En remontant de la Tour de la Reine Mathilde vers la Porte des Champs nous allons observer le passage graduel du Calcaire de Caen au Calcaire de Creully : Ce passage est visible dans les escarpements du fossé en contrebas de la Porte des Champs et de sa barbacane. Sous la barbacane de la Porte des Champs, les bancs massifs de la base de l’ancien front de taille appartiennent au sommet du Calcaire de Caen. Ils sont surmontés par des lits de silex et des bancs moins épais qui annoncent le Calcaire de Creully.
Au sommet du front de taille, des lits de silex s’intercalent dans les bancs calcaires et soulignent les joints de stratification.
Des Chevilles siliceuses dans les bancs du sommet du Calcaire de Caen correspondent à d’anciens terriers ou à des racines silicifiées de végétaux de mangrove. L'Escarpement du fossé Est faisant face à l’affleurement situé sous la barbacane, à gauche de ce qui serait un ancien four a chaux, expose les mêmes bancs. L’affleurement nous montre des bancs de calcaire massifs à sa base et des lits de silex dans sa partie supérieure.
Les nodules siliceux sont fréquents au sommet de la formation. Ils résultent d’une concentration et d’une précipitation de la silice au niveau d’hétérogénéités du sédiment en voie de consolidation. La silice mobilisée dans le sédiment est essentiellement d’origine biologique, provenant de la dissolution du squelette des éponges siliceuses qui peuplaient le fond marin ; une autre partie provient de l’altération des terres émergées qui a fourni des solutions siliceuses transportées jusqu’au bassin de sédimentation. Certains accidents siliceux, de forme allongée et en position verticale, ont été interprétés comme des anciennes racines de palétuviers, caractéristiques des mangroves.
A mi-hauteur de l’escarpement, au-dessus des bancs massifs subhorizontaux du Calcaire de Caen, les calcaires présentent une stratification oblique caractéristique du Calcaire de Creully. Le Calcaire de Caen correspondant à un milieu de dépôt calme passe au Calcaire de Creully qui témoigne d’un milieu de dépôt soumis à des courants.
3 - Utilisation de la Pierre de Caen.
De nombreux édifices sont construits ou ont été rénovés grâce à la pierre de Caen, bien au-delà des limites du département et de la France :
- A Caen
-
- Abbaye aux Hommes (Hôtel de Ville)
- Abbaye aux Dames (Conseil Régional de Basse Normandie)
- Les églises de Caen, notamment l'église Saint-Pierre
- Le château Ducal
- En France
-
- Abbatiale de la Trinité de Fécamp
- Église de Saint-Jacques de Dieppe
- Prieuré de Graville
- Cathédrale Notre-Dame du Havre (sauf sa tour en pierre de Vernon)
- Écoinçons et arcades du cloître de l'abbaye du Mont-Saint-Michel
- Palais du commerce de Rennes
- Église de Saint-Pol-de-Léon
- Tour de Londres
- Tower Bridge de Londres
- Cathédrale Saint-Paul de Londres
- Abbaye de Westminster
- Cathédrale de Cantorbéry
- Buckingham Palace
- Palais royal de Bruxelles
- Cathédrale de cologne ( élements de décors)
- Cathédrale Saint-Patrick de New York
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Plain of Caen limestone was formed during the Mid Jurassic period, in the advance up to the sea, the Bathonian stage (-167.7 ± 3.5 to -164.7 ± 4.0 million years). This limestone is formed in a shallow sea near a shore (mangroves type) of shelly sand cemented by carbonate mud (menus organic debris cemented by microcrystalline calcite). This limestone is limited to the west and south by the Armorican massif.
The climate is tropical (vegetation, fauna) and the warm waters are favorable to the precipitation of calcium carbonate (the origin of the formation of microcalcitique cement).
THE STONE OF CAEN
City of Caen is built at the confluence of the Orne and Odon who dug their valley in a tray formed by the superposition of Jurassic limestone dipping slightly to the NE, towards the center of the Paris basin.
The bottom of the valley is filled by recent Quaternary alluvium. The slopes have a staging of lower and mid Bathonian limestones.
