The peloton to discover Breton archaeological quirks

Beyond sport. In addition, the Tour de France also gives the opportunity to (re) discover our landscapes and sometimes their geological quirks. Furthermore, The route of the seventh stage of the Tour de France 2025 begins in Saint-Malo to join Murs-de-Bretagne. Moreover, Just after Yffiniac, the cyclists will go through Pledran, a town known for its Péran camp, whose walls are … In addition, vitrified. Meanwhile, Explanations.

On the stone walls. Similarly, the rocks are most of the time held together by a mortar: made to consolidate the construction, the latter is a pasty mixture made up of mud, lime or hydraulic cement with water. Furthermore, The whole, which hardens while drying, then acts as a glue. Moreover, The types of mortar. For example, their uses have varied over time: there are traces of their use since the Neolithic (10,000 peloton discover breton archaeological quirks years), but their composition is diversified and specialized in -4000 BC. Moreover, EC in ancient Egypt.

Constructions have also been established without the use of mortar: this is the case among the Greeks. Consequently, which used the only vertical gravitation force, or in the Incas, which used polygonal stones but perfectly adjusted with the neighbors in order to stabilize the construction.

Rarer. however, are the constructions in which the stones are well glued to each other, but without contribution of an external material: they are then directly transformed and welded on site. In addition, This is called vitrified walls. Nevertheless, which are found at the Fort de Péran, in Brittany, near which are preparing to pass the cyclists.

The best preserved fort in France – Peloton discover breton archaeological quirks

The Péran camp. Nevertheless, in the town of Plédran, is identifiable with its fortified enclosure, perched on the first heights (160 meters) which dominate the Bay peloton discover breton archaeological quirks of Yffiniac, 9 km southwest of Saint-Brieuc, in the Côtes-d’Armor. Formerly known in the region under the name of Burned stonesthis site has provided archaeological parts (spoons, parts, etc.) since 1820-1825. But the first publications relating are those of Jules Geslin de Burgundy, in 1846.

It was supposed that the place had been a Gallic oppidum, before being transformed into a Roman camp. The excavation campaigns have made it possible to confirm. specify its intuition: it is estimated that the camp, now classified as historic monuments, date of the culture of the Tène (approx. 450 to 25 BC), apogee of Celtic culture which ends with the Roman conquest.

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After the Gallic era. the place was occupied by the Vikings, whose incursions were identified around 9^e and x^e centuries, as well as in the 13th^e century as attested with terracotta tiles.

Elliptical in shape, it covers approximately one hectare (160 meters by 140) and has five concentric defensive structures.

The enigma of vitrified walls

This fort. which is best preserved in France, has the specificity of the vitrified stone bulwark of one of its five defensive structures: the stone stones are welded because a part has melted on the periphery of the blocks, forming a glass which cemented the rocks between them. Such walls, often associated with forts, exist throughout the old continent, but particularly in northern Europe. In France. we know about twenty, from Brittany to Alsace, peloton discover breton archaeological quirks with a notable concentration in Limousin, the Creuse and the Loire, all of the granitic countries.

These vitrifications intrigue since Antiquity. From the middle of the 18th peloton discover breton archaeological quirks century^e century such a wall was reported in the cellar of a house in Sainte-Suzanne. in Mayenne. We then wonder: what fire was violent enough to melt the stone and thus vitrify it? And, was this fire intentional or the fruit of an accident? The only known natural vitrifications were those linked to volcanism and. to a lesser extent, those caused by lightning (Fulgurites) or impactites (explosion of an impact in the atmosphere, which has not reached the soil but whose energy has melted the sand, such as the famous Libyan glass used to make the scarab of the Toutankhamon pectoral).

The first to offer combustion vitrification are Auguste Daubrée (1881) then Alfred Lacroix (1898). Indeed, observations on granites (rich in silica, therefore) reveal a partial fusion, more or less advanced at relatively low temperature. If a granite. a gneiss melt around 950 ° C in surface conditions, the presence peloton discover breton archaeological quirks of water allows fusion at a lower temperature (from 840 ° C).

Peloton discover breton archaeological quirks

Beam fire

We know today how this vitrification was obtained: by the combustion of wood beams that armed the Gallic walls. Indeed. the Gallic ramparts which equipped the oppidums but also certain villas (the farms of aristocrats), were constructions which associated intertwined layers of horizontal beams filled with earth with a facing of “dry stones” (without mortar).

The fire of the beams clearing water lowered the melting point of the granite which formed a glass by cooling. Under the earth’s surface conditions, peloton discover breton archaeological quirks a “dry granite” melts around 950 ° C and a “hydrated granite” from 840 ° C. It is therefore the presence of water that would have allowed this merger of the granite.

For their very majority, the vitrified forts are located in granite regions. It is undoubtedly not a coincidence. because the melting temperature of the granites is relatively low in comparison with that of the basalts, which occurs rather around 1450 ° C. The intentional, or accidental character by fire, nevertheless remains a debate point.