When the diabases were formed in the Middle Devonian (380 million years before today), a sea extended over the entire area of today's Rhenish Slate Mountains including the Sauerland. Large rivers transported erosion debris from a mainland far to the northwest, the Old Red continent, into this sea basin. Mainly clays and sands were deposited on the seabed, which solidified in the course of time. Thin-bedded basaltic lavas rose from the Earth's mantle (from a depth of about 150 km) via conveyor fractures and penetrated the rock along bedding joints. Upon contact with seawater, they rapidly solidified. This lava turned to stone is called intrusive diabase. In the course of mountain building 300 million years ago, these layers were then strongly constricted under high pressure, folded and lifted out. A mountain range was formed: the Rhenish Slate Mountains. This was subjected to weathering and erosion over a period of millions of years, resulting in the landscape we see today.
Theweathering-resistant diabases remained as witnesses of volcanic activities between the clayey-sandy rock layers and form the summits of many mountains as hardened rocks.
Age of the rocks : Diabase: Upper Givetian main greenstone train; Givetian stage, transition Middle to Upper Devonian (about 380 million years before today)
Access: The cliffs can be reached by a short path from Hillebachsee