09-2018, tome 115, 2, p. 289-308 - Dominique Sellier - Formes d'érosion prémégalithiques et postmégalithiques sur les menhirs de calcaire du causse Méjean (Lozère)

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09-2018, tome 115, 2, p. 289-308 - Dominique Sellier - Formes d'érosion prémégalithiques et postmégalithiques sur les menhirs de calcaire du causse Méjean (Lozère)

Résumé : Les mégalithes portent des marques d???érosion antérieures à leur construction, lorsqu???ils proviennent de blocs rocheux initialement situés à l???affleurement, et des marques d???érosion postérieures à leur élévation, organisées à partir de leur sommet. Les premières sont des formes prémégalithiques (vasques latérales, rainures,...) et renseignent sur l???origine des blocs utilisés comme mégalithes. Les secondes sont des formes postmégalithiques (vasques sommitales, sillons, lapiés...) et renseignent sur les modalités et sur la vitesse de l???érosion au cours des derniers millénaires, l???édification du monument servant en l???occurrence de marqueur chronologique. Le causse Méjean comprend des menhirs de calcaire dont les marques d???érosion ont été examinées selon une méthode appliquée par ailleurs sur des mégalithes de granite ou de schiste. Cette méthode comprend quatre niveaux d???analyse relatifs à la forme générale des blocs utilisés comme menhirs, à la morphologie des pans (notamment des faces d???affleurement et des faces d???arrachement), à la caractérisation des formes prémégalithiques ou postmégalithiques et aux états de surface des menhirs. Elle s???applique ici à des formes d???origine karstique ou périglaciaire. Elle contribue à illustrer l???intérêt d???une double approche, archéologique et géomorphologique dans l???étude des mégalithes.

 

Mots-clés : menhirs, calcaire, causse Méjean, vitesse de la météorisation, karstification, gélifraction.

 

Abstract: The article deals with the interpretation of the observable forms of erosion on the standing stones erected on the limestone plateau of Causse Méjéan (Lozère) using the geomorphological analysis method. These forms are the result of two successive morphogenetic sequences, separated by the erection of the standing stones. Some are pre-megalithic and were created on the limestone blocks used as standing stones at the time they were outcrops. Others are post-megalithic and have continued to evolve since the stones were placed in their upright position and transformed into standing stones.

Causse Méjean is a limestone plateau of about 1,100 m in height, made from Jurassic limestone from two main categories: massive limestone and sublithographic limestone. A lot of standing stones have been destroyed in the past. Fifteen of them, that are still standing and have for the majority been restored, have been examined. The analysis focuses on four different topics: the overall form of the blocks used as standing stones, the morphology of the pans, the inventory of pre-megalithic and post-megalithic forms and the conditions of the surfaces.

The standing stones of Causse Méjean have two parallel faces, of opposite morphological characteristics, determined by bedding planes. Some were outcrops or were sticking out from the ground before the megalith was erected and therefore correspond to the exposed sides. Others were on the side where the rock that was previously used, rooted in the substratum, before being extracted and erected as standing stones and those correspond to the quarried sides. Exposed sides, having been subject to erosion for a long time before being used as standing stones are often very karstified. The quarried sides have only been exposed at the time of the erection of the standing stone and therefore have only been exposed to the actions of weathering for a few thousand years.

Pre-megalithic forms of erosion mainly come from the dissolution process of limestone, occurring on the surface of the banks or deeper. Pre-megalithic basins associated with the exposed sides, are the result of a punctual dissolution of the limestone. Formed horizontally, on the surface of the limestone banks, they are found in a vertical position after the erection of the standing stones. The other pre-megalithic forms are microflutings, biopedogenic karrens and cryptokarst cavities. Pre-megalithic karst erosion forms can be exokarst-shaped, they are then created on the rock outcrop (basins, microflutings) or endokarst-shaped where the forms are created on the inside of the rock (biopedogenic karrens, crypto-karst cavities and also perforating cavities that spike through some standing stones).

Post-megalithic erosion forms are the result of the process of karst dissolution and frost wedging. The markings of post-megalithic karst erosion started to develop after the limestone blocks were placed in their upright position as standing stones. They formed at the top of the summits or from the summits down. Rock top basins are depressions formed at the top of the monument. Gutters are the microrelief in linear hollows. Karrens are a result of the coalescence of several basins or gutters. Post-megalithic flutings are gutters carved from the summit of the standing stones along their faces. Moreover, frost wedging markings, either inherited or functional, can be observed on sublithographic limestone standing stones. They are made up of frost wedging chips on the summits of standing stones and frost wedging scales formed on their faces or sides.

Causse Méjean???s standing stones have contributed to showing the interest in a dual approach, geomorphological and archeological, of megalithic sites. The method originally elaborated from granite menhirs (Sellier, 2013), has turned out to be applicable on limestone standing stones, providing that the properties of these rocks are taken into consideration. Causse Méjean???s standing stones, taken as a research subject, allow the two elements of this method to be applied: the morphological distinction between exposed and quarried sides and the chronological distinction between pre-megalithic and post-megalithic erosion forms. The breaking down of the different types of forms depends on the type of pans. All the rocks that have been examined, show the impacts of two generations of erosion markings, some predating, others postdating the erection of the standing stones.

The method used has several applications from an archaeological point of view. It determines the origin of the rocks and the way the megaliths have weathered since their erection. It can also help deal with attribution problems raised by certain blocks that now lie on the ground. The presence of opposite exposed and quarried sides as well as the presence of two different generations of erosion types indicate the prehistoric origin of an isolated rock; the forms that have developed at the summits indicate the direction of standing stones before falling.

The method presents conversely, several applications from a geomorphological point of view. It indicates the ways in which the limestone has weathered in temperate climates since the erection of the standing stones over the last few thousand years. Measuring post-megalithic microreliefs in hollows, such as basins and gutters, provides information on the weathering rates of the limestone. The depth of post-megalithic basins varies between 3 and 11 cm. That of gutters and karrens is between 3 and 25 cm. The speed of the corresponding erosion could be of 0.6 to 5.5 cm per thousand years, which is a reasonable order of magnitude. However, the erection of megaliths, being an archaeological event, becomes, in this case, a significant chronological marker that explains the conditions of the evolution of geomorphological processes.

 

Keywords: standing stones, limestone, causse Méjean, weathering rate, karst, frost actions.