The structure of the stone from Caen, lower floors, is chalky and thin, which makes sculpture work easier but is vulnerable to erosion.
Median floors are more compact and dense, and provide the building stone, called "Caen stone" with it international renown.
1 - Deposits
They are located in the towns of Caen, Bretteville -sur -Odon , Carpiquet , Fleury -sur -Orne and Cintheaux. The first deposits of Caen stone were readily available , as they are located in the hills surrounding the terraced alluvial plain where was built the city of Caen and in the immediate vicinity of the city.
Some old quarries are still visible today in the city same as the one we will see around The Castle.
The Ducal Castle was built with Caen stone extracted during the digging of ditches that served as quarries.
Scarps and ditches are former quarry faces to discover the upper limestone of Caen and its gradual transition to higher formation : the limestone of Creully.
The best outcrops of limestone Caen lie south-east of the castle , on either side of the Tower of Queen Matilda , avenue de la Libération. Outcrops extend into the base of the cliffs in the castle moat to The Door of Champs, and beyond.
2 - Outcrops
a) Tower of Queen Matilda - WP1 = N49°11.139' W000°21,641'
At the outcrop located between the two towers we can observe the upper part of the Formation of Caen limestone which was extracted Caen stone used to build the castle and ramparts.
The upper part of this formation outcrops in the form of massive layers of fine-grained limestone and beige color. This limestone comes from the consolidation of a carbonate mud consists of very fine debris of marine organisms (shells, sponge spicules, skeletons).
We can also observe a small fault that shifts layers of Caen limestone (outcrop at the left of the Tower of Queen Matilda)
b) La Porte des Champs - WP2 = N49°11.212' W000°21,681'
Going back from Tower of Queen Mathilde to the Porte des Champs we will observe a gradual transition from limestone Caen to limestone Creully :
This passage is visible in the cliffs of the gap below the Porte des Champs and its Barbican.
Under the Barbican , the massive layers of the base of the old gauge are the top of Caen limestone . They are surmounted by beds of flint and thinner layers announcing the upper Limestone of Creully.
At the top of the face, flint beds intercalated in the limestone beds and highlight the bedding planes .
Siliceous anchors in the pews of the top of limestone Caen match old burrows or roots silicified plant mangrove.
Siliceous nodules are common at the top of the formation. They result from a concentration and precipitation of silica in sediment heterogeneities during the process of consolidation. Mobilized silica in the sediment has essentially a biological origin and comes from the dissolution of the skeleton of siliceous sponges inhabiting the seabed; another part comes from the weathering of the land surface which provided siliceous solutions transported to the sedimentation basin. Some siliceous accidents, elongated and upright forms, have been interpreted as ancient mangrove roots, characteristics of mangroves.
The Escarpment of the East ditch facing the Barbican outcrop located to the left of what was to be a lime kiln, exposes the same layers. The outcrop shows massive limestone layers at its base and flint beds in its upper part.
Halfway up the escarpment above the massive subhorizontal layers of Caen limestone, layers have an oblique stratification characteristic of limestone of Creully .
The Caen limestone is corresponding to a environment of calm passes deposits, Limestone Creully reflecting a depositional environment more agitated.
3 - Use of the Caen Stone
Many buildings are constructed or have been renovated with Caen Stone, well beyond the limits of the department and France :
- In Caen
-
- Abbaye aux Hommes (City Hall)
- Abbaye aux Dames (Regional Council of Lower Normandy)
- Caen churches, including St. Peter's Church
- This Ducal Castle
- In France
-
- Spandrels and arches of the cloister of the abbey of Mont-Saint-Michel
- Trinity Abbey of Fécamp
- Church of Saint-Jacques de Dieppe
- Priory of Graville
- Notre-Dame du Havre (except the tower Vernon)
- Outside France
-
- Tower of London
- Tower Bridge London
- St. Paul's Cathedral in London
- Westminster Abbey
- Canterbury Cathedral
- Buckingham Palace
- Royal Palace of Brussels
- Decorative elements of the Cologne Cathedral
- St Patrick's Cathedral in New York