Seventeen bridges railway viaduct

At first glance, it may seem strange that a relatively small stream like the Pedebeek has carved such a wide valley. That is why an impressive railway viaduct had to be built here to cross the Pedebeek Valley.

Whether such a railway viaduct should be constructed, as opposed to simply allowing the train to descend and climb out of the valley, has less to do with the depth of the valley — which is only about 40 meters deeper than the surrounding hills — but mainly with how steep the valley walls are. A train can only handle gradients of less than 2%. In Belgium, even 1% (10 ‰) is considered a significant limit for heavy and frequently used lines. And that is the case here.

Since this is a relatively young stream valley, a bow valley effect occurs here, with valley walls that are quite steep and often too steep to navigate by train.

You can clearly notice how steep the valley walls are at the edge of the valley when you climb them on foot or by bike. There are even famous climbs here such as the Keperenberg, which are part of cycling classics like the Brabantse Pijl. But the Rustberg, where we find the supporters' café of Remco Evenepoel, is also quite a leg burner.

More details about the development of this valley

Let's start with the hills forming the valley walls of this stream. These clay and sand layers were deposited when there was a sea here during the Eocene (Paleocene). The world had already been recovering for about 20 million years from one of the six pronounced mass extinctions that marked the transition from the Cretaceous to the Paleocene, 66 million years ago. It is estimated that about 75% of all species went extinct, including all non-avian dinosaurs. We know the main cause: the Chicxulub impact (asteroid) in present-day Mexico, which darkened the atmosphere through dust, soot, and aerosols, nearly halting photosynthesis and causing a chain reaction in the food chains;

But why was there a sea here and not land? Was the sea level much higher than it is now, causing us to be submerged? Or was the land much lower than it is now? Both are true. It was a combination of a high global sea level and regional land subsidence in a lower-lying basin than now. Only later, associated with the Alpine mountain formation and the pushing of the African plate against the European plate, not only the Pyrenees and the Alps in Switzerland, Italy, and Austria were uplifted, but also the more northern areas where we are now received a push upwards, causing the land to become dry towards the end of the Tertiary.

But there was also a second reason. During large parts of the Paleogene, including the Eocene, global sea levels were high due to the warm climate and nearly absent large ice caps, which expanded the oceans and raised the average sea level meters to tens of meters above the current level.

But then comes a subsequent, reversed phase.

During the ice ages of the Pleistocene, continental ice did not reach Belgium, but what had a major influence on this valley was the fact that the sea level was 100 to 120 meters lower than it is now, as enormous amounts of water were stored as inland ice. The effect of this was that valleys were cut much deeper. This was certainly the case.

A cold, periglacial climate prevailed, in which the soil regularly froze and thawed.

To make it a bit more complex, this did not happen once but several times in different ice age waves.

Because the sea level was about 100 meters lower than now, this also meant that the North Sea bed was largely dry. The sand, silt, and clay particles were blown away and deposited in Flanders. The heaviest particles (sand) settled first. The lighter particles (sandy silt and silt) drifted further with the wind.

If the wind blew from a fixed direction for long periods during the ice ages, the ridge here acted as a natural windbreak. And we have quite an effective windbreak here. That is the ridge between Dilbeek and Ittebeek, over which the Ninoofse Steenweg runs largely.

On the windward side (where the wind hits), a lot of dust blew away immediately, but behind the ridge, on the leeward side, that dust (loess, silt) could drift quietly out of the air and accumulate.

This also happened here. The leeward side of this hill is the slope that looks eastward, so in this valley, it is the slope to the west of the stream, where later Itterbeek and Schepdaal emerged. On that east-facing, sheltered slope, a much thicker layer of wind-deposited silt and dust could form than on the west-facing slopes, which received the full force of the wind and where material was blown away faster.

That is why the slopes facing east are usually much steeper than those facing west. Ride down IJsbergstraat and up Doylijkstraat and Pedestraat to Vlezenbeek. Then do the reverse. You will notice that the slope is a bit steeper.

There is also another effect at play. Warming of the slopes due to orientation. An east-oriented slope receives colder morning sunshine, which means that a frozen top layer or a snow cover will thaw less quickly. A west-oriented slope receives afternoon and evening sun that warms more. As a result, deposits are more eroded on those west-oriented walls than the opposite. And this means that the valley shapes are more developed when you look at the west-oriented slopes than the eastern ones. We know that, how further you go in development usually evolves from V-valley, to bow valley, and then flat valley. In other words, you get more of a flattening.

Of course, vegetation also plays a significant role.

If we calculate with contemporary erosion models, then you should consider 10 cm per century of soil material eroding. So 2 meters by the beginning of our era.

The combination of moderate height differences, clay-rich soils, and increased runoff during rainfall resulted in efficient water erosion, allowing a pronounced bow valley with steep valley walls to form despite the limited discharge of the stream itself.

Pascal Brackman