On the western slope of the sandstone plateau known as the "Heiderücken" between Holsthum and Peffingen, there is a geological feature: a rock area with calcareous sintering. Crust-like limestone deposits have created a bizarre grotto.

Why is there such a phenomenon here? When rainwater seeps through the thick sandstone layer of the plateaus, it dissolves the calcareous binder that holds the quartz grains of the rock together. Where spring water emerges from slopes and valley cuttings, but also in the banks of streams or rivers, the carbonic acid escapes from the soluble lime compound, the water evaporates and what remains is an almost pure limestone. It is called calcareous sinter or calcareous tufa. The lime sinter is deposited along the flow paths of the water and covers mosses and plant parts with a stony crust. This creates fascinating rock formations. The process of calcareous tufa formation continues, the rocks "grow" like stalactites.

Another calcareous tufa grotto is the Tränenlay near Dillingerbrück.

Our tips:

• The Holsthum circular route (7.1 km) runs in the area of the Huwelslay.
• Make a stop at the Huwelslay during a cycle tour along the Prümtal cycle path. The path leads almost directly past the grotto.