1) Lessivage et dissolution
L’effet combiné de la dissolution chimique et de l’action érosive provoquée par le lessivage des eaux de pluies émousse les reliefs et fait disparaître les formes, les zones exposées sont plus claires. Les visages et les mains des statues sont lissés. Une usure différentielle peut aussi creuser les parties tendres et accentuer les parties dures, créant une surface alvéolaire :
2) Encroutement et colonisation par les végétaux
Les zones humides abritées sont propices à la colonisation par les végétaux (mousses, lichens). Des particules atmosphériques peuvent aussi s’agglomérer et conduire à la formation d’une croûte noire. Elles sont majoritairement constituées de gypse, de sulfates, de particules d’hydrocarbures, de goudron et de poussières. Elles peuvent aussi être constituées de calcite recouvrant la pierre d’une patine ocre.
3) Desquamation et détachement en plaques
Cette dégradation correspond à un écaillage en mille-feuilles de la surface du calcaire qui se décolle progressivement de la paroi souvent dû à des écarts de température et aux cycles gel-dégel et à la présence de sels. La roche devient très friable et se fragmente en plaques.
The Basilica of Saint-Denis (French: Basilique royale de Saint-Denis, or simply Basilique Saint-Denis) is a large medieval abbey church in the city of Saint-Denis, now a northern suburb of Paris. The building is of singular importance historically and architecturally as its choir, completed in 1144, shows the first use of all of the elements of Gothic architecture.
The site originated as a Gallo-Roman cemetery in late Roman times. The archaeological remains still lie beneath the cathedral; the people buried there seem to have had a faith that was a mix of Christian and pre-Christian beliefs and practices. Around 475 St. Genevieve purchased some land and built Saint-Denys de la Chapelle. In 636 on the orders of Dagobert I the relics of Saint Denis, a patron saint of France, were reinterred in the basilica. The relics of St-Denis, which had been transferred to the parish church of the town in 1795, were brought back again to the abbey in 1819.[2]
The basilica became a place of pilgrimage and the burial place of the French Kings with nearly every king from the 10th to the 18th centuries being buried there, as well as many from previous centuries. (It was not used for the coronations of kings, that function being reserved for the Cathedral of Reims; however, French Queens were commonly crowned there.) "Saint-Denis" soon became the abbey church of a growing monastic complex.
Geological reminder
Alteration is the combination of changes in the physical-chemical properties of minerals, and thus rocks, by atmospheric agents (meteoric alteration), by groundwater (subsurface alteration) and thermal waters (hydrothermal alteration). It depends in particular on the climate, the temperature of the water, the nature of the rocks and their degree of fracturing.
The materials used in the construction of historic monuments could suffer multiple attacks by the urban environment, causing several types of alterations on the façades and statues. The nature and intensity of degradation depend on the variety of the rock, as well as their positioning on the monument
Damages are of three types: physical, chemical and biological. The deterioration factors are: water (frost, humidity / dry cycles), temperature differences, soluble salts, biological colonization, air pollutants.
Some alterations forms :
1) Leaching and dissolution
The combined effect of chemical dissolution and the erosive action caused by the leaching of rain water dulls the reliefs and shapes that disappear, the exposed areas are clearer. The faces and hands of statues are smoothed. A differential wear can also dig the soft parts and accentuate the hard parts, creating a honeycomb surface :
2) Crusting and colonization by plants
The sheltered wetlands are suitable to colonization by plants (mosses, lichens). Atmospheric particles may also agglomerate and lead to the formation of a black crust. They are mainly composed of gypsum, sulfates, hydrocarbon particles, tar and dust. They may also be formed of calcite overlying the rock of an ocher patina.
3) Flaking and slab detachment
This degradation corresponds to a peeling of the limestone surface which gradually separates from the wall often due to temperature variations, to freeze-thaw cycles and the presence of salts. The rock becomes very brittle and breaks into plates.
Questions pour valider cette earthcache :
D'après vos observations et à l'aide du descriptif,
Questions to validate this earthcache:
Based on your observations and using the description,
WP 0
Joindre à votre log une photo de vous, votre GPS, votre GC perso ou tout autre objet vous représentant dans le jeu devant la Basilique (sans montrer les réponses !).
Les simples photos de la Basilique ne permettent de valider cette condition de log.
Cette tâche est obligatoire et autorisée par les guidelines mises à jour en juin 2019.
Attach to your log a photo of you, your GPS, your personal GC or any other object representing you in the game in front of the Basilica (without showing the answers !).
The simple photos of the Basilica do not validate this log condition.
This task is mandatory and allowed by the guidelines updated in june 2019.
WP 1
a. Quel(s) types(s) d'altération pouvez-vous observer dans la zone orange ?
a. What type(s) of alteration can you observe in the orange area ?
b. Quels en sont les agents responsables ?
b. What are the responsible agents ?
Envoyez-moi vos propositions de réponses soit via mon profil, soit via la messagerie geocaching.com (Message Center) PUIS loguez cette cache "Found it", je vous contacterai en cas de problème.
Assurez vous de répondre à TOUTES les questions ! La condition principale pour loguer une Earthcache étant de répondre aux questions posées dans le descriptif, tout log "Found it" enregistré sans envoi préalable de vos propositions de réponses sera supprimé. Il en sera de même pour les logs enregistrés sans la photo requise.
Send me your proposals for answers either via my profile or via the geocaching.com messaging (Message Center) THEN log this cache "Found it", I will contact you in case of problem.
Make sure to answer ALL questions ! The main condition to log an Earthcache is to answer the questions asked in the description, any log "Found it" registered without prior submission of your proposals of answers will be deleted. It will be the same for logs saved without the required photo.
Grand Merci à Visyl pour son accord de lui reprendre son texte !