🌍Le secret des murs de l'église Saint Pierre EarthCache

📝 Missions de terrain (Questions de validation)  :

Question 1 : Observer les « contreforts » des murs de l'église (wp1, wp2 et wp3), en vous basant sur leur apparence (couleur, présence de trous, dureté apparente, taille du grain…) ainsi que le descriptif ci-dessous, quel type de roches pouvez-vous identifier ?

N'hésitez pas à poser votre main sur les différentes pierres pour comparer leur texture et leur dureté.

Question 2 : Observer les zones noires aux wp1 , wp2, wp3. A quoi sont-elles dues ? Sont-elles situées sur les parties les plus exposées à la météo ou au contraire dans les zones protégées (creux, sous les corniches) ? Pourquoi, selon vous ? Quel effet la météo a-t-elle eu sur le calcaire au fil du temps ?

Question 3 : Au wp2 : sous le rectangle de couleur : de quelle couleur est la pierre ? Selon vos observations, cette couleur est-elle répartie uniformément sur toute la pierre ou semble-t-elle se concentrer à certains endroits ? Que nous dit ce phénomène sur la composition chimique de la pierre ?

Question 4 : Au wp4 : 🧩 Mission de terrain : Détective des pierres : comparez la pierre des murs avec celle des pavés au sol, laquelle est plutôt poreuse ? Laquelle est plutôt très dure ? Quel est le nom de la pierre du sol ? Pourquoi les bâtisseurs ont-ils choisi des pierres différentes pour le sol et pour les murs ? Si l'on avait construit l'église avec la pierre des pavés, quel aurait été l'avantage ? Et si l'on avait pavé la place avec la pierre de l'église, que se serait-il passé au bout de quelques années ?

Question 5 : Une photo de vous, ou d’un objet caractéristique vous appartenant prise devant la porte d’entrée de l’église à joindre soit en commentaire soit avec vos réponses. Conformément aux guidelines mise à jour et publiées en juin 2019, des photos peuvent être exigés pour la validation d’une earthcache.

Exemple de Photo-Preuve

 Flat et nous-même nous vous souhaitons une excellente découverte géologique !

Une fois que vous l'aurez validé et si vous le souhaitez vous pourrez mettre ce souvenir sur votre profil en copiant le code

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📝 Field Missions (Validation Questions)

Question 1 :  Observe the "buttresses" of the church walls (WP1, WP2, and WP3). Based on their appearance (color, presence of holes/voids, apparent hardness, grain size, etc.) and the description below, what type of rock can you identify? Do not hesitate to touch the various stones to compare their texture and hardness.

Question 2 : Observe the black areas at WP1, WP2, and WP3. What causes them? Are they located on the parts most exposed to the weather or, on the contrary, in sheltered areas (recesses, under cornices)? Why, in your opinion? What effect has the weather had on the limestone over time?

Question 3 :  At WP2, under the colored rectangle: what color is the stone? Based on your observations, is this color distributed uniformly across the entire stone, or does it seem to concentrate in specific areas? What does this phenomenon tell us about the chemical composition of the stone?

Question 4 :  At WP4, 🧩 Field Mission: Stone Detective: Compare the stone of the walls with the stone of the ground pavers. Which one is more porous? Which one is very hard? What is the name of the stone used for the ground? Why did the builders choose different stones for the ground and for the walls? If the church had been built with the paving stone, what would have been the advantage? And if the square had been paved with the church stone, what would have happened after a few years?

Question 5 : A photo of yourself or a characteristic object belonging to you taken in front of the church entrance must be attached to your log or sent with your answers. In accordance with the guidelines updated and published in June 2019, photos may be required for the validation of an EarthCache.

Example of Photo-Proof

Flat and ourselves wish you an excellent geological discovery!

Introduction: The secret of the walls: A dive into the marine past

Welcome to the Saint-Pierre church in Plaisir. This building is not just a historical site; it is also an open book on the geological history of our region. By observing its walls, you will discover the "Lutetian limestone," a rock formed 45 million years ago in a tropical sea, right where you are standing today.

🌊 Plaisir, in the heart of a tropical sea Millions of years ago, this place was very different. Back then, the region was covered by a warm, shallow sea—an ideal environment for abundant marine life.

The Lutetian Limestone Lutetian limestone (or "Paris stone") is composed of shell debris and marine microorganisms (mollusk shells, remains of microorganisms) that settled at the bottom of this ancient sea. Over millennia, this organic matter accumulated to form a sedimentary rock that is both robust and easy to carve. It is this material, often called "Paris stone," that allowed for the construction of most historical monuments in the Île-de-France region.

🔎 How to "read" the stone? By observing the walls, you can see the richness of this material:

• Porosity: The small holes you see are spaces left between shell fragments during formation, or traces of chemical dissolution by rainwater over the centuries.
• Fossils: The elongated or irregular shapes that stand out in relief are the remains of seashells that lived in this sea. Natural erosion "exposes" these fossils because they are often more resistant than the surrounding limestone.

