Mines of Paris

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.[3]

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.

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.

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[4] 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.

Open-air quarrying became quite difficult and even costly when the desired minerals lay below the surface, as sometimes enormous amounts of earth and other unwanted deposits would have to be removed before it could be extracted. One means of avoiding this problem was to dig horizontally into a hillside along the mineral strata from where it was visible in its flank, but the Paris area had few mineral deposits, save gypsum, the disposition of which fulfilled these conditions. There were few open-air stone quarries by the 15th century; instead, miners would access the targeted stone deposit through vertical wells, and dig into it horizontally from there. Although it seems that well-mining method only began then, there is evidence that Romans used this technique to mine clay under Paris's Left Bank Montagne Sainte-Geneviève hill.

No matter the means they used to access the underground mineral, miners had to provide a means of maintaining the enormous weight of the ceiling over their horizontally-burrowed excavations. The earliest means to this end, in a technique called piliers tournés, became common from the late 10th century. A tunnel would be dug horizontally along the deposit, tunnels perpendicular to the first were opened along the way, and tunnels parallel to the initial tunnel would be opened through these. The result was a grid of columns of untouched mineral deposit, or piliers tournés, that prevented a mine's collapse. In areas where a mineral was removed in a wider swath than the rest of the mine, usually towards the edge of the exploitation, miners would complement the natural mineral columns with piliers à bras, or stacks of stone creating a supporting column between floor and ceiling.

Gypsum mines, the origin of the famous plaster of Paris, used this technique with an added third dimension: as some of northern Paris's gypsum deposits measured 14 metres (46 ft) thick in some places, miners would create their tunnel grids in the top of the deposit, then begin extracting downwards. A gypsum mine in a particularly thick deposit had almost a cathedral-like air upon depletion, because of the towering columns and arches of mineral remaining. Only one example of this sort of gypsum-mining remains in Paris, in a renovated "grotto" under the Buttes-Chaumont gardens.

This method of burrowing was effective for the short-term, but over time the relatively soft mineral, subject to the elements and the earth's shifting, could erode or fissure, endangering the solidity of the mine.

Another technique appearing towards the early 18th century, hagues et bourrages, was both more economically and structurally sound. Instead of tunnelling into the exploitable mineral, miners would begin at a central point and extract stone progressively outwards; when they had mined to a point that left a wide area of the ceiling unsupported, they would erect a line of piliers à bras, continue their extraction beyond that line, then return to build a second parallel line of stone columns. The space along both lines of columns was then transformed into walls with stone blocks, or hagues, and the space between filled with packed rubble and other mineral detritus (or bourrage). This technique allowed much more of the targeted mineral to be extracted, and provided a support that could both settle and shift with the mine ceiling it supported.