International audienceThis paper evaluates the paraglacial evolution of a sediment-mantled slope in a polar maritime environment. The intensity of paraglacial processes is estimated through quantifi cation of erosion and dating of fi eld sectors with the help of photographic archives. Gully erosion has been estimated using morphometric parameters and by surveys of vegetation cover. The rapid melting of dead-ice cores controls gully formation. This leads to slope form modifi cation: gully profi le gradients are reduced from a mean of 35° to a mean ranging between 10° and 15°. Profi le evolution results from the collapse of glacier lateral moraine. All data (mean slope angle of individual gullies, frequency distribution of slope angles, fractional distance to the apex, gullying index, volume of debris mobilized, vertical erosion rate) tend to increase with increasing deglaciation age and the duration of paraglacial activity. Vegetation colonization is a response to stabilization of the ground surface and the drying up of the ground surface due to dead-ice melting. The full sequence of paraglacial slope adjustment (gully incision-stabilization) may occur rapidly at the study site, i.e. within two decades. Finally, a lateral morphogenic sequence is proposed showing the importance of paraglacial processes at the onset of the deglaciation
An analysis of megalithic alignments according to geomorphic methods can provide information about their environment before the Neolithic period and about the origin of the stones that were used to build them. The Kerlescan alignments (Carnac, Morbihan) show three types of granité blocks used as menhirs (blocks without weathered faces, extracted from the substratum, blocks with opposed fresh and weathered faces, blocks without fresh faces, issuing from the reutilization of rocks formerly cleared out by erosion). Microforms resulting from the weathering of granitic premegalithic outerops (domes, notches, platforms, rock basins) enable identification of three types of rocks existing on the site of alignments or their surroundings (rounded rocks, flared basai rocks, pedestal rocks). The quantitative study of these rocks is of limited significance since it applies to a partly ruined monument. Their numbers and their distribution show however that the highest part of the site included large granitic outerops and many residual rocks, such as are to be found on granitic slopes in régions of temperate climate. Indeed, almost 75 % of menhirs still extant on the site of Kerlescan alignments originate from outerops or from blocks that were stickin gout of the ground by several decimetres, and up to two metres. This type of environment favoured the building of a great megalithic monument.
Large stone monuments testify to the presence of a large human population in the Carnac region during the 5th and 4th millenia. Core samples were obtained from sediments of a former filled-in bay enclosed between interfluves on which several funeral constructions exist. These dimentology and palynology revealed by these samples were the basis for a first attempt at reconstituing the landscape and paleoenvironment before, during and after the neolithic occupation. The results show that the deposits at this site do not go back any earlier than the early Bronze Age. Resumption of the marine transgression between 3400 and 3000 BP seems to have contributed to the formation of a sait meadowat the beginning of the armorican subatlantic period. Cereals were present during the Bronze Age and were developed more extensively during Gallo-Roman times. Chestnut and walnut pollens were apparent starting at the base of the diagram, which poses a problem concerning the date and processes by which these plants were introduced into the Massif Armoricain.
Granite reliefs offer patterns of micro-erosion, differing from one another by their morphotypes and their répétition. These patterns, in the range of one mètre to one decimetre, are due to selective érosion. They are well- known, but it remains difficult to assess the rate of their progression in the absence of any definite chronological indicator. Megaliths offer one such indicator. Those in Carnac constitute a privileged subject for analysis, owing to their morphological variety. They represent a statistically significant sample, because of their number and in spite of their partial restoration. This article deals with the preliminary results of a study of the alignment of Kerlescan, according to geomorphological methods, making use of various scales. It aims at bringing informations about the siting of the alignment, about the rate of erosion of the granites in a temperate oceanic environment and about the conditions of the deterioration of the megaliths. This study is based on an inventory of patterns resulting from action of weathering on the standing- stones.
Active stony earth circles occur on the less coarse cells of relict sorted nets developed in quartzite debris above 600m ASL on Muckish Mountain, north‐west Ireland. The earth circles are underlain by deformed and truncated podzolic soil horizons that resemble cryoturbations produced by differential frost heave of adjacent sediments. Grain‐size distributions indicate that the soils are frost‐susceptible and possess negative frost‐susceptibility gradients. This infrequent association of coarse quartzite debris and fine‐grained frost‐susceptible matrix probably arises from the weathering of thin inter‐bedded pelites (schists) within the quartzite. Recent activity may have been triggered by climatic deterioration during the Little Ice Age, although present climate on Muckish appears capable of maintaining the surface morphology of the stony earth circles. These features, along with forms described previously, suggest that most if not all upland areas in Ireland above 600‐700m ASL possess climatic conditions favouring frost‐related soil processes.
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