Anyone approaching the village of Dedenborn, in the municipality of Simmerath, from the east will immediately recognize that this place is something very special. The rock ridge extends boldly above the Rur. In 1952 the regional geologist Dr. Wolfgang Schmidt from the Geological State Office of North Rhine-Westphalia in Krefeld, made an astonishing discovery during geological mapping work. Accidental, he found a rock that had a so-called mullion structure in the slate of the “Rurberg strata” at the south-west exit of the village of Dedenborn. It was a unique tectonic structure that is otherwise no longer found in the entire Rhenish Slate Mountains. It consists of the division of a “Grauwacken” bench into parallel bulges that lie one above the other on a steep rock face. This structure has never been described in Germany before. It was discovered for the first time in England and has passed into the literature there under the name "Mullion structure" (mullion = bundle of pillars on Gothic church windows). It is therefore a monument, unique in Germany, of a tectonic (= caused by mountain pressure) deformation of a “Grauwacken” bank.

Those who want to visit the Mullion structure can do this in combination with a hike. The village tour Dedenborn (No. 06), the local circular hiking trail “Eichheck-Runde” (No.16) and the “Heckenland-Route” lead along the special rock.