Ice, wind, rivers, the sea, and man: the young geology of the Vuelta-Holanda

The first three stages of the Vuelta a España of 2022 will not take place in the 300 million year old belt of the Iberian peninsula, or in the 80-20 million year old mountain ranges of the Pyrenees or the Sierra Nevada, but two prehistoric continents farther to the north, in the Netherlands. To see the rocks of the prehistoric continent below the Netherlands – known as Avalonia – you’d have to go to Belgium or England. In the Netherlands, this continental crust has been stretched and subsided and is covered by kilometers of sediments. In the oldest of these are the Carboniferous coal deposits of South Limburg (360 million years old). On top are sandstones from the Permian (270 million years old) that include the compacting gas reservoirs below Groningen, and marine claystones from the Jurassic (180 million years old) that generated the oil that was once pumped to the surface in Drenthe. But we don’t see these rocks in the Netherlands: the Dutch landscape was primarily formed in the last 150.000 years, in an endless dance of rivers and the sea, wind and ice, and a slowly subsiding land.

The second stage: the first categorized climb

The second stage (Den Bosch-Utrecht) leads the peloton over a geologically defined boundary that for millennia has formed a political and cultural divide: the River Area. To the north, we find the Utrecht Hill Ridge, and towards the east the hills of the Veluwe, with the Gelderland Valley in between. In Roman times, this northern area was occupied by the Germanic tribes, and from the 16^th century this was the dominantly Protestant part of the Netherlands. The River Area itself and the area to the south was part of the Roman Empire, and in the middle ages remained mostly Catholic. Experienced climbers will probably not be particularly impressed with the elevation of the Utrecht Hill Ridge, but they form a striking feature in the modern landscape. And 150.000 years ago they formed a spectacular and impregnable barrier: the Utrecht Hill Ridge are morraines consisting of pushed up and tilted layers of clay, sand, and gravel that had been deposited by the Rhine and Meuse rivers. Pushed up? By what, there wasn’t a continental collision here, was there? No, this push is not the result of plate tectonics, but by the growth and motion of ice. During the Saalian – the period of the penultimate ice age – hundreds of meters thick land ice glaciers reached to the central part of the Netherlands. That ice was easily flowing over the subsurface of the north of the Netherlands, but plowed itself into the sand and gravel of the River Area. These sediments were bulldozered in front of the ice and the ice came to a halt like Max Verstappen in the gravel traps. The resulting hills not only make for nice forests and short cycling climbs that the peloton has to cross 8 times (steeper on the side of the ice in the northwest and because of erosion and formation of fans shallower in the southwest). On one of these, the Hill of Amerongen, will be the first categorized climb of this Vuelta. And the Gelderland Valley? That was the location of one of the most southern glaciers, like a sunbathing snake in the landscape. The Rivers flowed along the hills and the glacier, from east to west, and they still do.

   For if you want to check out the region of Stage 2 for yourself

The giant river with multiple branches

South of the moraines are what the Dutch call their ‘great rivers’, but this term is a bit misleading. It is actually one giant river – the Rhine-Meuse delta – that has multiple branches. Each of these branches moved in the past, and merged and split into new branches. The rivers as they are now have only been in place since Roman times. About 900 years ago, the Dutch started to dam the rivers so that they can’t migrate anymore, and each branch has gotten a name. Not entirely as a coincidence, this is the same period in which the first castles were built: because of the dams, these would find themselves in the middle of the Meuse 50 years after they were built. Between the modern rivers are ancient branches. The finish of stage 2 near Utrecht is located on such a branch.

              So how do we know where these old branches were located? We can tell from the sediments in the subsurface. We can reach these by drilling: to reach the sediments of the ancient river branches of the Rhine-Meuse delta, we only must drill a few meters deep. River water flows fast and will carry along the finest-grained sediments that it transports – silt and clay: that material is suspended in the whirling waters and are not deposited in the riverbed. Sand and gravel grains are larger and heavier and those lie on and roll over the riverbed and form deposits: sandbars. When we find elongated, connected sand tracks in the subsurface, those are likely old riverbeds. When we find clay or peat instead, then we are in the ancient floodplains between the rivers. Those used to be extensive swampy landscapes between the river channels. After the rivers flooded during heavy rains or melting ice, water in the overbanks was stagnant and silt and clay would be deposited that forms fertile soil for grasses and plants. After the dikes were built in the Middle Ages, only part of the floodplains occasionally get flooded.

   Studying physical geography or earth science?

Old river sands, water levels higher than surrounding lands and slopes

When you ride the Dutch landscapes on your bike, you wouldn’t recognize the old river sands from the floodplain deposits you’d think. But that isn’t true: the old village centers are often built on the slightly elevated sandbars of the old river channels, and the oldest roads are also on the sand tracks. Many of the newer roads between villages, on which much of stages 2 and 3 will take place, are on the clay grounds. Because of human intervention, ground water levels have been lowered. Have close look during the stages: the water levels of many a brook or rivers is higher than the surrounding lands on the other side of the dike: the famous Dutch polders. And there is more in the present-day land use that is determined by the subsurface. Orchards are located on the sand tracks, grasslands on the clayey and peaty sediments. And you thought everything was flat land? Careful now! When we ride up-stream, the slope is easily 0.15%!

La Vuelta the third stage

The third stage will lead the peloton through the the southwestern part of the River Area, in the province of Noord-Brabant. And also there, we will see relief: the Brabant Wal, also known in Dutch as the ‘Brabant Zoom’. This ridge has nothing to do with ice, but is the result of more than one million years of slow erosion of the Scheldt River that was once flowing in front. The Scheldt, which was also migrating back and forth like all meandering rivers, was captured in the Middle Ages by a creek of Zeeland that we now know as the Western Scheldt. The Brabant Wal is protected by a clay layer at the top from which one can make a decent brick: the old houses in the region have a characteristic red color that results from baking bricks from this clay. And on top of the Wal are dunes made from sand that in the last ice age was picked up by the wind in the valley of the Scheldt, making the ‘Bergen op de Zoom’ – the name of the town next to the Brabant Wal, meaning Mountains on the Hill. The largest part of the province of Brabant after the 15 m climb up on the Brabant Wal consists of sand that was once covered with peat that almost everywhere has been dug away for heating. The sand landscape is cut by several river valleys that flow north from Belgium. Not much climbing is involved here, so the peloton will probably stay together. Just like the riders during the bunch sprint that seems inevitable, the rivers sprint in a straight line to the sea these days. But their deviant lines and continuous crashed in the past still ake their mark in the Dutch landscape, and left their traces in the Dutch cultural and political history.

The authors Kim Cohen, Wim Hoek and Douwe van Hinsbergen are all working at the Faculteit Geowetenschappen at Utrecht University. Jeroen Schokker works at TNO (Netherlands Organisation for Applied Scientific Research) at the department Geologische Dienst.

Kim Cohen, Wim Hoek, Jeroen Schokker, Douwe van Hinsbergen