GEOLOGICAL MAP OF RAS BENAS
| Ball. Geography & Geology of South-Eastern Egypt. | PLATE XXI. |
Photo-Metal-Process. Survey Dept. Cairo 1910. (60-190)
The Nubian Sandstone.
The Nubian sandstone covers a comparatively small portion of South-Eastern Egypt, occupying only about one-tenth of the total area under consideration. The greater portion of it forms the eastern termination of the vast sandstone plateaux which stretch eastward from the Nile towards the igneous and metamorphic back-bone of the Red Sea mountains, but there are in addition some small patches on the eastern side of the watershed which are interesting as showing that the deposition of the sandstone was not confined to the western side of the mountain ranges.
The most northerly exposure of Nubian sandstone in the district here described extends with some breaks southward from near Gebel Sufra to near Gebel Homr Akarim, forming low hills. Further south, it comes in again on both sides of the Wadi Garara, forming the hills of Felieiti and Abu Hashim. In the sandy plain around the Wadi Timsah are outlying sandstone hills of considerable height and extent, the principal being Gebels Nuggur, Mulgata, and Ziraga. Further south-west, the Nubian sandstone forms an extensive high broken plateau stretching southward from near Gebel Zergat Naam to near Gebel Um Reit, sending out a long tongue south-eastwards across Wadi Dif to near Gebel Nigrub el Tahtani. This tract of broken sandstone plateaux bears various names in different parts, Gebels Um Harba, Um Khafur, Dagalai, Shebakhit, Awamtib, Um Sididad, Abraq, Hodein, Dif, Anfeib, and Kala, all forming parts of the same great sandstone mass, though separated by wadis from each other. Small outliers of Nubian sandstone cap the hills of Gebels Reietit, just north of latitude 23°, near the meridian of 34°, and Gebel Seiga, in latitude 22° 44′, longitude 34° 16′.
On the eastern side of the watershed, Nubian sandstone extends for some distance near the coast opposite the Gulhan islands, and also forms small thin outliers south of the Wadi Hodein. The most southerly point where the sandstone has been met with in the area is a small outlier in latitude 22° 36′, a few kilometres east of Gebel Hamra Dom.
In petrographical characters the Nubian sandstone is remarkably uniform, consisting of medium-sized silica grains set in a more or less ferruginous cement; the colour varies from nearly white, through various shades of brown, to nearly black, according to the amount of iron present. It is generally well bedded, and frequently much jointed. There is usually a marked absence of the clay beds which are associated with the Nubian sandstone in other parts of Egypt. The basal beds are generally pebbly, forming conglomerates in some places. Concretions are found in the rock at some points, notably to the north of Gebel Um Harba. Some of the concretions are hollow, the shell being of hard dark ferruginous sandstone while the interior is filled with a powder of snow-white chalky matter containing sand grains. The only fossils observed in the sandstone within the area described were collected by Mr. Charteris Stewart in the plain of Um Harba; they consist of crocodile scales and various shells. From these and the fossil shells found further west in sinking a well in Wadi Abu Rahal,[124] it may be concluded that the Nubian sandstone here, as in the Nile Valley, is of Cretaceous age.
Intrusions of igneous rock into the sandstone have only been observed at two points in the area, viz., near the western foot of Gebel Awamtib, where a basic dyke runs through the sandstone, and on the Red Sea coast, near Bir el Ranga, where Dr. Hume has found andesite interbedded in the sandstone. A bed of diabase, now much altered, underlies the sandstone near Wadi Muelih, and is possibly of contemporaneous origin.