V – Paris Geology/ - The rise of Paris
The earth's history is long, and still evolving. Carbon dating drastically changed the geological time scales in the mid-20th century, and again through more recent techniques and new innovations and theories on the formation of earth's crust.
I had originally written this chapter as the first stop to Paris' underground. My studies about the formation of the Parisian basin carried me progressively towards the beginning of earth's history. Many of the books I have read concerning Paris' underground speak of its mines, but few mention in detail the reason why the mined mineral was there in the first place. One exception is Emile Gerard's book 'Paris Souterrain,' but as it was published in 1906, much of its geological and fossil information is outdated. Most of this chapter is taken from this work with the necessary modern modifications. This information, though it is unattached to the more 'dramatic' events of Paris' underground, helped me greatly to understand the origin and exploitation of Paris' mines, and even mining in general.
The principles of Sedimentation
The base of the earth's crust is made up of a very hard rock. It was created through the eruption and cooling of lava that was a mix of silica, aluminium oxides, potassium, sodium, calcium, iron, and magnesium. These base elements - depending on their mixture and the speed at which they cooled - formed different crystalline compositions of different density. Granite is the most widely exploited rock of this type.
Oxygen and hydrogen freed from volcanic activity mixed as condensation to form clouds, which in turn produced rain. This created seas and oceans, and the still-constant movement of the earth's crust alternately raised and lowered surface rock above and below their surface. Bodies of water captured and sifted the earth's base minerals to form layers on their bottoms in sedimentary deposits. More importantly for Paris, Oceans were the origin of the earth's first life: the residues of algae, plant, and then animal species added new ingredients to the above mixture.
Sedimentation varies depending on the depth and agitation of a body of water. A sea's wave action washes sediment grains to and fro towards its shore and "sifts" it; grains of sand and fossilized matter fall where they are thrown according to their density and/or form, whereas silt only settles in deeper waters that are almost dead calm. This separation forms bands of homogenous sediment in a process called "stratification." The water itself affects these deposits: the salt and acids in seawater dissolve organic matter, and freshwater allows natural decomposition by oxidisation or dissolution. Water works in this last way not only above the earth's crust, as it settles to fill any cavity until depths where the earth's heat is too great for its hydrogen and oxygen to exist in a combined form. This action carries some minerals to mix with those in lower levels in veins: an obvious example is marble.
The Rise of Paris
Paris remained relatively flat through earth's history because of its position away from the edge of the tectonic plate that makes most of today's Western Europe. It spent time above and below water during the separation of the continents, but mid-way through this evolution, in the Cretaceous period (around 145 million years ago), it had one final long period of immersion before gradually rising above sea level.
The sedimentation that resulted from this long time underwater measures over four hundred metres thick under the Parisian basin. It appeared in the final stage of the Mesozoic era ("Meso" meaning Greek "middle," and "zoic" "life" or "animal"), in the "Supra-Cretaceous" sub-period of the Cretaceous period. This deposit is representative of sea sedimentation: primitive sea invertebrates, after their demise, fell to the calm of the deep sea bed to decompose, leaving only their carbon content and shells to be mixed with the calcium salts of the sea brine. The result, after millions of years of accumulation and compacting, was a band of an unconsolidated (amorphous) form of calcium carbonate, or chalk.
The end of the Mesozoic era, around 65 million years ago, was marked by the disaster that eliminated 80% of all life on earth including dinosaurs and almost all forms of sea plankton. The next era, the Cenozoic ("Ceno" - Greek "Kainos" - meaning "recent") that covers about 64 million years until the earth's most recent history, began with the gradual rise of northern France to its above-water altitude. Only “modern” Paris would exploit the sedimentation accumulated from this point onwards.
The divisions of the Cenozoic era are more complicated than those before it, but its "stages," the smallest subdivision of the geological calendar, suffice to describe the formation of Paris until today. Still, some mention should be made of the intermediary divisions of eras into periods, sub-periods, epochs, and stages: the Cenozoic era is divided into two periods, the Tertiary (third) and Quaternary (fourth); the Quaternary period has no sub-periods, but the Tertiary period has two: the Palaeogene ("ancient-origins") and the Neogene ("new-origins"). It is necessary to detail the only the epochs of the Palaeogene sub-period here, as it was then that Paris' sedimentary stone appeared: its oldest epoch is the Palaeocene (Greek "ancient-recent"), then the Eocene epoch ("dawn-recent") from around 56 million years ago, and finally the Oligocene epoch ("fairly-recent") which began about 35 million years ago. The names of the stages of these three epochs change according to the evolution of their consistence and fossil content, and will be detailed with the evolution of the Parisian basin's geological activity.
Northern France's base for its future sedimentary stone appeared, as already mentioned, during the Supra-Cretaceous sub-period that marked the end of the Mesozoic era. It was this four-hundred metre gain in altitude that caused the large variation of deposit to come in later epochs; as the rising and lowering of the sea was less extreme from then, northern France was exchanged between sea and land at a larger frequency, and had an immersion at shallower depths than during previous eras.
