Excursion in the Boulonnais / France - 2 - Cap Gris Nez.

Few kilometers and about 50 million years separate the Cap Blanc Nez and Cap Gris Nez; we pass from the Cretaceous at the Blanc Nez  to the Jurassic at Gris Nez.

The Plage de La Sirène / Mermaid Beach, near Cape Gris Nez, is a real "museum of plicative tectonics". One can discover a beautiful anticline, whose important flexure layers is related to Alpine folding. The dip of the layers is increasing as they approach the foreshore (to the north) to reach a subvertical position.

An anticline is a fold of a geological layer convex, dome-shaped; the latest layer is outside the fold.

We are in the presence of clay and limestone marl black from the Kimmeridgian, the "clays of Châtillon" and portlandiens sandstones, the "Grès de la Crèche", yellower (Tithonian)
 

Cap Gris Nez - the Mermaid Beach - photo © 2015 Bernard Duyck

Cap Gris Nez on the Mermaid beach - tne anticline - photo © 2015 Bernard Duyck

The anticline of Cap Gris Nez, the different layer - the green Star marks a break, highlighted on the right picture - a click to enlarge - photo © Bernard Duyck 2015

Kimméridgien (-152 / -157 Ma) and Tithonian (-145 / -152 Ma) in the Upper Jurassic

The Banc of the killer whales / Banc des Epaulards mark the existence of an eroded anticline  cut flush to one of its sides.

Tectonics is complicated here: as stated André Holbecq, we find that there is not a single fold, but folds intersected by faults.

Cap Gris Nez / the Mermaid Beach: Banc of killer whales, an eroded anticline - photo Patrick De Wever

Cap Gris Nez / the Mermaid Beach: Banc of killer whales, from the level of the beach - Photo © 2015 Bernard Duyck

Cap Gris Nez / the Mermaid Beach: subvertical dip rocks - photo © 2015 Bernard Duyck

The cliffs of Gris Nez, and those of "Cran aux oeufs"  / Egg's cran, further south, represent the only example in Europe of cliff with a marl bedrock of Kimmeridgian, covered with sand and sandstone lenses. These egg-shaped blocks come from the landslide rubble of sandstone of the "Grès de la Crèche", which are barefoot in the sand formation.

 

Cap Gris Nez - "Egg" of sandstone, fell on the beach - Photo © 2015 Bernard Duyck

Cap Gris Nez - hummocky sandstones- photo © 2015 Bernard Duyck

Cap Gris Nez - hummocky sandstones- photo © 2015 Bernard Duyck

Sources :

- Geologie info- Un aperçu de la géologie du Boulonnais – par André Holbecq - link

- Le GR du littoral, de Bray-Dunes à Berck – par Pierre Leflon – link

- Planet Terre ens-lyon - Un musée des mini-plis dans le Jurassique supérieur du cap Gris-Nez, Pas de Calais  / Pierre Thomas - link 

- Structure et évolution des falaises gréseuses et argileuses du cap Gris-Nez (Boulonnais, France) – Pierre Guillaume - link

- Parc Naturel  Régional des Caps et Marais d’Opale – Biodiversité en Caps et Marais d’Opale – link

- Etude morphologique des falaises du Cap Gris Nez – JF Vangilve  / Univ. De Lille Géographie physique - link