The typical rock basin, called a _galt_[120] by the Arabs, is a smooth-sided cavity in the rocky floor of a steep gorge draining a large mass of high hills. Galts are generally difficult of access, being situated in the higher parts of very stony wadis, so that one has to tramp often over miles of steeply rising stony ground to reach them. Galts abound in all high mountain-masses in South-Eastern Egypt, and furnish the greater portion of the water supply of the population. The capacity of galts varies very much; some hold millions of litres. Often there is a chain of successive galts at intervals along the length of a gorge, and after rain the whole series is filled, while much overflows and runs to waste. The origin of the rocky basins is generally due to pot-holing action on a large scale. Galts may occur in rocks of any hard type, but are most commonly met with in eruptive rocks. Sandstone galts never remain long full, owing to the permeable nature of the rocks. The large Galt el Aguz near Gebel Um Harba, which from the inscriptions near it was evidently known and used in Ptolemaic times, is in sandstone, receiving in fact the superficial run-off from the same high mass of sandstone hills as furnish the collecting area for feeding by percolation and underground flow the adjacent springs of Bir Abraq and Abu Saafa; but the galt is seldom full for more than a month or two, while the springs are constant. The water supply of galts depends firstly on the rainfall of the district; secondly, on the existing drainage system; thirdly, on the presence and size of eroded basins along the lines of drainage; fourthly, on the nature of the rocks forming the basin, and, lastly, on the degree of exposure to evaporation. The rainfall in the Eastern Desert is often very local. A series of galts full one year may be dry the next, while those of another area may show a reverse state of things. The drainage system most favourable to galts is one formed by the union of long narrow steep-sided gullies into a single gorge. The presence of basins is conditioned by the steepness of the drainage and the hardness of the rocky floor; if the rocks are soft or much crushed, galts are very unlikely to occur. Again, if the basin is formed of permeable though hard rock, there will be rapid loss by infiltration. Evaporation is generally far less rapid at galts than in open country, because being in narrow gorges galts only receive the sun’s rays for a few hours of the day, and are, moreover, sheltered from winds.