Forest of La Houssière

The sands and gravels on which the Forest of La Houssière lies were formed by deposits from ancient river and sea arm/groove systems. These sands belong to the Brussels Formation, a geological unit from the Middle Eocene (about 48 to 41 million years ago).

The plateau on which the Forest of La Houssière lies is a textbook example of relief inversion. Relief inversion means that a geologically old low-lying area (such as a river valley or tidal groove), filled with resistant sands or sediments, today appears as a height in the landscape. The Brussels sands were originally deposited as marine (sea arm, tidal) deposits in depressions and groove shapes. Because these sands are often more resistant to erosion than the surrounding material, they remain as a plateau while the surrounding landscape is further eroded. The Brussels sand groove, now a plateau, typically lies at an elevation of about 120 to 150 meters. The former hilltops may protrude dozens of meters above this — estimates vary from about 30 to 60 meters higher than the current plateau.

This groove flowed northward, and the Brussels deposits (Brussels Formation) extend into Brussels, where they also play an important geological and morphological role. In Brussels, these deposits are clearly visible due to the steep slopes of the city center (for example, around Royal Square, Warandepark, and the Palace) and the higher Brussels Plateau. There, the Brussels sands locally reach up to 40 meters thick, providing a robust foundation for prominent buildings such as the Royal Palace.

Pascal Brackman