The context
L'idée de ces EarthCaches "Urbans Fossils" provient d'une très belle cache de fafahakkai : Urban Fossils à Bordeaux (lui-même s’étant inspiré de -Ruby- et la cache RBY 7 - Urban fossils in Lille). Comme c’est en plus lui qui nous a fait remarquer la diversité et le nombre de fossiles lors de notre rencontre, cela valait bien un petit clin d’œil ! ^^
Definition and formation of fossils
To know the definition of the term "fossil" and how they are formed, you can refer to our other EarthCache : Urban Yellow Fossils - Recognition.
Some fossil species
Among the tens of thousands of fossil species identified, marine fossils are the easiest to find. Why? There are two main reasons for this:
- life has been living in the oceans for about 3.85 billion years, but on land we had to wait: the first plants only appeared about 440 million years ago. The first animals capable of moving arrived 360 million years ago, and for mammals it is still necessary to wait almost 150 million years;
- it is most often sedimentary rocks that contain fossils. However, these rocks are formed on the bottom of the oceans.
Among the very many known marine fossil species, we can mention (see image below):
- ammonites: contrary to appearances, it is not the ancestor of the snail, but of the octopus! ;
- the nérinées: they are the ancestors of snails;
- Pectinidae: rather flat shells, often with wide and thick streaks. They can be compared to our current scallops;
- gryphées: the ancestors of our mussels (unfortunately, French fries did not yet exist...), as they are often found in colonies of a large number of individuals;
- rhynchonchonella: they could be confused with other bivalves such as pectinids, but they are generally much more rounded and their striations finer and narrower;
- belemnites: ancestors of squid, generally only their rostrum is found, the only hard part of these organisms.
An ammonite is not a snail!
Seeing an ammonite and its characteristic winding, one would be tempted to say that it is a "kind of snail", but this is not the case at all! Already, ammonite is a cephalopod (feet on head) while snails are gastropods (stomach in foot). Then, their growth and structure are very different: ammonites have a shell wrapped in a spiral on a plane, while snails have a spiral wrapping in space. In cross-section, we will therefore distinguish the spirals of the ammonite and sometimes the empty boxes, unlike the gastropod which will reveal its helical winding:
Sedimentary rocks and fossils
from various particles (sand, gravel, dust, remains of living beings,...), which are transported to the bottom of lakes, seas or oceans and then assemble before consolidating. They are generally rocks with a bedding (layered formation), characterizing the successive deposition of particles at the origin of these rocks.
There are two main subfamilies of sedimentary rocks:
- biogenic sedimentary rocks, formed by the chemical precipitation of elements and the deposition of remains of living organisms in significant quantities. It is characterized by a significant presence of fossils that have contributed to the formation of the rock. In this family there are many forms of limestone ;
- detrimental sedimentary rocks, formed by products resulting from the erosion of a previous rock, whose debris is transported and then deposited. These elements are cemented together at depth by diagenesis to form a detrital sedimentary rock like sandstone. They generally do not contain fossils.
Calcification: a common fossil mineralization
The calcification process can be illustrated as follows:
It can happen that the skeletons are partially or completely dissolved by the water flowing through the rock. This dissolution leaves a cavity in the rock, which retains the shape of the skeleton. This cavity can then be filled with another material, such as calcite cements for example: by circulating in a limestone rock (composed of calcium carbonate CaCO3) more or less amorphous (without crystals), the water is charged with calcium carbonate, which is then redeposited by precipitation. When this precipitation takes place in the open air, a "travertine" or "tufa limestone" can form (petrifying fountains, stalactites, etc...). In a closed environment, a mineral with bright crystals, calcite (see picture below) can form:
Questions
Careful reading of the cache description, as well as an observation of the field elements and a little deduction are normally sufficient to answer the questions of this EarthCache.
Go to N45° 46.575' E3° 04.933' (listing coordinates, waypoint 1) and locate the yellow highlighted slab in the "Zone A" photo.
1. Evaluate the proportion of fossils in the rock. Is it biogenic or detritic?
2. What is the overwhelming majority species?
Go to N45° 46.605' E3° 04.930' (waypoint 2) and locate the very beautiful fossil under the blue square of the photo "Zone B".
3. Which species could it be?
4. Describe the appearance of the contours of this fossil. Is it travertine or calcite?
5. What has happened to this fossil? What can we deduce from this about the nature of the rock?
6. Has this phenomenon occurred outdoors or on the ocean floor?
Optional: if you see a remarkable fossil, send us its picture ^^
Optionnal: a picture of you or your GPS on site is not mandatory but highly appreciated ^^
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