Géomorphologie : relief, processus, environnement vol. 16 -n° 2 | 2010 Des géosites aux géomorphosites : comment décoder le paysage ? Processus géodynamiques, modelés et formes du relief, environnements passés et actuels L'analyse intégrée du relief et la sélection déductive des géomorphosites : application à la Charente-Maritime (France) Multidimentional analysis of relief and deductive selection of geomorphosites: application to Charente-Maritime (France) Dominique Sellier Édition

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... Référence électronique Dominique Sellier, « L'analyse intégrée du relief et la sélection déductive des géomorphosites : application à la Charente-Maritime (France) », Géomorphologie : relief, processus, environnement [En ligne], vol. 16 -n° 2 | 2010, mis en ligne le 01 juillet 2012, consulté le 03 mai 2019. Résumé La vulgarisation de la géomorphologie a pour objectif de diffuser des informations sur les reliefs et sur leurs origines. Elle procède de trois étapes fondamentales : l'analyse préalable du relief, la recherche d'une pédagogie adaptée au public concerné, l'application de moyens de vulgarisation appropriés. Elle passe aussi par le choix initial de sites significatifs, qui peut inspirer trois démarches non exclusives. La « démarche sélective » a recours à la notion de géomorphosites et implique des méthodes d'évaluation élaborées, mais n'épargne pas du risque de privilégier les sites les plus spectaculaires. La « démarche généraliste » implique que tous les reliefs, y compris les plus ordinaires, peuvent justifier un commentaire, mais n'évite pas non plus toute sélection. La « démarche déductive », appliquée ici au cas de la Charente-Maritime, part d'une analyse multiscalaire du relief et trouve son principal domaine d'application en amont de l'évaluation des géomorphosites. La première étape de cette démarche consiste à définir les propriétés générales de l'espace considéré (en l'occurrence le département de la Charente-Maritime). La deuxième vise à identifier ses composants géomorphologiques majeurs (ici un espace continental propre à un bassin sédimentaire, le Nord de l'Aquitaine, et un espace littoral témoignant de fluctuations eustatiques, la mer des Pertuis). La troisième consiste à déterminer les unités de relief élémentaires présentes à l'intérieur de ces composants majeurs, en tenant compte de leur représentativité et de leur complémentarité (plateaux aclinaux, cuestas, boutonnières, blocs faillés, d'une part, îles et côtes rocheuses, marais et côtes d'accumulation, d'autre part). La dernière aboutit à une sélection déductive de géomorphosites ponctuels, significatifs de ces reliefs (en l'occurrence une douzaine). Elle intègre les critères de classification usuels de nature scientifique (exemplarité, intérêt didactique) ou touristique (accessibilité, lisibilité). La méthode, fondée sur une analyse géomorphologique intégrée, c'est-à-dire sur une analyse des reliefs à plusieurs niveaux d'échelle, peut déboucher sur des pédagogies adaptées aux publics impliqués. Elle peut aussi contribuer aux études de faisabilité conduites en amont des opérations de vulgarisation. Mots clés : vulgarisation de la géomorphologie, bassins sédimentaires, reliefs littoraux, façade atlantique, Charente-Maritime. Abstract The aim of popularising geomorphology is to make information about relief accessible to a wider public. It is the result of three fundamental stages: the prior analysis of the relief, the search for an educational method adapted to the public, and making the necessary means available in order to achieve a given objective. The choice of visited sites involves three non-exclusive processes. The first, the 'selection process', is based on the concept of geomorphosites, which involves complex methods of evaluation, though it is not exempt from the danger of favouring the most spectacular sites. The second is a general or 'broad-based approach', which implies that all sites, although of varying interest, deserve a mention although it does not completely avoid selection. The third is guided by 'deductive reasoning' and is to be found in the process prior to the evaluation of geomorphosites. The present approach, illustrated by examples from Charente-Maritime, is based on a prior multi-scale analysis of the relief. In the first stage, the general characteristics of the area under consideration are described ( in this case, North Aquitaine and the Mer des Pertuis). The second is the analysis of the main geomorphological components (in this case, a continental area typical of a sedimentary basin and a coastal area subject to eustatic fluctuations). The third involves identifying the relief inland from these components with reference to their representative and complementary characteristics (aclinal plateaus, cuestas, wealds, fault blocks, on the one hand and islands and rocky coasts, wetlands and accumulation coastal areas on the other). The last leads to a deductive selection of geomorphosites, characteristic of each relief (about a dozen). Thus, it links up to the usual scientific classification criteria (exemplary nature, didactic nature) or of tourist interest (easy access, clarity). The methodology, based on a global geomorphological analysis, can lead to didactic processes adapted to various types of public. It can also contribute to feasibility studies carried out prior to popularisation processes. Abridged English Version The aim of popularising geomorphology is to make information about relief accessible to a wider public through appropriate means. It follows from an initiative and, in principle, responds to a demand, and is the result of three fundamental stages: (i) the prior analysis of the relief, (ii) the search for an educational method adapted to the public visiting the site, and (iii) carrying out specific popularising activities or projects. In order to popularise geomorphology, through direct contact with the terrain, the sites, which constitute the places of observation, explanation and transmission of knowledge must first be selected. The sites will often coincide with viewing points on a circuit within a given area. The choice of sites involves three non-exclusive processes. The first, the selection process, is based on the concept of geomorphosites, which involves complex methods of evaluation. The second is a general or broad-based approach, which implies that all sites deserve to be mentioned. The third could be described as an integrated approach being based on the prior multi-level analysis of the relief leading to a selection of geomorphosites. Guided by deductive reasoning its place lies mostly in the stages prior to the evaluation of geomorphosites. This process lies at the heart of the present article. It includes four stages and implies the drafting of a geomorphological map which is an indispensable tool for analysis. In the first stage the geomorphological features of the area to be visited will be defined. The area, whose boundaries should be justified, will comprise fundamental characteristics. In the second stage, the key geomorphological components will be identified with the aim of recognising several sub-sets having similar dimensions but different characteristics on the basis of topographic (relief), structural (lithology), hydrographic (catchment areas) or palaeographic (heritage) features. The third stage is the result of the subdivision of each main geomorphological component into elementary units of analogous dimensions and equivalent geomorphological significance on the basis of their morpho-structural representativeness, their morphological, palaeographic or dynamic relations and the extent of their evolution. Finally, each basic geomorphological unit will be represented by one or more geomorphosites whose selection will be justified on the basis of its relation to taxa of the same or a higher rank. The final choice of geomorphosites is based on geomorphological criteria: their exemplary nature, didactic interest, rarity, integrity, representativeness, as well as tourist-oriented criteria such as ease of access and perception. The method leads to a deductive selection of geomorphosites that are representative of each of the fundamental geomorphologic units. It could, for example, be applied in the Charente-Maritime on the northern shores of the Bay of Biscay between the mouth of the Sèvre Niortaise and the Gironde. The region presents three general geomorphological characteristics. In the first place it is part of an area of low limestone plateaux, mostly less than 100 m. It lies at the contact between a sedimentary basin and a basement which means it comprises the four structures typical of sedimenta-