Mines of Paris
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[edit] Paris' minerals — geological composition and formation
Paris lies within the Paris Basin, a geological bowl-like shape created by millennia of sea immersion and erosion. Much of north-western France spent much of its geological history as a submerged sea water coastline, but towards our era, and the formation of our continents as we know them today, the then relatively flat area that would become the Paris region became increasingly elevated, alternately invaded and sculpted by both sea water, inland sea water "lagoons" and fresh water, in addition to above-water air and river erosion. This cycle resulted in the rich and varied strata of geological formation containing many minerals that played a major role in the settlement and wealth of the Paris region.
[edit] Stratification
Paris' earliest history of long deep-sea water immersion resulted in a thick and relatively level layer of chalk; this would play a role in the area's ability to retain freshwater lakes and "lagoons" of sea water from receding ocean waters.
Paris became a receptacle for inland freshwater lakes that created a thick strata of clay. In the next major geographical development, ocean reclaimed the area once again to leave lagoons of immobile sea water, itself often contaminated with inland freshwater; this resulted an equally major strata of limestone (15-20m), then a layer of sand, and finally a band of low-quality limestone. In what is perhaps Paris' most interesting geological formation, the evaporation of inland sea water lagoons, itself periodically invaded by doses of freshwater, resulted in a 40-metre thick deposit of plaster, divided into four successive bands separated by thin deposits of marl. The whole is capped with another layer of clay, itself topped with sand then organic landfill at the surface.
Paleozoic-era continental shifts, particularly the Variscan orogeny geological upheavals, in an extension of the "Meudon bump" land formation, buckled the land to the south-west of the city into a ridge; it was this development that made Paris' limestone deposits exploitable, as it raised them to an altitude above Paris' river network and water table. Erosion sculpted all of the above formations into a form still visible today: over time, sea and freshwater eroded most all of Paris' Left Bank water-soluble gypsum deposits away, leaving a surface of sand and limestone; protected from rainwater erosion by a waterproof cap of clay deposits, the flanks Right Bank gypsum-rich hills of Montmartre and Belleville were the only high ground preserved from river erosion by the ancestor to what it today's river Seine.
[edit] Mining techniques - the exploitation of the Parisian basin
Erosion (by the ancestors of its Seine, Marne and Bièvre rivers exposed many layers of Paris' underlying rich sedimentation deposits to open air. Available from the surface were: in the banks - now hillsides - left by Paris' earliest river erosion, the plaster deposits in the upper reaches of the Right Bank hills of Montmartre and Belleville; below that are sand and limestone deposits, mostly available on Paris' Left Bank. Clay strata was accessible from around Paris' river network, that is to say in the lower reaches of the Seine, Marne and Bièvre valleys.
Once all mineral visible from the surface was exhausted, and the cost of "scraping" any unwanted detritus above it became too dear, miners began to burrow horizontally into hillsides along the vein of desired mineral. From the 12th century, when this technique in turn became too dear or too dangerous, appeared the technique of digging a ramp downwards from the surface towards the level of desired mineral; this soon led to the digging of vertical "well" shafts to the same level. Although the latter technique appeared only to the end of the 13th century, remains of what is thought to have been a Roman-era clay mine were discovered in the 19th century twenty metres under Paris Left Bank Montaigne Sainte-Geneviève hill.
Stone-mining in the Paris area fell into decline towards the early 10th century, because of the population's move to the Right Bank: rather than undertake the onerous task of finding new deposits and opening new mines, Parisians exploited the vast Left Bank ruins (of buildings levelled by 9th-century Viking raids) as an easily accessible source of building material.
Gypsum, the origin of the famous Plaster of Paris, was most often extracted through the flank of the hill that held it. Tunnelling would begin from where the mineral was visible on the surface, and continue horizontally along the strata; the created cavities would be consolidated as the mining progressed. Where the gypsum strata was relatively important, for example the haute masse deposits that reached a thickness of fourteen metres in places, mining would begin at the top of the strata, burrow in, and progress downwards until the bottom of the deposit. The technique used here was piliers et hagues: often piercing the mineral horizontally along the mineral from the flank of the hill, a first series of parallel tunnels were later bisected laterally by a second row of parallel tunnels; the columns of un-mined mineral remaining were the support for the excavation ceiling. For gypsum mines, a widely-spaced tunnel grid was created at the top of the mineral deposit, and once made, miners would "lift" gypsum from the tunnel floor and walls, making the tunnels progressively wider as the excavation progressed downwards. In Paris particularly, in its major haute masse gypsum deposits, an exhausted exploitation had the airs of a cathedral because of its immense height and narrowing columns culminating in arches. Unfortunately, due to fragile gypsum's tendency to crumble when exposed to air and humidity, the mine columns could not retain their stability for long and would often collapse. Only a resemblance one such cavity exists in Paris today, an arch of former mine renovated into a "grotto" attraction for Paris' Buttes-Chaumont gardens.
