Grand Erg Oriental - Natural Phenomena

Natural Phenomena

The Grand Erg Oriental is a desert region receiving very little rainfall. It is the largest Erg in Algeria, the next in size being the much smaller Grand Erg Occidental ('Western Sand Sea'). The largest erg of the Sahara is probably As-Sahra al-Libiyah, which straddles the inland border of Libya and Egypt. Erg is a Tamachek Berber word, and also a geographic term of art.

The Grand Erg Oriental used to be associated with the Wadi Igharghar, a mostly dry and buried river with a sizable network of tributaries which, should it possess any water, would flow north into the erg from the Ahaggar mountains of the central Sahara. Yet such dry, anciently-made river beds, lying seemingly useless beneath the desert sands, can preserve the infrequent rain water, by carrying it off underground and so rescue the moisture from an otherwise "intense and almost instantaneous" evaporation.

A buried river bed "not only serves in certain cases to carry into the heart of the desert the waters of distant rains which have fallen outside the desert domain, but in it the waters of the local storms are concentrated and carried swiftly to the alluvial basins where they are imbibed by the lighter earth and form lasting reserves within its depths. The result is that what vegetation survives is localized along the wadi beds or in their basins; in fact the words wadi and pasturage are interchangeable in the language of the nomads, who habitually reside in such places."

For these and similar reasons concerning the desert ecology of water, the geographer and historian of the Sahara E.-F. Gautier once wrote referring to the Grand Erg Oriental as "the great Igharghar Erg". Accordingly, surface moisture, seasonal pastures, and wells may reflect unseen conditions beneath the sands. What appears as an entirely inhospitable erg can, in stark contrast, elsewhere offer the fruits of "some buried wadi".

To the north of the erg, the Aurès mountains provide abundant runoff. These waters feed the artesian aquifer of the Jerid, despite its surface covering of salt lakes. These conditions lie adjacent to the grand erg. Here grow "the finest dates of all the Maghrib". In winter, winds blow from the northwest and the north. The erg "appears to have been pushed forward on the east and southeast slopes" toward Ghadames at the Libyan border.

Winds over time will sweep desert sand into heaps which, given enough sand, form into a series of hills. In some types of dunes the slope on the windward side is gradual, on the leeward steep, and such dunes may "roll" forward being blown in the direction of the wind. Vegetation does not survive in such spreads of hot dry sand. Only in rare areas where moisture can endure is there life.

Common in the Sahara desert are seif dunes: here the air currents form sand dunes parallel to the prevailing direction of the wind. These dunes have long, sharp ridge lines. Cross-winds may alter the height or width of such lengthy dunes, with the slope being steep on both sides. Seif dunes may thus form long rows whose parallel ridge lines follow the wind's direction.

There are other types of dunes, as well as "complex" dunes. Saharan winds are also known to clear an area of sand altogether, leaving bare rock (hamada) or gravel (reg).