Stage 21 | Thoiry - Paris / The Lutetian age

The last stage of this Tour de France brings us back to the Paris Basin. Paris itself is built on the youngest rock formation that were deposited in this basin. These rocks, with an age of approximately 45 million years old formed during a period known as the ‘Lutetian Age’, are dominated by limestones that were deposited in shallow, warm seas, like a tropical version of the North Sea, and contain enormous fossils, such as 70 cm long gastropods (spiral-shapes shells) (see figure).

The Lutetian Age, named after Lutetia (the Roman name for Paris)

As explained before, sedimentary rocks can be dated based on fossils that existed during a period of geological time, and that are so abundant that they are easily found and recognized. Those are often microfossils, since they occur in great abundances, but also ammonites are useful fossils for dating. On basis of the presence of charactertistic fossils, the geological timescale is subdivided into intervals that are named after type localites where these rocks are particularly abundant, or where they were first described. The Lutetian is such a time interval, and this is named after Paris that was known in Roman times as ‘Lutetia’. (Remember, in the blog of stage 2, we explained two earlier Epochs: the Maastrichtian named after the Dutch city of Maastricht and the Danian, named after Denmark, separated by the meteorite impact that led to the extinction of the dinosaurs, 66 million years ago).
So the time intervals in the geological timescale are largely based on fossils, and originally, we only knew the relative occurrence of these time intervals: a series of rock in which a certain fossil existed is followed by a series in which it didn’t, marking that the fossil species probably went extinct. The geological timescale was built this way in the course of 200 years of painstaking research. Only in the second half of the last century did we really start to piece together how old these intervals were in millions of years. Through measuring the radioactive elements in minerals, we can determine the absolute age of these time intervals. This showed that the Lutetian Age started at 48.6 million years ago, and ended at 40.4 million years ago.

The paving rocks of the Champs Élisées

And in case you wondered whether the iconic paving rocks of the Champs Élisées consist of limestones from the Lutetian: no, they do not. These are pieces of Brown Najran granite from a rock quarry in Beer Askar and Aakefah in Saudi Arabia. Goodness knows why: granite is not exactly in short supply in France…

I am a geologist and I study plate tectonics and the driving mechanisms in the Earth’s mantle, mountain building processes, and the geography of the geological past. I enjoy geological fieldworks all over the world, and translating the results to science and a broad public.

Douwe van Hinsbergen