[edit] Paris' growth over abandoned mines - a city over an abyss
There are no concrete proof of any mining activity before the late thirteenth century. The earliest text we can find is but a mention in the town commerce register: Paris had 18 "quarriers" in 1292. The first written act concerning any mine dates from almost a century later, 1373, in an authorisation that a certain Dame Perrenelle be permitted to operate the plaster mine already existing in her property to the lower flank of Montmartre.
The majority of Paris' stone deposits were in its Left Bank, and at the time of the city populace's 10th-century move to the Right Bank, well to the suburbs of the former Roman/Merovingian city. As the stone from the abandoned ruins became depleted from the 13th century, new mines began to open further from the city centre. Earlier mines closer to the city centre, when discovered, sometimes served a purpose: when Louis XI donated the former Chateau Vauvert, a property in an area today the northern part of today's Luxembourg Gardens, to the Chartreuse order in 1259, the monks renovated the caverns under their property into wine cellars, and continued the exploitation of stone to the ancient mine's extremities.
By the early 16th century, there were stone excavations operating around today's Jardin des Plantes, Boulevard St-Marcel, Val-de-Grâce hospital, southern Luxembourg (by then the Chartreuse Coventry) and in areas around the rue Vaugirard. Paris' then suburban plaster mines remained for the most part to the Right Bank Montmartre and Belleville hills.
It was only with its expansion past its 13th-century walls that the city began to build on previously-mined land. The left bank faubourgs were the most at risk: in the 15th century, the largest were the faubourg Saint-Victor (from the eastern extremity of the rue des Écoles and south down the rue Geoffroy St Hilaire); the faubourg St Marcel (rue Descartes, rue Mouffetard) and the faubourg Saint-Jacques (along today’s rue Saint-Jacques below the rue Soufflot); lastly, the faubourg (then bourg) Saint-Germain-des-Prés below today’s church of the same name.
Although seventeenth-century Right Bank Paris had in five centuries expanded past three successive arcs of fortifications, Left Bank Paris was nowhere near as dense in comparison within its unchanged but crumbling 13th-century city walls. Many royal and ecclesiastical institutions came to the area during this period, but by then it seems that the mined state of the Paris faubourg underground had been forgotten by then: The Val de Grâce coventry and the Observatoire observatory, built from 1645 and 1672 respectively, were found to be undermined by immense caverns left by long-abandoned stone mines; reinforcing these took most of the budget consecrated to both projects.
Faubourg growth remained along the main routes from the city, but began to expand at a faster rate with the rise of traffic along the routes to the Fontainebleau and Versailles castles. The route de Fontainebleau (extending to the south of today's Place Denfert-Rochereau) would be the site of one of Paris' first major mine collapses in 1774.
[edit] The consolidation period
The above disaster was in part responsible the Conseil du Roi's decision to create a special division of architects responsible for the inspection, reparation and maintenance of the ground under royal buildings within and without Paris. Another division of inspectors created around the same time, but under the direction of the Ministry of Finance, claimed the role of assuring the safety of the national roadways that were their jurisdiction. Created officially on the 24th of April 1777, the Inspection Générale des Carrières entered service on the same eve after a new collapse of the route de Fontainebleau (Avenue Denfert-Rochereau) outside of the barrière d'Enfer city gateway. Although the Ministry of Finance continued to claim jurisdiction over damaged roadways, this rather inept service was eventually succeeded by the Crown-appointed IGC.
As the centuries of mining under Paris' underground went mostly uncharted, thus largely forgotten, the real extent of former mines was unknown then. All important buildings and roadways were inspected then: any signs of shifting were noted, and the ground underneath sounded for cavities. Roadways were particularly problematic: instead of sounding the ground around the route, inspectors instead tunnelled directly under the length of endangered roadway, filling any cavities they found along the way, and reinforcing the walls of their tunnels with solid masonry to eliminate the possibility of any future excavations and disasters. When a length of roadway was consolidated, the date of the work was engraved in the tunnel wall under it, next to the name of the roadway above; today Paris' underground tunnels, dating to as early as 1777, are a living testimony to Paris' old street names and roadways.
[edit] Bibliography
History
- (French) Gérards, Emile (1908). Paris Souterrain. Sides. ISBN 2840220024.
