Sahara megaflood debate splits geologists over eroded landscape

The Sahara carved by a flood of continental proportions

Satellite images of Mauritania show striations that, for many, cannot be the work of wind. The hypothesis of a megaflood that swept the desert some 12,000 years ago has returned to the debate with unusual force, and this time the geological community has not reacted with indifference: it has reacted badly. The chronology points to the Younger Dryas, that period of abrupt cooling that some place at the end of the last ice age.

The erosion band stretches some 400-500 kilometres wide and up to 1,500-2,000 kilometres from the Atlantic coast. Those who defend a water origin point out that the line is too sharply defined and the undulations too massive to be the product of the trade winds. Wind, they argue, erodes uniformly; water, when it arrives with the force of a flood, leaves directional marks that are hard to mistake. The very video that sparked the controversy even claimed the ocean was there tens of millions of years more recently than science had considered possible.

The precedent of Lake Missoula

The obligatory reference is Lake Missoula in the American West. There, there is consensus: the overflow of a glacial lake, formed by the melting of the Laurentide ice sheet, generated floods of a magnitude hard to imagine. Science communicator Randall Carlson, author of the original video, has made these megafloods his specialty. Writer Graham Hancock, better known for his Atlantis theories, has contributed to the subject being viewed with suspicion by academia.

The problem is that that erosion was studied in situ for decades, with the ground exposed. In the Sahara, so far, there are only satellite images and screenshots from Google Maps. No one has done the fieldwork that would confirm the striations are from water and not from wind-borne granulometry. That does not stop some from recalling that the desert was not always sand. The rock paintings of southern Algeria and Libya are full of hippos, and the subsoil preserves fossil water reserves in astonishing quantities. There was a humid period in the Holocene, between 11,000 and 3,500 BC, with rivers, lakes and vegetation. From there to claiming that a single flood swept the desert is a leap mainstream science is not willing to take.

Richat, the Eye of Africa and the background noise

The controversy also intersects with the Richat Structure, that circular formation in Mauritania visible from space. Some link it to a meteorite, some explain it by uplift and differential erosion, and some use it as a supposed remnant of a lost civilization. Academic geology favours the second explanation: a bulge that erosion has gradually shaped into rings.

The problem is that, tangled up in this mess, serious analysis has been contaminated by creationist noise. Some want to see in the images confirmation of the biblical flood, and others respond with the same dogma in the opposite direction. The result is that a fascinating question about the hydrological history of the Sahara is trapped between two camps that do not want to talk about the same thing. The evidence of great waters in the past is hard to dispute; what should not be a trench is a good geological question.