User:ThePromenader/sandbox
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[edit] Paris' minerals - geological composition and formation
Paris lies within the Paris Basin, a geological bowl-like formation created by successive millennia of sedimentary deposits and erosion. Upon with a thick layer of chalk disturbed by the Variscan orogeny (an extension of "the Meudon bump"), Paris' exploited mineral deposits were, in order of deposit: a thick strata of clay, an equally major strata of limestone (15-20m), sand, a band of low-quality limestone, three to four successive bands of plaster (together totalling 40m in places) 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.
(what era) erosion sculpted all of the above into a form still visible today: both sea and freshwater eroded most all of Paris' Left Bank water-soluble gypsum deposits away, leaving a surface of sand and limestone; protected from the surface by a waterproof clay cap, it was river erosion that cut the flanks Right Bank gypsum-rich hills of Montmartre and Belleville.
[edit] Mining Techniques
The first minerals mined in the Paris basin were those visible on the surface: clay and sand in the banks of the strata-eroding river, then limestone where it could be found in the same. Plaster could be found in the flanks of the hills of Montmartre and Belleville, but these would not be exploited to any great extent before the beginning of the first millennium. Pit-mining was the next common step to expose new layers of mineral once it was exhausted on the surface.
Mineral scavenging and rough pit-mining most likely took on a more industrial aspect with the Roman era. The Romans brought other techniques as well: Roman potters are thought to have made deep wells discovered near the Left Bank Pantheon, as a means to attain the thick clay deposit underlying the Montaigne Sainte-Geneviève hill. Mining in this method became rare from the decline of the Roman Empire, and pit-mining would become the norm once again until the early 11th century. Another reason for a drop in mining activity around Paris was its move to the Right Bank from the early 10th century; the Roman and Merovingian town, having been destroyed by 9th-century Norman invaders, became a mine in itself when its abandoned and toppled buildings were recycled as building materials.
Gypsum, the origin of the famous Plaster of Paris, was most often exploited through the flank of the hill that held it. Tunnelling would begin from where the mineral was visible on the surface, and continue along the strata, and 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: a first series of tunnels would be bisected by a series of parallel tunnels lateral to these, and the columns of unexploited mineral remaining would serve as of support for the excavation ceiling. The tunnels would become wider as the excavation progressed downward; an exhausted haute masse exploitation, because of its immense height and towering columns culminating in arches at the excavation cieling, would often have the aspects of a cathedral. Only one such cavity exists in Paris today, renovated into a "grotto" under one of the hills of Paris' Buttes-Chaumont gardens.
The technique of peircing vertical "sounding wells" from the surface to the later of desired mineral became a general practice only when the mineral accessible through other means was exhausted.
