Geomorphosite selection in the Brière natural park (Loire-Atlantique, France): natural heritage and cultural heritage - Archive ouverte HAL
Communication Dans Un Congrès Année : 2013

Geomorphosite selection in the Brière natural park (Loire-Atlantique, France): natural heritage and cultural heritage

Le patrimoine géomorphologique du Parc naturel régional de Brière (Loire-Atlantique) et la sélection de géomorphosites : héritages naturels et héritages culturels

Résumé

The Brière is a big swamp of fresh water, close to the sea level (1 m), which belongs to the wetlands of the French Atlantic front. It is situated north of the estuary of the river Loire, at the back of the bay of La Baule, in one of the most touristic sector of the armorican Massif. In Brière, landforms are little contrasted but they nevertheless constitute a natural heritage deserving being popularized within the framework of the Natural Park established in 1970.
A swamp is at first an environment which involves three factors: some water - regularly supplied, a natural dam - without which a stream occupies only a valley, and a sedimentary mound without which the dam holds down only a lake. Waters of Brière come from the river Brivet, the last tributary of the Loire, which comes from the Northeast after having crossed the hillside Brittany Line and which floods the Brière basin before joining the estuary at Saint-Nazaire. In this basin, landforms are incorporated in a set of big tilted blocks, stemming from deformations of the armorican platform during the Tertiary era. These asymmetrical blocks are lined by fault scarps in the Southwest and form backslopes that disappear under subsiding depressions in the Northeast: Brière appears as a basin which subsidence is connected with the toppling of the Guérande plateau in the Southwest (61 m) and of a stripping of the Brittany Line in the Northeast (91 m). The twenty meters mound proceeds of marine sediments, alluviums of the river Brivet and from the Loire estuary. It is held to the Southeast by the levees of banks, which isolate Brière from the estuary, transforming the basin into a swamp of fresh water, finally subjected to the development of the peat.
Besides, Brière is a joint ownership swamp: the property is shared by the inhabitants of the peripheral municipalities and the collective management is recognized by letters patent of the duke of Brittany for the XVth century. This status, links by specific rights (exploitation of peat and reed, hunting and fishing, breeding on islands), involves a permanent management of the water levels according to the various swamp uses.
In these circumstances, the least variation of water levels modifies the configuration of every landform, and the selection of geomorphosites has to take into account all natural and cultural inheritances. Such a selection, led within the framework of the popularization of geomorphological heritage of the Park, supply the opportunity to use an integrated method, already applied in other places, basing on a landform analysis at different scales and leading to a deductive selection of geomorphosites.
The method includes several stages. The first one concerns the characterization of general properties of the studied area in the operation of popularization, such as it has just been made. The second stage concerns the identification of the major geomorphological components, of similar dimensions, but that present additional properties. In Brière, those major landforms are those of the land (Brière’s lands), of the islands (Brière’s islands) and of the swamps (Brière’s swamps). The third stage concerns the subdivision of each of those major components in elementary geomorphological units. For Brière’s lands, those units are steep slopes, backslopes and mounds; for Brière’s islands, the "islands", vast and inhabited, and "mounds", isolated and uninhabited; for Brière’s swamps: banks (occupied by water meadows), reed bed swamps and smooth expenses of water (piardes). The next two stages concern the final selection of one or several representative geomorphosites for each geomorphological unit, whose choice considers scientific criterias (representativeness, didactic interest), then the determination of viewpoint which localization takes into account usual touristic criterias (accessibility, legibility). The choice of geomorphosites results from a deductive selection which involves a series of taxons, presented similar scientific and educational values.
In the case of Brière, geomorphology and history are conjugated for the geomorphosites selection, from tectonic steep slopes (major landforms) up to smooth expenses of water (human made swamps). This selection is significant of a conversely proportional influence of natural and cultural factors.
La Brière est un marais d'eau douce proche du niveau de la mer qui se situe au Nord de l'estuaire de la Loire dans l'un des secteurs les plus touristiques du Massif armoricain. Son relief constitue un patrimoine naturel digne d'être vulgarisé dans le cadre du parc naturel régional qui y a été créé dès 1970. Ce relief s'inscrit dans un ensemble de blocs basculés au cours des déformations du socle au Tertiaire. La Brière se présente ainsi comme une cuvette dont l 'affaissement a été solidaire du basculement du plateau de Guérande au Sud-Ouest et d'un rejeu du coteau du Sillon de Bretagne au Nord-Est. La Brière est un marais indivis dont la propriété et la gestion sont partagées entre les habitants des collectivités périphériques depuis le 15e siècle. Dans ces conditions, la moindre variation du niveau des eaux pour répondre aux utilisations du marais modifie la configuration de chaque relief Une sélection de géomorphosites, menée dans le but de faire mieux connaître et comprendre le patrimoine géomorphologique du parc, doit ainsi tenir compte des éléments naturels et d'héritages culturels. Elle peut fournir l'occasion d'employer une méthode intégrée, reposant sur une analyse du relief à plusieurs niveaux d'échelle et sur une sélection déductive des géomorphosites. Les premières étapes consistent à caractériser les propriétés de l'espace impliqué dans l'opération de vulgarisation et à y identifier des composants géomorphologiques majeurs. La suivante concerne la subdivision de ces composants en unités élémentaires, ou géomorphotypes. Les deux dernières concernent la sélection de géomorphosites représentatifs de chaque géomorphotype et de stations d'observation des géomorphosites. Dans le cas de la Brière, la géomorphologie et l'histoire se conjuguent pour déterminer des milieux dont le classement est significatif d'une influence inversement proportionnelle des facteurs naturels et des facteurs culturels.
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Dates et versions

hal-04683738 , version 1 (02-09-2024)

Identifiants

Citer

Dominique Sellier, Claire Portal. Geomorphosite selection in the Brière natural park (Loire-Atlantique, France): natural heritage and cultural heritage. International Symposium on Geosite Management (ISGM) : Managing Geosites in Protected Areas, Laboratoire Environnements, dynamiques et territoires de la montagne (Le Bourget du Lac, Savoie), Sep 2011, Chambéry, France. pp.163-170, ⟨10.3406/edyte.2013.1255⟩. ⟨hal-04683738⟩
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