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Indroduction :
“Craie”, chalk in French, comes from the term "Créta" in Latin and it is a white rock, quite friable, composed of two elements:
- From a biological perspective, this rock is made of more than 50% of coccolith (word coming from the Greek "stone"). These are small circular calcite plates serving as protection of marine unicellular algae: the coccolithophorids.
- From a chemical point of view, it consists of a majority (90%) of calcite (which is one of the crystalline forms of calcium carbonate) and some clay. Calcite is a carbonate which has a role in cementing the rock.
Formation :
The chalk is a sedimentary rock formed by marine remains of unicellular algae "shells". These algae, called coccolithophorids, are part of the foraminifera group. They live in the lagoons where the water is shallow and warm.
Their test (limestone skeleton) is formed of an assembly of plates, called coccolith, which are made of calcite. To create these plates, they use the calcium dissolved in the water and the hydrogen ions.
When they die, their tests accumulate on the seabed and form a carbonated sludge. Over time, these remains compact and assemble to form a calcite cement.
The Earthcache :
We are on one of Abscon quarries, the quarries of Plombs and Peupliers have two geological curiosities.
The first is the juxtaposition of two distinct sedimentary layers. The working reveal two types of rocks:
- A significant layer of chalk similar to the one of the Cap Blanc Nez. Dated from Late Cretaceous (Senonian: about 70 to 90 million years), it has many vertical fractures that allow the movement and storage of water. These fractures illustrate the importance of the chalk as an aquifer tank and its vulnerability.
- A brown-green sandstone fairly brittle. Directly applied on the chalk, it is a Landenian limestone or "tuffeau from Valenciennes". This rock is dated from the base of the Tertiary (about 55 million years).
Between these two periods of sediments, over 20 million years have pass, and they have left us no testimony! This lack of sediments of a representative period is called a sedimentary gap. The Tuffeau of Valenciennes is covered by recent sediments (Quaternary silt).
The other richness of the place corresponds to a fault discovered by Professor Francis Meillez from the University of Science and Technology of Lille. This hitherto unknown fault shows an abrupt collapse of geological layers and indicates that the region has gone through several tectonic movements.
Reminder on "Earthcaches": There is no container or logbook on the given coordinates. Just visit the site and answer the questions by e-mail. There is no need to wait for my reply, feel free to log immediately after. If the answers are incorrect, I will let you know. Happy caching!
To validate your visit:
1-. Look North-West, you’ll see part of the fault. To which side does the fault dips? Right or left?
2-. What colours do you see?
3-. What is the reason why there are these two colours?
4-. Walking on the path, what do you feel under your feet? Briefly explain the feeling.
5-. Continue along the path a few meters, you will find a wooden object, what is it?
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