RIYADH – A total of 1,826 caves and "duhool" (plural of ‘dahl') scattered across a dozen provinces in the Kingdom have attracted the attention of many earth scientists, geographers, historians and archaeologists. Earth scientists describe Saudi Arabia's landscape as an open geological museum, as it has rocks dating back to billions of years. It begins with the igneous and metamorphic rocks of the Arabian shield, the oldest of which dates back to more than 1.5 billion years, through the sedimentary rocks of the Arabian Shelf including sandstone, limestone, mudstone, evaporites and others that were deposited during approximately 542 million years, a Saudi Press Agency (SPA) report said. The types of rocks in the Kingdom include igneous and metamorphic which are rich in mineral ores such as gold, silver, copper, and zinc; and sedimentary rocks whose pores store water, oil, and gas. This natural resource, experts say, is considered as an important source to support the national economy, along with the wealth of information it provides to scientists and scholars specializing in the earth sciences. Several geological and morphological factors have contributed to the formation of towering, majestic mountain ranges and deep valleys in the Kingdom. Meanwhile, ancient and recent climatic conditions of all kinds -- continental, marine, glacial, rainy and dry periods -- have acted to form various types of terrains and topographies that have turned many sites to be rare natural scientific museums, apart from becoming the destination for researchers, amateurs and tourists from all over the world. Caves and "duhool" were formed in the Kingdom with the power of Almighty Allah as a result of the various geological processes in the Kingdom. The Kingdom enjoys a number of important hydrological, economic and cultural advantages, and is a scientific wealth for geological, geomorphological, geographical, biological and archaeological studies. The Bedouins living in the desert areas use the term "dahla" or "al-dahlah" on the voids located below the surface of the earth and contain water. As for the definition of caves and "dahl" from the geological point of view, a professor of geology, Dr. Abdulaziz Bin Laboun, said that they are similar in their geological formation. Most of them result from folding, cracking and further cracking of the layers of rocks due to the ground movements. Then by the flow of water through those joints and cracks, and the dissolving effect of the water, especially acidic water, on the soluble rocks, especially limestone and gypsum. He explained that most of the region's caves and "duhool" were formed during the rainy and dry (arid) ages to which the Arabian Peninsula was exposed since a million years ago, causing gaps and channels of various dimensions and shapes. They are known today as "duhool". However, the terrain and climatic conditions, and sedimentary formations within the duhool differ. Their forms (shapes) vary from oblique and vertical to winding, not to mention the difference in temperatures and humidity levels and the existence of water in them, and the living beings in them such as pigeons, owls, foxes, wild cats, dogs, hyenas, scorpions and snakes among others. Earth scientists have distinguished between the cave and the dahl, as Prof. Bin Laboun said: "The cave is the wide gap in rocks or mountains above the surface of the earth while the dahl is the gaps and caves that lie below the surface of the earth. This is in terms of distinguishing between the two types, and our literary and historical heritage is rich in documenting their importance and locations." After a whole year, the Ministry of Environment, Water, and Agriculture recently completed a study on caves and duhool in the Kingdom. Its results have shown that there are 1,826 caves and duhool in the Kingdom distributed in 12 regions, concentrated in the layers of limestone and gypsum rocks, such as those in the Dammam, Umm Al-Ras, and Umm Radhma, Arama, Heet, and Al-Arab formations. The study showed that most of the duhool that were discovered were in the rocks of the Eastern Province, totaling 680 caves and duhool, and in the Northern Borders Region with 542 caves and duhool. This was due to the nature of limestone formation and topography of the two regions. Meanwhile, in Riyadh Region 308 caves and duhool were found and in the rest of the areas there were 296 caves and duhool. The study conducted by the Ministry of Environment, Water, and Agriculture concluded that the number of caves and duhool in the banks of water courses and valleys in the Kingdom totals 156, spread in six regions of the Kingdom: the Eastern Province (80), the Northern Borders Region (24), Riyadh (34), Al-Jouf (11), Al-Qassim (3), Hail (3) and Madinah (1). Adoption of appropriate classification for duhool for harvesting rainwater and replenishing the underground reservoirs (aquifers) and the sources of some springs was adopted at their locations within the drainage basins of valleys and ravines. The duhool classification also covered what can be harvested from rainwater and floods and directing the harvested water toward the openings of those duhool. It became clear that the selection of the appropriate duhool for feeding the subterranean strata (aquifers) was based on their location in the regions, governorates, geological formations, the drainage of valleys and ravines, the amount of water in the water basins, the levels of the duhool and their dimensions, and the locations of the duhool, according to their proximity to urban and agricultural areas. Among the most important duhool is Dahl Al-Najmah in Sakaka, Dahl Al-Jarbou'iah in Al-Qurayyat, and Dahl Abu Totah in Rafha. As to the duhool attracting tourism, they are 105 in number, and nine duhool that are being used in academic studies and research are distributed in five regions.