Paris emerged partially from the ocean during the Palaeocene epoch's first age, the Danian, and then was submerged again during its second epoch, the Montian. The receding ocean's wave action during the Danian epoch had carved shallow bowl formations out of the chalk bed and left a sediment of roughly sorted sand grains, and its return filled these new formations with a mix of sediment and dead sea life.
These inland pockets, treated by some as "lagoons," were important in Paris' the formation of sedimentary stone. They formed a long and shallow coastline, and aided the waves to "sort" sand grains and other debris into various densities as they carried them towards land. Lagoons retain seawater if the sea recedes permanently, and then become freshwater lakes if rain is abundant. This would be the case in Paris' next stage of sedimentation.
The ocean began to recede again at the beginning of the Thanatian stage, the last of the Palaeocene epoch. This was also a time of global warming, and high humidity caused much inland rain. The precipitation replaced the saltwater content of any lagoons trapped inland by the receding sea, and filled them to a point where they merged to form a lake that covered the whole of the Parisian basin.
Calm freshwater created an environment proper for a new deposit, clay, that filled all crevices and valleys and made Paris flat once again. The most common form of clay, and that under Paris, is a mix of microscopic grains of alumina and silica sand which, possible only in freshwater, become glued together by amorphous calcium and becomes a hydrous aluminium silicate mix. Aluminium is sometime combined or replaced with magnesium or iron, which can result in varying colours. Water is an essential part of this mineral, and it can only be removed through heat or a prolonged exposure to air. While wet it is "waterproof" to any additional humidification and can be moulded into any form, and when dry it becomes hard and brittle. This bed of clay was the first mineral in the Parisian basin to be exploited by man.
The next period of change to the Parisian basin began with the "Ypresian" stage, the first of the Eocene epoch. The earth's crust, still in movement then, made a "wrinkle" with its cumulus in Meudon, whose slopes stretched until the western side of Paris' left bank. This barrier blocked the advance of the sea when it returned from the north-east once again, and forced its sediment to gather on its shoreline on today's right bank. The deposit brought by this immersion was a mix of silica and quartz sand from earlier erosions.
This partial immersion was the first tides of what would transform Paris into a complete undersea environment during the next "Lutetian" stage (named for Roman Paris, as it is seen as a classic example for the sedimentation trends of its time). As the sea advanced and receded, gaining progressively more territory with each return, it left grainy sand deposits to mix with clay residues which formed indistinct bands of alternating freshwater and seawater deposits. The Lutetian stage gained in importance only after the sea managed to bypass the hill of Meudon's northern slope and claim the whole of the Parisian basin.
The ocean's rise during the Lutetian stage didn't cover Paris permanently. Its peak invasion was towards the middle of the stage where it covered France well towards its centre, but during its overall progression then recession, Paris was alternated between a partial and complete submersion. During long periods of inland stagnation, any lagoon deposits and saltwater were cleaned of brine by inland freshwater. In periods of exposure to the sea, lagoon sedimentation consisted largely of the remains of shell animals that were "sorted" according to their size and weight by the ocean wave action as they were carried inland. Agitated coastlines left a deposit of a larger granularity in varied sizes, but more towards the centre of the sea, a wave action's constant to-and fro movement towards the shore separates grains into bands of deposit of the same consistency - this sorting is what determines the quality or "maturity" of a sedimentary stone. In the case of the Lutetian stage, the base sea-animal sedimentation was of a new species, the "nummulite" (Greek "small medallion") that forms a major part of calcaire grossier.
This small animal appeared for the first time in the beginning of the Ypresian stage evolved into larger and more developed forms during the Lutetian. It was a silica-secreting invertebrate, and silica forms a "cement," when crystallised and compressed, that bonds the matrix of sand and residues of calcium shells (calcium carbonate) in a form called "calcaire grossier." The quality of a stone is determined by both its granular maturity and cohesion - Paris was lucky enough to have the right geological conditions for the formation of a series of banks that had the best of both. Paris' stone was of a very high quality, and increasingly so with each additional level of deposit until mid-way through the Lutetian cycle.
Paris' calcaire grossier is divided into three levels: the inferior Lutetian, the middle Lutetian, and the Supra-Lutetian. The first two are commonly divided into five subdivisions, and the Supra-Lutetian is divided into three banks. The best quality of stone appear from the last bank of the middle Lutetian until the lower "bancs francs" of the Supra-Lutetian. The end of the middle Lutetian banks is marked by a "banc vert" (green bank) which formed during a period where sea sedimentation spent more time under freshwater than seawater, in conditions that made a hard stone consisting of calcium shells and sand bonded both by clay and silex crystallisation. The Supra-Lutetian deposits were highly varied, two of which were suitable for building purposes, but the calcaire grossier deposit's exploitability ended halfway through the bank with an extremely hard layer of silica-based sandstone called "roche." The levels above were of a lesser quality because of the rough debris left by departure of the sea from the Parisian basin towards the end of the Lutetian cycle.
