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Author: Admin | 2025-04-27
Action of pressure and the carbonic acid content of seawater, a thick deposit of clay.[citation needed] The invasion of calcium-rich seas then covered this with an even more important layer of chalk.[citation needed] 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.[citation needed]After a long period above sea level that ended towards the Cenozoic era, Paris began a period of alternation between sea and land environments.[citation needed] 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.[citation needed] 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).[citation needed] 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.[citation needed]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.[citation needed] 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.[citation needed] 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.[citation needed] 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.[citation needed] Gypsum, an evaporite mineral, is known for its fragility against freshwater invasion, re-dissolving quite readily.[citation needed]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.[citation needed] This last deposit was important when the Paris basin rose
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