Stage 4 | Dunkerque - Calais / Michaelangelo’s hills & white marls

Anyone who has ever crossed the channel by boat knows the cliffs of Calais. Today's stage leads through the limestone and marl area of west Flanders and northwest France. Over the hills and along the cliffs. These rocks are from the Late Cretaceous, between 90 and 66 million years old.

Forming of plankton skeletons during period of ice-free Earth

The Late Cretaceous was a super warm geological period. At that time, the sea level was 80 meters higher than today, because the climate on Earth was so warm that all the ice that is stored in polar ice caps today was in the sea. The Earth was ice-free. Moreover, Europe then was much more southerly, in a drier climate zone, where today the Mediterranean Sea is located. In fact, the cliffs of Calais were formed in a kind of subtropical shallow sea, in which plankton and other marine life had a very good time. The cliffs consist almost entirely of microscopic fossil plankton skeletons. Of calcium carbonate, which is why the cliffs are so bright white.

Spectacular coastel cliffs easy to erode

These coastal limestone cliffs look quite spectacular: they loom vertically. With such vertical cliffs you quickly think of rock hard rock, but nothing could be further from the truth. Actually, the mergels of the chalk cliffs near Calais are quite easy to erode: by slightly acidic rainwater, and by rivers that cut into the chalks. This is also the reason that todays stage goes up and down a lot: the heavy annual rainfall in this area have cut out valleys from the soft chalk. And here comes another fun fact. Where normally mountains are formed because tectonic plates collide with each other, and a crumple zone of those 2 plates is created (for example, the Alps, more on this when the TdF gets there), in this area we see relief because rivers have cut into the landscape. The rock layers of the Cretaceous are almost horizontal in this area, and therefore not folded as in the Alps, but the relief is caused by erosion. This can sometimes cause viciously steep ascents, as we also know from the Limburg landscape, where this phenomenon also occurs.

Witness Hill: The Kassel Mountain

There is 1 more funny geological phenomenon that is visited in this stage: the Kassel Mountain, is a special kind of hill. The Dutch word for it would translate to “witness hill”. It is the first climb of the stage, after 28 km of racing. A witness hill is actually an anti-mountain. It could have been made by Michaelangelo: his credo was that you have to free sculptures from the block by removing the excess rock. That is what nature thought when forming the witness hill near Kassel. The mountain lies in the landscape because nature has cleared away the excess rock, through erosion processes. And Kassel Mountain was witness to that erosion, hence the name. Apparently the rock under Kassel was slightly harder than the surrounding stone, and because rivers choose the path of least resistance, the surrounding rock was eroded, and the Kassel Mountain remained.

I study climate and ocean conditions on and around Antarctica, during the Earths most recent 100 million years. Specifically, I study sediment cores to reconstruct the onset and development of the Antarctic circumpolar current around, and the ice sheet on Antarctica. Check the TdF-team.

Peter Bijl