Extraordinarily vast (50 km in diameter, with elevation changes of 30 to 40 m), this complex formation is nearly impossible to comprehend from ground level… From the ground, you'll encounter only a succession of gently curving hills, tracing concentric rings, with at its center a sebkha flanked by a hill composed of breccia blocks. The ensemble sits in the East of the Adrar approximately 35 kilometers from the city of Ouadane, at the northwestern edge of this 'empty quarter' known as Majabat al Koubra (here, the Erg Warane).

The plural term Richat or Richât (Rich in singular) derives from a Hassaniyya word (the Arab dialect spoken in Mauritania) meaning "feathers," alluding to the way the circular cuestas appear to overlap like feathering. "Semsiyyât," another local term meaning "children," designates a smaller annular formation (3 km in diameter) situated 25 km west of Guelb, and 6 km east of the Akhmakou ravine.

Discovery and Location

This structure was first described in 1916 with considerable approximations (though aerial photography and GPS did not yet exist—only compass, tablet, and pencil!) by French military personnel, and later mentioned by Ernest Psichari in his 1920 book "The Voices Crying in the Desert": he describes the depression bordering Richat to the west, between the formation's outer ring and the cliff face bordering the Chinguetti plateau. The Guelb er Richât first appeared on a map in 1922 (Geographic Service of French West Africa). In 1934, Th. Monod became the first scientist to visit this extraordinary formation (a facsimile of his exploration notebook entry from July 7, 1934 was published in B. Lecoquierre, 2008, Traversing the Earth, L'Harmattan, p. 113). He returned countless times, published a scientific work on the subject in 1973 (with Charles Pomerol), and conducted his final mission there in December 1998 at the age of 96. Guelb er Richat also caught the attention of early space missions because of its distinctive "eye" shape in an otherwise monotonous desert landscape, making it a crucial landmark for cosmonauts.

Scientific Interpretation

The most widely accepted explanation is that a magmatic dome formed at the intersection of fault lines approximately one hundred million years ago and ceased growing before becoming a volcano. The ancient Cambrian sandstones lifted by the dome's formation subsequently underwent fracturing and erosion, creating the concentric cuesta system that gives Richat its somewhat mysterious character today. After initially interpreting it as an astrobleme (meteorite impact structure), geologists considered symmetric uplift (circular anticline). But Th. Monod, who studied this phenomenon with colleagues, published hypotheses nearly half a century ago (Monod and Pomerol, 1973) remarkably close to the explanation commonly accepted today. Thus, since around 1980, it has been established that Richat's structure is that of an "alkaline complex" resulting from a particular form of volcanism dating to 100 million years ago (Cretaceous: late Secondary era) that created a magmatic dome associated with hydrothermal manifestations (circulation of extremely hot water at very high pressures). This complex formed during the opening of the Atlantic Ocean.

  Deep faults are also observable, which permitted the rise of carbonatites—rocks associated with alkaline rock complexes and localized along major faults in the Earth's crust.

The deep faults reach the Mohorovičić discontinuity *. This discontinuity, located approximately 35 km below the surface, marks the boundary between the Earth's crust and upper mantle. * It is, in fact, a physicochemical discontinuity: the Earth's crust is composed of basaltic rocks (at ocean level) and granitic rocks (at continental level), while the mantle consists of peridotites (ultrabasic magmatic rock containing garnets, amphiboles, olivine, etc.).

The bedrock situated above the dome, composed of sediments deformed by uplift, would have collapsed following prolonged, slow karstic-type erosion**, which explains the presence of breccias at the formation's center. ** Karst phenomena result from the dissolution of limestone, named after a region of Istria that provides the finest example (Combaluzier, Introd. géol.,1961, p. 83).− [kaʀstik]. − 1st attestation 1902 (Nouv. Lar. ill.); from karst, suffix -ique*. The quartzite sandstones*** of the Primary period composing the structure's layers, alternating with limestone, proved more resistant to erosion and today form the concentric residual rings ("rich") of Guelb. *** a massive siliceous rock composed of cemented quartz crystals. It exhibits conchoidal fracture (similar to glass fracture). Its color is generally pale. Other similar phenomena (alkaline complexes), though of smaller dimensions, are now recognized notably in Mali (Tadhak, west of the Adrar des Ifoghas), but also in Algeria and Chad. One might wonder at the precise knowledge of these massifs' distribution and that of carbonatites, but while detailed prospections have been conducted in various sectors, it is simply because carbonatites are typically associated with diamond deposits.  

