Imaggeo on Mondays: The Chalk Cliffs of Étretat

Étretat is a coastal region in northern France, well known for its stunning geological landscape. Particularly the headland you see here.

“The chalk cliffs of Étretat” by Chiara Arrighi, distributed by the EGU under a Creative Commons licence.

Headland erosion is perhaps one of the best known processes in coastal erosion, where a crack in the headland is opened and enlarged by hydraulic abrasion. Continued wave action causes the widened crack or cave to break through the headland and form an arch. As erosion continues, the arch collapses, leaving behind a stack (or needle) that erodes down to its base to form a much smaller stump. There are some great examples of other coastal processes describes in The British Geographer.

How headlands form and coastal arches erode. Source: Sbsgeog’s Weblog.

But that’s not all there is to Étretat, the chalky cliffs are composed of several layers, clearly distinguishable in Arrighi’s photo. These chalk layers are of varying hardness and can be clustered into three main strata: the lowest is a light, fine, stratified chalk aggregate, rich in foraminifera; the middle stratum is a compact chalk layer with beds that are tens of metres thick, and the uppermost stratum is composed of chalk nodules that, like the lowest stratum, is rich in foraminifera.

These strata provide insight into how the chalk of the region was laid down: the fine strata would have been a calcareous ooze, deposited by gentle currents and the nodular material would have formed as small “bullets”, before accumulating more calcareous material as they are transported and redeposited elsewhere. The sediments that now make up the Étretat chalk sequences were subject to several episodes of deformation and slumping before cementing to form the layered cliffs you see above.

References:

Dercourt, J. and Paquet J. (1985). Sedimentary Facies. Geology Principles and Methods, pp. 195-206.

Bromley, R.G. and Ekdale A.A. (2006) Mass transport in European Cretaceous chalk; fabric criteria for its recognition. Vol. 34, pp.1079-1092.

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