Stage 20 | Lacapelle-Marival – Rocamadour / Karst, fossils & evolution

This time trial will lead the riders individually over a limestone plateau where geologically recent (since 3.5 million years ago) erosion formed steep canyons creating the exciting end of today’s stage. Most of the limestone rocks found in the geopark ‘Causses du Quercy’ were formed during the Jurassic between 200 and 145 million years ago; a time when the Young Atlantic Ocean was growing and expanding.

Carbonate limestone plateau

Racing forward to 70 million years ago, the thick deposits of carbonate limestones were hefted above the sea surface and became subject to erosion and the formation of karst. At this time, the climate was very different, and resembled a tropical rainforest, with rivers and strong erosion, not dissimilar to those found in South America today.

Forming karst and rivers

Karstification is a process which regularly occurs in carbonate limestones as water dissolves minerals and in doing so forms cracks in the rock through chemical weathering. Cracks can grow into caves and through time these caves (like the Padirac Chasm, figure 1) can link to form huge interconnected cave systems containing stalagmites and even underground rivers. One of those partly underground rivers is the Ouysse river, its source in Lacapelle-Marival (the stage start), disappearing near Thémines before it resurfaces ~20 km further northwest near Rocamadour (the stage finish). Fortunately, the peloton can take a more direct route!

Fossils and evolution

Karst landscape features (such as caves) make ideal sites for animals to shelter or hibernate. Occasionally, the cave dwellers were unfortunate enough to be engulfed in floods, and the caves refilled by sediments leading to the burial and preservation of animal remains in the caves. Due to long continuous karst formation here (extending to 70 million years), the geopark is famous for its fossils, with more than 30 million years of evolution in bats, rats, bears and many more mammals uncovered by paleontologists (figure 2). This unique preservation enables scientists a valuable insight into such events as the Eocene-Oligocene extinction event (34 million years ago) in which the Earth experienced a major cooling climate change, where Antarctica started growing its ice-sheet.

The word karst is named after the Slovenian Karst Plateau (Kras), so the Slovenian riders might feel at home today and have a good chance for the victory!

I study geodynamics, more specifically the interaction between the mantle and plate tectonic processes on the surface and on a very large scale. As we can not go into the mantle, my computer models are based on data from rocks all around the world that tell the story of plate tectonics. Check the Geo-TdF-team-2022.

Erik van der Wiel