► [Géol' Tours] ◄
Tours is a metropolis located in the heart of the Loire Valley, classified as World Heritage by UNESCO.
Since the foundation of the Roman city of Caesarodunum, Tours has grown under the influence of the Loire and the riches of the Touraine basement. Its houses, religious buildings, castles or bridges come from stones extracted in the region.
[Géol' Tours] offers a series of EarthCaches describing rock natures, geological or hydrological phenomena, in order to discover the geological richness on the banks of the Loire and in the streets of the city.
► The Wilson bridge ◄
The Wilson bridge was built between 1765 and 1778. It is the oldest bridge in the city of Tours. It is 434-meters long, with 15 arches spanning the Loire.
First named the "Stone Bridge", it was renamed August 13, 1918 as a public tribute, to thank Woodrow Wilson, president of the United States from 1913 to 1921 for his country's participation during the World War I. Tours was then an important American base.
The Wilson Bridge has been classified as a Historic Monument since 1926.
The bridge has a chaotic history! Even before its completion, a vault collapsed in 1776, and a pile collapsed in 1777. Then four arches on the north side collapsed in 1789 under the pressure of a Loire with a torrential flow and carrying many blocks of ice during a very vigorous winter (the "ice break-up"). The reconstruction lasted another 20 years, before three arches collapsed in 1835. After a certain period of calm, the bridge was blown up twice during the Second World War: first on June 18, 1940 by the Tourangeaux themselves to protect the city from the German troops, then on August 22, 1944 by the Wehrmacht to ensure their retirement. The last incident took place on April 9 and 10, 1978, six arches and five piers on the south side collapsing successively, ie a third of the bridge. The bridge was returned to service on September 18, 1982.
April 9, 1978: Mayor Jean Royer in front of the collapsed Wilson Bridge.
Today, only one of its 15 arches, the seventh, is still original. Originally built in Touraine lake limestone, exploited in the old quarries of Athée-sur-Cher, near Tours, other rocks were later used for successive reconstructions: limestones of marine origin.
► The Touraine lake limestone ◄
During the first part of the Tertiary era (Paleocene, Eocene and Oligocene), the Touraine experienced a particularly hot climatic context, which we would describe as "tropical" today, with alternating and contrasting seasons. One is humid and ensures the leaching of soils and the transport; the other is dry and gives off stretches of limestone mud. A large lake, fed by the surrounding rivers, settled in Touraine, the basement of which was slightly sagging.
The evaporation of lake water during the dry season resulted in the deposition of limestone mud rich in calcium carbonate. This mud was greatly altered on the emerged edges: gas escape bubbles, cracking by drying, colonization by plants (grass), gravels displaced by runoff when the rains returned, etc. The Touraine lake limestone results from the hardening of this mud at the end of the Ludian (Priabonian) Eocene.
The Stone Bridge was built in the 18th century with Touraine lake limestone extracted from the quarries of Athée-sur-Cher, 20 km south-east of Tours. Collapsed stones from the bridge and other stones extracted from the same quarries were later used for reconstructions.
Lake limestone cut in large blocks is especially visible in the pillars and vaults of the first arches, north side of the bridge, and also in the parapets, along the entire length of the bridge.
Lake limestone is recognizable by its gray color, and the numerous small millimeter-sized cavities it contains: it is said to be vacuolar. Some of these cavities, of almost circular shape, are due to the degassing of the mud. Others are thin and elongated: these are the traces of ancient grass roots.
► Limestones of marine origin ◄
Many buildings of Tours, recent or old, were built in limestone formed at the bottom of the sea. These stones come from different French regions (mainly Touraine, the quarries being in the immediate vicinity of the construction sites, and the bordering regions) and at different geological periods of the Jurassic and Cretaceous during the Mesozoic era (Secondary). Current France is then gradually submerged by waters coming from the Atlantic and the North.
Continental and marine area in the Middle and Upper Jurassic.
For their hardness and resistance, the Wilson Bridge was partly rebuilt with oolitic limestone of Chauvigny and limestone of Verger.
► The oolithic limestone of Chauvigny
Chauvigny is a commune in the Vienne department, in the Poitou-Charente region, and located 90 km south of Tours. The limestone extracted from the quarries of Chauvigny dates from the geological era of the Bathonian.
This limestone is characterized by its beige / yellowish color, and its ooliths which appear in successive beds. Ooliths are millimeter balls, similar to fish eggs. They were formed under a few meters of agitated water, near coral reefs, by deposit of calcite around very small pre-existing grains. These ooliths were then welded together with a calcite cement giving a hard rock.
The oolithic limestone was used in the parapet of Pont Wilson. In addition to the bridge, many buildings in Tours were built in Chauvigny ollithic limestone, in particular the Town Hall and the Tours train station.
► The limestone of Verger
The limestone of Verger is extracted from quarries located in the town of Suilly-la-Tour, in the Nièvre department, in the Bourgogne-Franche-Comté region, and located 180 km east of Tours. This limestone is a facies of Burgundy stone, and dates from the geological era of the Kimmeridgian. Marine transgressions continue during this geological period, with successive more or less strong advances on Bourgogne. Sedimentation deposits marls, clays and limestones. The tropical waters are shallow and host abundant wildlife.
This limestone is fine grain and light gray in color. It is characterized by tubular shapes, bordered by a darker gray: these are former burrows of marine animals, in particular crustaceans (lobster type). Digged in the mud at the bottom of the sea by crustaceans, these traces were consolidated with the rock. We then speak of bioturbation.
This limestone was used for the reconstruction of the southern part of the bridge, under the vault of the first arch and several pillars.