Direct and Nearby Points of Interest

  Geology - 1) Various rocks of Richât: among the most remarkable...       . Breccia / Central Peak : spherules (cf. Boureau, 1983)            . Carbonatites : numerous veins throughout Guelb but especially to the west.        . Kimberlites : no kimberlite pipe within Guelb, but precursors, 25 km west at the Semsiyyât level (6 km before the Akhmakou ravine). Australian diamond exploration conducted in this area in 1999-2000.          . Gabbros : outcrops in small domes, to the west.    . Silexites : at Megsem Tamat level, to the west      . Various Salts : deposits at the "sebkha" level (Sebkhet el Guelb).     . Analcimolites : at the center, NE of the sebkha.   - 2) Sebkhet Touijinit : SW of Guelb, sebkha and strong deflation zone where "dreikanter" are observed. - 3) Megsem Tamat : beds of silexites in varied colors. - 4) Sebkhet el Guelb : salt deposits and analcimolite bench to the NE. - 5) Rich Serize : remnants of a fine esker (terminal Precambrian glaciation), carved by Guelb's uplift. This esker is contemporary with those of Zerga and Herrour. - 6) Carbonatite Veins : numerous west of Guelb. - 7) Gabbros

Biology: The rocks (cliffs, scree...) harbor numerous plant and animal species that find here (to live and reproduce) effective protection against excessive evaporation (sun, wind). Thus, much like in Adrar Tmar, we note:

• Plants:

When rains occur, a long list of species can be observed.

• Animals:

When rains occur, a long list of species can be observed. .

Invertebrates: insects (the list is extensive: beetles, lepidopterans...), arachnids,... .

Vertebrates:

Reptiles (lizards and snakes, including Malpolon moilensis, Moïla Whip Snake),     

Nesting Birds: Hirundo rupestris, Eurasian Crag Martin Falco biartmicus, Lanner Falcon Bubo ascalaphus, Pearl-spotted Owlet,…                     

Mammals Rhinolophus and Epseticus , Bat Mellivora capensis, Honey Badger Canis aureus, Golden Jackal Felis libyca, Sand Cat Fennecus zerda, Fennec  

Archaeology

- 1) The Fort of Agweidir (after R. Vernet, 2003, modified and supplemented). About twenty km to the ENE of Ouadane, near the excellent well of Hassi Thiba, stands a complex archaeological site.
A first ensemble comprises an extensive network of foundations of sun-dried brick walls, surrounded by a vast quantity of pottery sherds, bones, and stones (grinding material?). A second, in the northwest quarter of the tell, consists of a quadrangular structure also in sun-dried brick.
At the 4 corners stand towers in ogival form 3m in diameter for 2 m in height. The preservation state of the towers and vegetation elements within the bricks suggest late and perhaps ephemeral construction of the building. An accumulation of glass bottle sherds supports this theory. Both ensembles appear linked: small "village" established near a military camp set up by French camel-mounted troops (1935-1945). The construction "style" matches that of the camp at El Ghallaouiya (established near the well of Bir Zemran), 80 km east of the Hassi Thiba well. Earlier, according to information gathered in Ouadane, the Portuguese (credited with an ancient presence in this region) may have briefly chosen this location in the 15th century, then called Veregni, to draw closer to the caravan routes between the Niger bend and North Africa.

- 2) Hermitage of Cheikh Mohamed Fadel "Dar Md Fadel" is a dry-stone building situated approximately 8 km east of the Hassi Thiba well. Th. Monod discovered nearby very large hand axes (80 cm, 1 meter) described as belonging to the Fadelian.

- 3) Worked Pebbles: North and northwest of Guelb lie the oueds Akerdil and Bamouéré. On the surface of their alluvial fans, millions of worked pebbles are observable, testimony to paleolithic occupation of the area. (cf. Doc 5).

Conclusion / Interest : - Geological Interest: multifaceted Flora and Fauna Archaeological: paleolithic, neolithic, subrecent