Dry valley of the Rau du Fond du Wisselet

Here you see a striking geomorphological phenomenon: the dry valley of the Rau du Fond du Wisselet. Dry valleys are exceptional in the Ardennes, but form a logical consequence of the special geological structure in the Theux Window.

A dry valley is a valley shape without a permanent watercourse at the valley bottom. The valley has indeed been formed by flowing water, but that water no longer flows above ground today. In this case, you still see a small stream higher up. However, it disappears underground via a disappearing hole. As a result, the lower part of the valley receives no more water and remains “dry”.

This is characteristic of a karst landscape. Rainwater and stream water absorb carbon dioxide from the soil, which makes it slightly acidic. In limestone, this water can gradually dissolve cracks and fractures. This creates underground drainage pathways, disappearing holes, and sometimes caves. The water no longer necessarily follows the visible valley, but chooses the easier route through the underground.

The dry valley thus shows an earlier phase of surface drainage. When the stream was still flowing fully at the surface, it could cut into the valley. After an underground drainage system developed, the downstream path of its water supply was cut off.

Why is this special here? In most of the Ardennes, acidic and poorly soluble rocks dominate, such as sandstone, quartzite, and phyllite. These rocks provide streams, ravines, and cut valleys, but normally not pronounced karst with disappearing watercourses. A dry valley like this is therefore unusual.

The Theux Window differs from this. It is a geological window: due to tectonic thrusts, older rocks came to lie above younger layers; later erosion essentially cut an opening through the displaced rock mass. In this way, underlying Devonian layers became visible again at the surface. Part of this consists of limestone, deposited in a warm, shallow sea about 380 million years ago.

The Rau du Fond du Wisselet beautifully illustrates the interaction between lithology, structure, and relief development:

• The limestone enables underground drainage and karst formation.

• The disappearing hole leads the stream water away from the visible valley.

• The dry valley preserves the geomorphological “heritage” of an earlier surface stream.

• The phenomenon is a landscape indication of the exceptional limestone-rich substrate of the Theux Window.

In short: in a predominantly non-karstic Ardennes environment, the Theux Window locally allows for a very different type of landscape—featuring limestone, solution forms, disappearing holes, and, as here, dry valleys.

Pascal Brackman