Article and sketch by ©Bruno Lamarche - Détours Mauritanie Voyages

At the end of the Precambrian, 540 million years ago (Ma), the disposition and location of the continents were very different from those we know today.

In short, the continental portion that would become West Africa was located near the South Pole, covered with a thick layer of ice.

Just like the current Antarctic continent, West Africa was then topped with a vast inlandsis, a glacier with an average thickness of 2 km.

The flow of this gigantic glacier was steady and the erosion caused by its weight was considerable.

Beneath the inlandsis, meltwater (seasonal) circulated, traveling from the surface to the bedrock through glacial "mills" and their multiple channels, which converged into various subglacial torrents of great power and erosive capacity.

The erosion generated by the ice sheet as a whole, and by the subglacial torrents, produced considerable masses of debris of varying sizes (sand, gravel, pebbles, boulders).

When the glacier receded and then disappeared, these moraines (basal, frontal, and lateral) remained in place, as did those formed by subglacial water circulation, and were exposed to the air.

The deposits abandoned by the glacier display very diverse characteristics:

• For subglacial torrents, these are "eskers" (from Irish escir), highly elongated hills that are more or less serpentine in form.

• For peripheral circulations: drumlins (from Irish druim), elongated hills, and kames (from Scottish kame), mounds of varying dimensions and shapes.

Various depressions are also observable: lakes, kettles.

Obviously, the scale of these deposits is directly related to the thickness of the glacier that covered them and produced them.

The considerable dimensions of the Zerga ridge (50 km long, average height of 100 m) explain the term "mega-ridge" employed by M. Deynoux (1980): it is a giant esker but it originates from the activity of an inlandsis rather than a mountain glacier.

The Zerga ridge forms a very remarkable ensemble (aerial view taken by M. Deynoux toward the North: one can distinguish the Guelb er Raoui (mountain of the mouflon) and the northern summit of El Ateg-, and 6 -view toward the South-).

But this giant esker is not the only one in West Africa nor even in the Adrar T'mar.

One can thus observe various formations of the same type, but of lesser scale, and sometimes fragmented... we will mention, among the most obvious:

• 75 km to the West, the ridge of wadi H'Nouk,

• 90 km to the East, the ensemble named Herrour, in the immediate vicinity of the small oasis of Tanouchert,

• 60 km to the East of Wadan, in the East of the Hassi Thiba well, near the hermitage of Cheikh Md Fadel, segments of an esker (likely fragmented by the uplift of Guelb er Richat) are observable on the flank of Rich Serize and stretch to the South-East to the edge of Taguenzé.

Much further South, outside the Adrar T'mar, in the Hodh, 12 km to the NW of Tichit, is the great ridge of Kheneg ed Deheb and 35 km to the NW of the same locality, the ridge of the Zig well, which is highly characteristic...

Some are also observed on the Hank, far to the North-East.

Nothing particularly surprising about this, as the inlandsis covered most of West Africa!

The strong heterometric character (fine sand, coarse sand, gravel and pebbles, sometimes boulders) of the sandstone that constitutes these eskers leads to particular and characteristic erosion forms.

Thus the crest of Zerga is deeply dissected and presents itself as a maze of corridors winding between pillars of varying dimensions, a veritable labyrinth that led Théodore Monod to name this formation "corridor sandstone".

Specific Biology of the Zarga Ridge:

These corridors and caves (excavated in the pillars) shelter very numerous plant and animal species that find there (to live and reproduce) effective protection against excessive evaporation (sun, wind) and are hardly observed around it. One notes, among others:

Plants:

• Lavandula coronipifolia,

• Corbichonia decumbens,...

Animals: 

• Invertebrates:

  • Insects, the list is long:

    • coleoptera,

    • lepidoptera...

  • arachnids,...

• Vertebrates: 

  • Reptiles (lizards and snakes, including Malpolon moilensis, Moïla Colubrid), 

  • Nesting birds:

    • Hirundo rupestris, Rock Swallow

    • Falco biarmicus, Lanner Falcon

    • Bubo ascalaphus, Pearl-spotted Owlet,…

  • Mammals

    • Rhinolophus and Eptesicus, Bats

    • Mellivora capensis, Honey Badger

    • Canis aureus, Golden Jackal

    • Felis libyca, Sand Cat

    • Fennecus zerda, Fennec Fox

    • and in a more distant past Ammotragus lervia, Barbary Sheep (hence the name of the summit Guelb er Raoui = Guelb of the mouflon).

Geology

Nearby points of interest:

1. 12 km to the SW of Guelb er Raoui, Gleib or Hofrat Aouelloul, an astrobleme (meteorite crater) 250-300 m in diameter, around which numerous impactites have been found (drops of melted rock in the form of grey-green bubbly glass containing kamacite -a meteoritic mineral-). This astrobleme was discovered by Pourquié, J. Gallouédec and Th. Monod.

2. 30 km to the East-South-East of Aouelloul is Guelb Aouinet, a sandstone promontory with a flat summit which was recognized in 1986 by Th. Monod as the relief on which Captain Gaston Ripert discovered in 1916 the famous "meteorite of Chinguetti". 

He declared that his discovery (an actual meteorite) was resting on an immense iron meteorite, which triggered a veritable hunt for the object for many years. Indeed, when one observes the guelb from the right angle it appears to have a bluish and metallic appearance. These are iron oxide flows on the sandstone that create this effect. 

The vast plain that extends from the crater to Guelb Aouinet is also not without interest. Several small circular depressions, a few meters in diameter, filled with sand, suggest micro-craters, not visible on aerial photographs.

However, at sunrise, when the light is raking and shadows are strong, the depressed zones appear more pronounced and the crater-like aspect with sometimes a small peripheral rim becomes more apparent. More to follow.

Conclusion / Significance:

• Past climates

• Considerable size of the esker

• "Corridor sandstone" - particular erosion

• Flora and Fauna interest

• Geological interest of Hofrat Aouelloul and Guelb Aouinet