Stage 1: Steep climbs

The GeoTDF project shows you how how big or small geological events lead to the course we race on the upcoming four weeks: mountains, vineyards, steep climbs or winding rivers. We start this year’s Tour de France in the Basque Country, Spain, where big geological events led to steep climbs.

This part of Spain is a crossroads between the Iberian peninsula and continental Europe in many respects: both anthropologically and geologically. Evidence of human habitation in the region dates back to 40,000 years, or the Paleolithic period when ice sheets still covered much of northern Europe.

It is also a geological cross road marking the intersection of the Cantabrian mountains of northern Spain to the west, the Pyrenees to the east and the apex of the west-northwestward opening Bay of Biscay.  Of course, then we are talking about a timeline of many millions of years and big geological events.

The evolution of continents over hundreds and millions of years, and especially the role of the ‘supercontinent cycle’ which is the formation and demise of the supercontinent Pangea, led to the climbs we now have in the Basque Country. It also led to the Pyrenees later in the race and the beautiful vistas over the Bay of Biscay that will stay with us on the three opening stages of the Tour de France 2023 in Bilbao, San Sebastian and Bayonne.

Stage 1: Big geological events lead to steep climbs
A tectonic map showing the ancient continental domains that have come together to form Europe, including Armorica in Yellow.

The formation of continents

300 million years ago the geography of the Basque region was markedly different. Bilbao lay near the centre of the supercontinent Pangea. Pangea formed when the oceans separating the major southern and northern continents, Gondwana and Laurussia, closed. We call this tectonic event the Variscan Orogeny, or mountain building event.

In the oceans between the supercontinents were a number of long, narrow ‘ribbon’ continents, one of which was ‘Armorica’. Most of modern-day Spain and France consist of the remains of Armorica which now found itself trapped between Gondwana and Laurussia. That long and narrow continent buckled and bent like a car in a junkyard. The Cantabrian mountains in the northwest of Spain form the core of one of these major bends.

Stage 1: Big geological events lead to steep climbs
A geological map showing the bends of the Armorican ribbon continent including the Cantabrian in northern Spain which is continuous into the Central Iberian bend of central Iberia. Inset at upper left is a paleogeographic reconstruction showing the Amorica ribbon continent (A) between Gondwana to the south and Laurussia to the north at about 350 Ma prior to the Pangea forming continental collision that gave rise to the Variscan orogeny and during which Armorica was bent (via Douwe van Hinsbergen)

Seabed becomes steep climb

The geology of the hilly opening stage of this year’s Tour de France is the product of hundreds of millions of years of plate tectonic processes. When the riders and fans in Bilbao look towards to the northeast, they will see the often green hills that rise up 300 to 400 metres above the adjacent valleys. Deformation of the continental crust underlying the Bilbao region has folded the region’s formerly flat-lying layers, in places tipping the layering right on end. These highly inclined sedimentary beds form the steep-sided flanks of the elongate hills that march from the Pyrenees down to the sea, giving rise to many of the short but steep climbs that characterise the stage. The fifth, final and hardest King of the Mountain climb is the Côte de Pike.

Stage 1: Big geological events lead to steep climbs
At bottom, cross-sections through the Pyrenees today (a) and prior to closure of the Cretaceous Pyrenean Rift (b) showing the amount of shortening as well as subduction of Iberian lithosphere during the formation of the Pyrenees.

Spain almost joined North America

While the Cantabrian mountains and the Iberian peninsula owe their origins to the continental collisions that formed Pangea, it was the demise and break-up of Pangea, a process that started >200 million years ago and which is still ongoing, that gave rise to the Bay of Biscay and the Pyrenees.  

The magnificent views we will have of the Bay during today’s stage began to take shape 180 million years ago when the Atlantic Ocean opened up. As the Atlantic rifts migrated north separating what would become North America from Europe, Iberia’s initial inclination was to join with America. It drifted west, so away from Europe. 

Stage 1: Big geological events lead to steep climbs
At top, paleogeographic maps drawn at 121 million years ago (at left) and 79 million years ago (at right).  Green indicates oceanic domains, Iberia is indicated in yellow. Movement of Iberia away from Europe opens the Cretaceous Pyrenean rift and gives rise to the oceanic domain separating Iberia from Europe.  The subsequent jump of the main rift to west of Iberia causes counterclockwise rotation of Iberian, closing the rift and giving rise to the Pyrenees. (via Sciencedirect)

However, after only a small oceanic tract had opened between Iberia and mainland Europe, giving rise to the Cretaceous Pyrenean rift system, Iberia had a geological change of heart.  As the main locus of ocean formation jumped to the west of Iberia, forming what we now recognize as the Atlantic mid-ocean ridge, Iberia twisted and turned, opening the Bay of Biscay, closing the rift, and folding and faulting the sediments that had only recently been deposited in the basin that had briefly (from a geological perspective) separated Iberia and Europe.





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