The next cycle was the third of the Eocene period, the Bartonian. The seas rose rapidly then, a result of melting glacial formations from the last cycle. Much of the upper levels of Lutetian deposit were washed away by the rapid sea currents, replaced only by a thick layer of quartz and silica sand called "Sable de Beauchamp" in the Parisian basin. This formation was crowned by a thick layer of "Calcaire Saint-Ouen," a result of another series of departures and returns of the sea that left a series of lagoon and assorted sea deposits. The land formations created through the sea activity of the early Bartonian formed the base for the arrival of the next major banks of exploitable mineral, gypsum.
Gypsum is of chemical origin, a product of evaporated sea brine. The temperature was dry and almost tropical at the beginning of the Bartonian, which meant that any lagoon left inland by the receding sea retained its sea brine instead of having it washed away by freshwater rivers and rains. The mostly sodium and calcium content of the seawater, which mixed with any organic matter trapped by the pool, gradually became a crystalline "soup" of compact matter. An addition of anhydrite salts causes this mixture to retain some humidity, but it is very fragile to the invasion of additional water.
This deposit covered the Parisian basin in four successive periods through the Bartonian and Priabonian (last of the Eocene period) cycles. It was divided into by sea and freshwater invasions that left only thin layers of sandy and clayey residue, and each successive deposit increased in purity because its diminished exposure to water. The first bank to be deposited was washed away in many places under the Capital, as its overlying layer of "waterproof" clay deposit was too thin to protect it from freshwater. The next three layers, much more important in thickness and purity are numbered, in reverse order of their appearance, the first, second, and third masses of gypsum. The first and highest gypsum mass is over twenty metres in thickness under the hills of Montmartre, Belleville, Chaumont, and Menilmontant. It formed the centre of Paris' mining activity in the latter 19th century, as it has many uses after being baked, crushed, and sifted into a powder form commonly known as plaster.
Luckily for future mankind, a cooling of temperatures provoked rain and freshwater lakes that deposited a varied cap of impure and sandy clay, known as "glaise vert," over the gypsum deposits. This marked the end of the Priabonian stage, the end of the Eocene epoch, and the beginning of the Oligocene epoch that began 35 million years before our time.
Paris takes its final form
The Oligocene epoch was the last in which the sea would cover Paris. In its first stage, the Stampian (also Rupelian), the deposits are similar to the early Lutetian: thin layers of calcaire grossier alternated with argyle-laced freshwater sedimentary stone. This level has its own appellation, "calcaire de Brie." Next came a complete sea emersion that brought a thin layer of fairly pure but crumbly calcaire remarkable for its primitive oyster content that gave it the name "marne a huitres." The next deposit, the Chattian stage, was a thick layer of sand of quartz and granite; the rising ocean had captured above-land erosion from afar and deposited it near its shoreline, which left a layer of "sables de Fontainebleau" over Paris.
The end of the Chattian stage was also the end of the Palaeogene sub-period. The Antarctic was frozen my then, which meant a drop in the earth's temperatures, a drop in sea level, and the humidification of inland climate which resulted in the development during the next sub-period, the Neogene. Melting glaciers during the latter sub-period's first Miocene epoch filled higher altitude lakes to overflowing, which is held in part responsible for a diluvia rush of water that eliminated much of the Fontainebleau sand formations in the Parisian basin. Another cause was the continuing movement of the earth's crust, the same that formed the Alpian and Pyrennée mountains, which caused the Parisian formations to wrinkle and form potential riverbeds. New glacial formations in northeastern Europe humidified the climate again at the end of the Miocene epoch, which led to more diluvian downpours and torrential rivers during the Pliocene.
The Quaternary period, the next and last, began 1.34 million years ago. It had several cycles of warming and cooling, but France had reached its almost final shape and altitude by then, and the change in climate served only to vary the freshwater irrigation of the Parisian basin. The ancestors of today's Seine river were thought to have been over six kilometres wide at times, and their force eroded much of the above formations back to their early-Tertiary levels, which meant through 100 metres of sediment that had taken over 65 million years to accumulate - today's hills of Chaillot, Montmartre, Chaumont, Belleville, Menilmontant, Mount-Saint-Genevieve, the Butte aux Cailles, and Montrouge (flank of Montsouris) are all that remain. The Parisian basin had its final form by the beginning of the Holocene epoch 100,000 years ago, but had yet to be inhabited by man.
Our ancestor Homo sapiens was already present in Europe around 40,000 years before our time. His first tendency was to migrate costal regions, as sea life was a principal source of his alimentation, but hunter-gatherer tribes gradually moved inland along rivers. The last glacier period (Würm - 10,000 years ago) ended with a global warming which completely changed the habits of man; he invented agriculture, pottery, and navigation, but he hesitated to move further inland because flooding caused by the receding glaciers. He finally settled more northerly regions and established Western Europe's first villages around 5,000 B.C.