The mines of Paris (French: carrières de Paris – "quarries of Paris") comprise a number of abandoned, subterranean mines under Paris, France, connected together by galleries. Three main networks exist; the largest, known as the grand réseau sud ("large south network"), lies under the 5th, 6th, 14th and 15th arrondissements, a second under the 13th arrondissement, and a third under the 16th, though other minor networks are found under the 12th, 14th and 16th for instance. The commercial product was Lutetian limestone for use as a building material, as well as gypsum for use in "plaster of Paris". Exploring the mines is prohibited by the prefecture and penalised with large fines. Despite restrictions, Paris's former mines are frequently toured by urban explorers known popularly as cataphiles. A limited part of the network—1.7 kilometres (1.1 mi) in length—has been used as an underground ossuary, known as the catacombs of Paris, some of which can be toured legally. The catacombs were temporarily closed between September and 19 December 2009 due to vandalism, after which they could be legally visited again from the entrance on Place Denfert-Rochereau. The entire subterranean network is colloquially referred to as "The Catacombs".
Formation of the minerals mined in Paris
Paris lies within the Paris Basin, a geological bowl-like shape created by millennia of sea immersion and erosion. Much of northwestern France spent much of its geological history as a submerged seawater coastline, but towards our era, and the formation of our continents as we know them, the then relatively flat area that would become the Paris region became increasingly elevated. The region was alternately invaded and sculpted by both sea water, inland sea water lagoons and fresh water, in addition to above-water air and river erosion. These cycles produced a rich and varied geological strata containing many minerals that would become a source of growth and wealth for the Paris region.
Mineral formation The region of Paris has spent most of its geologic history under water, which is why it has such varied and important accumulations of sedimentary minerals, notably Lutetian Limestone. The Paris area was a relatively flat sea-bottom during the early Cretaceous period: first in a deep-sea environment, then under a more agitated near-shoreline sea towards the end of the same period, Paris's largely silica-based sedimentary deposits became, under the action of pressure and the carbonic acid content of seawater, a thick deposit of clay. The invasion of calcium-rich seas then covered this with an even more important layer of chalk. Paris emerged from the sea towards the end of the Cretaceous Period, and later Palaeozoic-era continental shifts, particularly the Variscan orogeny geological upheavals, created a series of hills and valleys throughout the Parisian basin, creating conditions ideal for the mineral deposits that would appear during the next eras. After a long period above sea level that ended towards the Cenozoic era, Paris began a period of alternation between sea and land environments. Paris was the middle of a shoreline of bays and lagoons of still seawater, an environment perfect for the silica-based sea life abundant then. As sea creatures died and settled to the lagoon bottom, their shells mixed with the deposits already present; pressure from additional sea-life sedimentation and the chemical action of the water transformed the result into a sedimentary stone quite particular to the Paris area, calcaire grossier (calcaire lutécien in more modern publications). Paris's most important deposits of this stone occurred during the Eocene epoch's Lutetian age; in fact, the age itself is named for the sedimentary activity in the Paris region, as Lutetia was the city's name during Roman times. Paris's next important mineral deposit came with the Bartonian age. After a period of land-sea alternation that brought layers of sand and low-quality calcaire grossier, the sea regressed again to return only occasionally to refill lagoons with seawater. The result was stagnating pools of evaporating seawater; the salts of these, mixed with other organic matter and mineral deposits, crystallised into the calcium sulphate composition that is gypsum. This evaporation cycle occurred several times during this age, creating several layers of gypsum divided by layers of mineral left by the sea's brief return. In all, Parisian gypsum deposits are divided into four "masses", with the last appearing, the haute masse, being the most important and most exploited in Parisian history. Gypsum, an evaporite mineral, is known for its fragility against freshwater invasion, re-dissolving quite readily. The sea returned one last time to the Parisian basin towards the end of the Paleogene period, leaving several layers of varied sediment capped with a thick layer of clay. This last deposit was important when the Paris basin rose from the sea, this time definitively, during the early Neogene, as the topmost layer would protect the soluble gypsum layers from erosion by air and weathering.
Erosion Paris began to take the form we know at present as huge rivers resulting from the melting of successive ice ages cut through millions of years of sediment, leaving only formations too elevated or too resistant to erosion. Paris's hills of Montmartre and Belleville are the only places where gypsum remained, as the ancestor of the river Seine once flowed, almost along its present path, as wide as half the city, with many arms and tributaries.
Mining techniques
Open-air quarries The most primitive mining technique was to extract a mineral where it could be seen on the surface, in places where millennia of erosion by the ancestors of the Paris basin's rivers Seine, Marne and Bièvre exposed many levels of Paris's underlying stratification to open air. Minerals available from the surface, beginning with Paris's highest elevations in the valleys created by this erosion are: the plaster deposits in the upper reaches of the Right Bank hills of Montmartre and Belleville; lower in the valleys are sand and limestone deposits nearest the surface on Paris's Left Bank. The underlying clay strata were accessible from the surface in the lowest points of the Seine, Marne and Bièvre river valleys.
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