Stage 2: Karst

You don’t always need big geological events for breathtaking scenery. Sometimes the subtle influence of karst can make all the difference. Let us explain how on day two of the Tour de France. We are still in the Basque Country with all its fascinating geological history. On the course, the riders traverse the foothills of Mount Txindoki. This majestic peak marks the northwestern boundary of the Aralar mountain range. These are the spectacular remains of thick limestone deposits that formed in shallow subtropical seas between 150 and 100 million years ago. This was during the Jurassic and Cretaceous periods when the dinosaurs roamed the planet and these seas as well. 

The limestones were later uplifted by tectonic forces. Over time, the uplifted limestone gradually eroded, transforming once rough and rugged mountain peaks to the climbs we now see. However, despite millions of years of erosion the climbs in the Basque country are still pretty steep. The non-climbers should wait a few million years more for a more gradual gradient.

Karst shapes the landscape

We tend to think that big significant changes in the landscape come with big tectonic forces. That’s not true. It can be much more subtle. The phenomenon geologists call karst (after the Karst region in Slovenia) is a very slow process but the results are significant. Karst is a delicate dance between ancient limestone and the force of water. When rainwater flows downhill and enters natural crevices, it carves sharp valleys and infiltrates deeper layers. This forms subterranean channels and caves.

Breathtaking scenery shaped by karst
Stage 2 through the Basque Country is a hilly stage, whereby the hills expose the oldest rocks (anticlines) and the valleys consist of somewhat younger Cretaceous rocks (synclines). Figure: Malin Niemöller (BSc student at University of Münster, Germany)

Seeing through time

Water is a skilled sculptor but can be a destructive agent as well. While gradually polishing rock faces until their foundations are excavated, it might lead to abrupt collapses that expose the underlying structure of the idyllic landscape above.

A north-south geologic cross section along today’s race route reveals undulating patterns similar to the ripples in a theater stage curtain. The upwarped “hills” are referred to as anticlines. The downwarped “valleys” are known as synclines. Within this layer cake of undulating rocks, the oldest layers belong to the Lower Cretaceous. They are situated beneath younger layers that belong to the Upper Cretaceous. 

The “Cantera de Margas” quarry in Olazagutia provides a look inside the anticlinal structures that make up the Basque Country hills. Moreover, this quarry is the worldwide reference for the base of the Santonian (Late Cretaceous) stage. Photo: David De Vleeschouwer

The upwarped anticlines are most susceptible to karst. They experience greater weathering compared to the more sheltered synclines. Consequently, large parts of the younger, Upper Cretaceous layers in the anticlines have been stripped away, revealing a geologic window into the older, Lower Cretaceous rocks below. It’s basically looking back in time without any digging. The riders race right through geological time today and get rewarded by breathtaking scenery.

Breathe in, breathe out

As the riders take in the breathtaking scenery of the the rolling terrain of the Aralar Range their bodies work tirelessly to take in oxygen and convert it into energy through the combustion of carbohydrates. The resulting carbon dioxide is then exhaled and replaced with fresh oxygen-rich air. The human body performs optimally at sea level, where the air contains almost 21% oxygen and less than 0.05% carbon dioxide. 

What would happen if the atmosphere itself experienced a sudden and significant shift in oxygen and carbon dioxide concentrations? The answer to that question can be found in the Aralar Range’s geological record. Rock layers near the Igaratza refuge and the village of Madotz, not far from Larraitz on today’s course, show the direct effects of a massive release of carbon dioxide into the atmosphere. These resulted from a sudden outburst of volcanic activity in the central Pacific, literally on the other side of the planet. 

We know this event as Oceanic Anoxic Event 1a. It occurred about 120 million years ago. It led to a destructive increase in carbon dioxide in the atmosphere and in the world’s oceans. The proof is in the coloring of the rock layers. The altered chemistry of the seawater prevented certain organisms from constructing their shells, causing them to perish. This event resulted in a significant shift in the appearance of the rock layers from light-colored limestones – of the skeletal remains of organisms – to dark marls and shales of the organic material that didn’t rot because there was hardly any oxygen in the seas. The event lasted for 1,5 to one million years before conditions returned to normal. The light-colored limestones returned and the darks marls and shales are gone.

Check the YouTube explanatory video by Douwe van Hinsbergen.





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