Geomorphologie

The littoral geomorphology

The Coast of alabaster extends over 140 kilometers, from the estuary of the Seine River to the estuary of the Somme. This name is only a reference to the white color of alabaster. The cliffs emerging a wall of 30 to 110 meters in height (average 70 meters) mainly carved into the layers of the Upper Cretaceous chalk, but other layers of the upper Jurassic and the engagement documenting are also presented.
These cliffs are lowered by places, nicked by some thirty glens suspended, called locally valleuses, or by the valleys reaching the level of the sea, sometimes drained by a watercourse (Valmont, Durdent, Dun, Saâne, Scie, Arques, Yeres, Bresle).

Relief of the Country of Callus and the Country of Bray (colors) and main rivers

Relief in false colors of the Maritime Seine - Notice that the highway A 29 follows the ridge line to avoid crossing the transverse valleys, except for Mirville where, as for the railroad, the crossing takes place by a viaduct or in Neuchâtel for the crossing of the buttonhole of Bray. Notice also the relief little raised at the heart of the syncline of Ailly.

The cliff moves back, from 10 to 50 cms a year, on average 20 cms / year, but in a very irregular way in the space and in the time, generally because of rough collapses of several hundreds of cubic meters. By moving back, the cliff loosens a rocky platier in its foot, of very low slope (2 %). The low waters of deep water sometimes discover an abrupt of several meters in height when hardened levels (hard-grounds) are present in the chalk. Deep, called sections "carniaux", are arranged in the sense of the slope, sheltering a rich fauna.

The evolution of the cliff came true under the dependence of the climatic phenomena alternants during the Quaternary:

  • The recoil of the cliff is maximal in period of interglacial high level;
  • The digging of valleuses is maximal in period of glacial low level.

The karstic cavities of the chalk are recut by the cliff, which offers excellent conditions of observation and analysis of the processes of karstification.

The recent recoil of the cliff (current interglacial period) can, under certain conditions, recut the upstream of valleuses built in the last glacial period. Two beautiful examples are given by the valleuse of Ignauval near Havre and valleuse of Jambourg to Etretat.

Valleuse of Ignauval - drawing colorized of R. Musset, 1951

Under the influence of the swell and the tides, the sea transports and gives evidence on the platier of the detrital veneers, dividing up granulométriquement according to the degree of energy and the arisen of storms:
-CCords of pebbles at the top of beach, at the top of about fifteen meter in width and of the order of 2 in 3 meters of thickness.
-SSands of bottom of beach, about 1 meter of thickness, especially plentiful and continuous between Saint-Valéry and Dieppe.
According to their profile, 4 big types of cliffs can be distinguished:

  1. The vertical cliffs or the simple cliffs, cut from homogeneous chalks (eg. Etretat)), observing on approximately 70 km
  2. Cliffs with projecting ledge or cliffs with base, when a more marly chalk fits (ex:Senneville),
  3. Cliffs on foot of clay, when marls kimméridgiennes or / and albiennes level (eg. Cauville),
  4. Cliffs with hat of clay, when clays and brown coals of the Eocene are protected (eg. Varengeville, Drink Municipalities))

Erosion by the swell in charge of pebbles (pass in full screen and resolution 1080p)

The current line of coast is only resultant ultimate of the progressive opening of current Manche which the quaternary glaciations patiently modelled.