The Lutetian limestone before you is a carbonate sedimentary rock, much softer and well-suited for carving regular blocks. It is this diversity that makes each block unique. Some are very dense and smooth, while others are true "marine graveyards" frozen in stone. Others, more porous and cavernous, show surface alteration due to erosion (dissolution of the limestone by slightly acidic rainwater).

Here is a detailed diagram illustrating the four key stages in the formation of Lutetian limestone, from the initial sedimentary environment to the final rock:

1. The accumulation environment: It all began in the Eocene, in a shallow epicontinental sea (the Lutetian Sea) under a tropical climate. This is where carbonate debris from marine organisms such as calcareous algae, foraminifera (miliolids), and shells accumulated.
2. Sedimentation and burial: These debris formed an initial carbonate mud that gradually sank under its own weight.
3. Compaction and cementation (diagenesis): With burial, the sediments were compacted, and pore waters rich in calcium carbonate circulated, causing the precipitation of a calcitic cement that bound the grains together.
4. The final rock: The result is a compact, hard limestone rock, the Lutetian Limestone, which is often rich in fossils and was widely used as a building stone, particularly in Paris.

The diagram includes close-ups of the initial sediment microstructure and details of the final rock to better visualize the components.

Millstone (Pierre Meulière) Millstone (very typical of certain areas of the Parisian suburbs, such as Brie) is a siliceous sedimentary rock. Millstone, or "meulière," was used until around the 1880s to manufacture grain millstones, hence its name. Only the densest parts of a millstone bank are suitable for manufacturing millstones, as they do not shed grains during wear (unlike sandstone).

It has a very different appearance: it is often irregular, very cavernous (like a frozen sponge), with a color that varies from reddish-brown to dark gray, and it is extremely hard, almost impossible to carve cleanly into a rectangular block. This vacuolar structure gives it a certain thermal and acoustic insulation capacity, which is highly valued.

Shell Limestone The Porous Rock: Shell Limestone (or "Vernon Stone" / local Tuffeau)

The highly porous, brown or yellowish rock, dotted with visible cavities, is a shell limestone. It is a marine sedimentary rock rich in fossil debris (shells, calcareous fragments). Over time, the softer elements (such as small shells or the initial cement) erode faster than the matrix, revealing these characteristic alveoli (pitted cavities).

This stone is very common in the region because it was easy to extract from local quarries. However, its high porosity makes it fragile against frost and humidity, which explains its sometimes "attacked" or honeycombed appearance.

Why are there dark stains on the stone?

The presence of black areas on Lutetian limestone is a classic phenomenon of weathering in urban environments or on ancient buildings.

By carefully observing the blocks of this structure, you will notice black stains embedded in the limestone. This phenomenon is not just a simple accumulation of dirt, but the result of a complex interaction between the nature of the rock and its environment. Here are the main causes:

1. Atmospheric Pollution This is often the predominant factor, particularly in urban areas or near industrial zones.

• Particle deposits: Soot and carbon particles resulting from combustion (heating, engines, industries) settle on surfaces. The natural porosity of the limestone traps these particles, which become difficult to remove by simple rainfall.
• Sulfation: This is a more serious chemical transformation. Sulfur pollutants in the air (sulfur dioxide) react with calcium carbonate (the main component of the stone) in the presence of humidity. This transforms the stone's surface into gypsum, creating an often-friable "black crust" that can eventually peel off.

2. Biological Growth The stone is a living medium, especially when exposed to humidity.

• Micro-organisms: Persistent humidity promotes the proliferation of algae, moss, fungi, or lichens. Some of these organisms produce dark pigments (notably certain black algae) that embed themselves into the stone's surface.
• Biopitting: Certain micro-organisms can also "dig" into the stone, creating small cavities where dirt and pollutants accumulate even further.

3. The Action of Water and Humidity Water acts as a vector and catalyst:

• Runoff and evaporation: Rainwater flows over the facade, carrying dust and salts. When it evaporates within the pores of the stone, it leaves behind these deposits, which accumulate over time.
• Capillary rise: Humidity coming from the ground can transport mineral salts which, as they migrate toward the surface, can alter the appearance of the stone and promote blackening.

Why these stones in particular? Here, we notice that some stones are very honeycombed (presenting cavities). These are likely porous limestone rocks (such as tuffeau or shell limestone), which are very sensitive to these alterations. Their rough texture facilitates the adhesion of pollution particles and provides ideal water-retention zones for the development of micro-organisms.

These areas are very informative: if you see black in recesses sheltered from the rain, it is because pollution has accumulated there without being cleaned away by natural runoff. It is a witness to the history of the air that the stone has "breathed" over the decades!

🎨 The Chemical Signature: Why are some stones pink?

Iron oxide is a fascinating component of geology that explains the coloration of many rocks and soils you encounter. Sometimes, amidst the gray or beige limestone, you will notice stones with pinkish or reddish hues. This is not an anomaly, but the signature of iron oxide ($Fe_2O_3$).

What is iron oxide ($Fe_2O_3$)?

