult of access, and afford but a precarious source of water supply because they are liable to dry up if a long period passes without rain. The alluvium of the wadis absorbs the rainfall and protects it from evaporation, so that even in very dry years water may be found by excavating in it at suitable places to depths of a few metres. Almost all the wells which are so important to travellers crossing the desert from the Nile are of this character. The abundance of trees which flourish in so many of the wadis likewise derive their nourishment from the water conserved in the alluvium of the wadi floor.
Calcareous Tufa.
Small deposits of calcareous tufa of recent origin have been noted in the Wadi Um Tundeba and in a gorge on the east side of Gebel Ghuel, as well as round the little trickling spring called Megwel Hamida in the south part of the region.
At Um Tundeba the deposit occurs in a little gully close to a well known galt or pool of rain water. The deposit is not extensive, and has doubtless been formed by the evaporation of trickling drainage-water which had absorbed lime from the rocks. The tufa (10,374) is a pale brown rock of rather porous nature; it envelops fragments of schist-debris from the surrounding rocks.
The calcareous tufa of Gebel Ghuel is a more impressive deposit, though its total mass is not very great. It occurs at a point of sudden fall in a narrow rocky gorge leading to the Wadi Ghadir in about latitude 22° 53′. Proceeding up the gully one is confronted with a great curtain-like mass of tufa covering the face of a high ledge like a solidified cascade. Here also the origin is clearly due to trickling of lime-laden drainage-waters over the ledge of rock, which forms a step in the wadi floor.
Fragments of tufa were also seen round about Gebel Allawi, though the rock was not traced to its source. As only a relatively small number of rocky gullies have been explored, it is likely that similar deposits to those above described occur in many other places.
Gypsum and Gypseous Limestones.
The occurrences of gypsum and gypseous limestone beds in South-Eastern Egypt are restricted to the coastal regions. The most prominent of the deposits are those on Ras Benas (see map on [Plate XXI]), where the beds form white hills rising to 188 metres above sea-level. Further north, gypseous deposits have been found by Dr. Hume to exist near the sea at Bir el Ranga, and by Mr. Ferrar the same beds have been recorded as occurring near the coast in the neighbourhood of Wadi Igli. It appears probable that the gypseous strata form a continuous or nearly continuous strip extending along the coast down to latitude 24° 22′, as shown on the geological map on [Plate XX,] but more complete observations may show that the distribution of the beds is somewhat different from that indicated. On the coast-plain south of Ras Benas, gypseous beds are not exposed except in the extreme south-east corner of Egypt, where they form small patches at Halaib and round the wells of Ti Kureitra.
At Ras Benas, where the beds have been studied in most detail, they consist of gypsum and anhydrite [11,513][123] alternating with sandy marls and marly sands, forming hills much cut-up by steep sided narrow ravines. The weathered faces of the rocks are very soft, and the disintegrated material forms a stretch of soft gypseous sand, into which one’s feet sink four or five centimetres at every step, between the hills and the shore. At Halaib, gypseous limestones crop out from under the gravel of the coast-plain and form low banks; the gypsum is here associated with calcareous grits [12,152, 12,114] and conglomerates, the latter having boulders of igneous rock set in a calcareous matrix. Near Ti Kureitra wells, the gypsum exposures contain much crystalline selenite, blocks of this material being used in the masonry lining of the wells. Both at Bir el Ranga and on Ras Benas the gypsum is found to contain small pockets of native sulphur.
No fossils have been found in the gypseous strata, and their geological age is uncertain. The beds are younger than the Nubian sandstone, since they overlie that formation at Bir el Ranga. Their occurrence only near the coast leads one to regard them as having been formed after the Red Sea occupied its present position; but whether they originated as direct gypseous deposits, or were produced by the alteration of pre-existent Cretaceous or Tertiary limestones is not yet quite certain. It is noteworthy that at Ras Benas, where the gypseous strata rest on diorite and hornblende granite, the igneous rocks are considerably altered as if by weathering, with a strong brick-red colour due to the oxidation of ferruginous matter.