In thickness, the Nubian sandstone attains a maximum of about 350 metres in several places round about Bir Abraq and the Wadi Hodein. Its thickness must at one time have exceeded this, for the upper surfaces have suffered much denudation, and are not capped by younger rocks.
Some interest attaches to the maximum altitude reached by the Nubian sandstone beds. The highest point at which it has been observed is at Gebel Seiga, whose sandstone cap is 905 metres above sea-level. The highest point reached by the rock on the great plateau round about Wadi Hodein is Gebel Kala, 846 metres above sea.
In contradistinction to the same beds which form the plateau between longitude 34° and the Nile, the Nubian sandstones within the area here described show considerable disturbances from their original horizontal bedding, the dips being as a rule greater and more variable the nearer one approaches to the watershed mountain-ranges. These tectonic disturbances, which are important in connexion with the geological history of the region, will be considered in [Chapter XI.]
[122]Geol. Mag., Decade V, Vol. VI (1909), p. 271.
[123]The numbers in square brackets are the specimen-numbers in the Cairo Geological Museum.
[124]Wadi Abu Rahal is a small feeder of Wadi Abad, joining the latter from the south in latitude 25° 0′, longitude 33° 30′. At the point of junction of the two wadis, which lies on the usual camel road from Edfu to the Baramia mine, a well was sunk by the Mines Department in 1906, in the hope of obtaining a water supply. I visited the well in May 1906, when it had attained a depth of fifty-four metres. The strata passed through were ten metres alluvium, then thirty-seven metres of sandstones and clays, followed by a thin band of bituminous shale, and seven metres of dark grey clays. From near the bottom of the well I collected specimens of Lingula and a mytiloid shell which Mr. Bullen Newton compared to Septifer linearis; the latter shell differs but little from specimens obtained from the English Gault, and thus tends to show that the Abu Rahal beds are of Cretaceous age. (See Hume, Preliminary Report on the Geology of the Eastern Desert of Egypt. Cairo, 1907. p. 29). Since my visit, the well has been deepened to seventy-three metres in sandstones with a bituminous seam, but water was not reached, and the well has been abandoned.