Iron is a very abundant element in the Earth's crust. When exposed to oxygen (in water or air), it undergoes an oxidation reaction. It is exactly the same phenomenon as the rust that forms on an old piece of metal left out in the rain. In rock, this oxidized iron acts as a natural pigment that colors the sediments. It colors the stone pink or orange. The presence of these tints in the limestone of Plaisir indicates either an influx of iron-rich water through the porous rock, or the presence of ferrous sandy deposits during the marine formation of the limestone.

Color is an excellent indicator of chemical composition. Where you see pink, red, or brown in the stone, you are seeing the chemical trace of iron that has "oxidized." It is a color change that transforms the stone into a kind of "geological color chart"!

The Pavers

Environmental Context

• The ground (pavers): The pavers at the base of the wall confirm an urban environment or an ancient building.
• Material: These pavers are made of sedimentary rock or hard rock (such as sandstone or silicified limestone), chosen for their resistance to foot traffic and passing vehicles.
• Erosion: The cavernous appearance of certain blocks clearly shows the process of chemical dissolution of calcium carbonate. Over time, rain erodes the rock, exposing the harder fossils that were initially included in the mass.

Here is what can be said about these pavers:

1. Geological Nature: Sandstone or Silicified Limestone. In the Paris Basin, this type of paver is traditionally made of sandstone (often Fontainebleau sandstone) or silicified limestone.
2. Why this choice? Unlike Lutetian limestone, which is soft, sandstone is an extremely hard rock resistant to wear, humidity, and frost. It was essential for supporting the passage of carts, horses, and later vehicles, without disintegrating.
3. Appearance: You may notice that they are less porous than the church stones and have a smoother surface, although patinated by time and repeated traffic.

Architectural and Historical Role

• Protection: Placing these pavers around the church has a crucial utilitarian function: protecting the foundations. They allow rainwater to be quickly drained away from the base of the walls (the plinth), thus preventing the soft limestone from being constantly saturated with humidity and becoming fragile.
• Layout: The irregular arrangement of the pavers (what is called "sharp-joint" or sand-joint laying) is typical of historical urban planning. It allows the ground to remain "draining".
• Link with the environment: The presence of these pavers at this specific location testifies to the development of the church courtyard over the centuries. They often come from local or regional quarries, and their accumulation around the building reinforces its historical anchorage.

Sandstone Sandstone is a detritic sedimentary rock resulting from the aggregation of mostly sandy-sized grains (0.063 mm to 2 mm) and consolidated during diagenesis. The grains that make up sandstone generally come from the erosion of pre-existing rocks, which largely determine its composition, consisting mainly of quartz and feldspar. Depending on the degree of cementation and composition, it can form a very friable or coherent rock. Sandstone is found in a wide variety of depositional environments, from continental (rivers, beaches) to marine (turbidites). Its unconsolidated equivalent is generally called sand.

Construction Technique and Implementation

• Masonry: You are looking at a "rubble masonry" wall or a buttress. The stones are arranged to ensure structural stability.
• Mortar: The ocher/yellow color of the binder between the stones suggests the use of lime mortar, traditional for this type of construction. It is well-suited because it allows the stone to "breathe," unlike modern cements that would block humidity and could cause the limestone to burst.
• Natural alteration: The contrast between light gray blocks (fresh or protected) and darker or yellowish blocks (heavily weathered or exposed to the elements) testifies to the natural aging of Lutetian limestone when faced with atmospheric conditions.
• Construction design: In Plaisir, Lutetian limestone was the local reference resource. Builders used dimension stone (often extracted locally or in the vicinity of the Seine/Oise valley) for the structure, which is very visible on the buttresses of your church. Millstone, on the other hand, was often used for "blocking" or facing, but it does not allow for the clean, regular cuts observed in your photos.

Why this mix? The presence of these three rocks (millstone, Lutetian limestone, shell limestone) in the same wall is not a coincidence, but a reflection of traditional construction economics:

• Recovery and cost: Materials available in the immediate vicinity (geological proximity) were used. Lutetian limestone was often reserved for noble structural parts (corners of buildings, window frames), while rubble for filling was composed of whatever was found on-site.
• Millstone for solidity: Millstone is extremely hard and resistant to humidity, often used for plinths or to reinforce the wall structure.
• Limestone for implementation: Easier to work with, limestone allows for filling the spaces between harder blocks.

Other Visible Elements

• Lime mortar: The binder is likely lime mortar (often ocher/yellow here). Its color often comes from the sand used locally for the mix. It is crucial for the survival of these walls because it lets the stone "breathe," unlike modern cement mortars that trap humidity and cause these soft rocks to burst.
• Brick: Red brick is also visible here (notably on window frames). It was often added during more recent restorations or alterations to reinforce corners or openings.

📷 Photo Credits

• Field photos: All shots were taken by Denfire and Veda on-site.
• Educational diagrams: The schematic illustrations explaining the various phenomena were designed with the help of Flat.
• Geological sources: The technical information presented here is based on regional geological data from the BRGM (Bureau de Recherches Géologiques et Minières).

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