جيومورفولوجية منطقة البيضا بالمدينة المنورة و دلالتها المناخية

Abstract EN

This study aims to investigate whether Al-Baydah area, Al- Abyar valley and Abul-Dood depression used to compose a lake or several old lakes.

To reach facts about these depressions Geographic Information System (GIS) was utilized.

Borders of these depressions were determined; the drainage network in this area was studied carefully; fault system was analyzed; rock formation in it was recognized; a sample of the old lake sediment was analyzed in order to recognize its mineral composition and texture.

In addition, deep ground water of the area was chemically analyzed.

The area consists of three basic drainage basins.

These are: Al- Hamdh valley, heading west; Al- Naqmi valley, directed north to south; and Al-Abyar valley directed to the north-west and falls in Al- Hamdh valley, but outside the area being studied.

Three hypotheses were put to explain the phenomenon of the three depressions (Al- Baydah 1, Al-baydah 2, and Abul-Dood) and their sediments.

It was hypothesized, first, that Al-Hamdh valley was blocked.

The second hypothesis suggested that Al-Hamdh did not reach as far as where it is now.

Thirdly, Al- Abyar valley separated from Al-Hamdh valley, but the two valleys used to meet at Al-Baydah, unlike the current situation – they join at Melaileeh.

This blockage caused composition of a lake or several lakes.

This was in the Pluvial ages at the Pleistocene and early Holocene periods.

It is supposed that these pluvial periods ended 5000 years ago; and the lake lasted for 6500 years.

A field study and analysis of maps and satellite photos and laboratory analyses revealed several facts.

The most important of which were: the fact that the area was someday full of pure water; and that it got empty by means of water gaps at Al-Hafeerah and near wadi Al-Abyar outlet.

Furthermore, fine sediments had been removed and only what was difficult to carry by the running water remained.

Morphometric analysis indicated that the drainage basins in the area were still at the youth stage and the relief ratio was high.

Laboratory analysis indicated that the basic components of the lake sediments were mostly quartz, feldspar, chlorite and iron oxide.

They were mostly harsh sediments whose edges have not been polished yet, due to the short distance they transported.

underground water in the area was void of chemical pollution.

Salt rate was within the standard allowed locally and internationally.

However, the study could not determine accurately the age of the sediments, due to absence of any fossils in the area.

The study recommends conducting more and deeper historic research, using modern methods and technology to detect specifically the age of these basins and sediments.

It also recommends making a road to Al-baydah depression 2; and restricting digging wells so that the ground wealth be preserved, and the duration of benefit be extended.

Finally, the study recommends increase in tree plantation with the same type of trees that existed there.