When the diabases were formed in the Middle Devonian (380 million years ago), a sea stretched across the entire area of today's Rhenish Slate Mountains with 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 over time. Thin, basaltic lavas rose from the Earth's mantle (from a depth of around 150 km) via fractures and penetrated the rock along bedding planes. On contact with the seawater, they quickly solidified. This lava, which turned to stone, is known as intrusive diabase. In the course of mountain formation 300 million years ago, these layers were then strongly constricted, folded and uplifted under high pressure. A mountain range was formed: the Rhenish Slate Mountains. This was subject to millions of years of weathering and erosion, resulting in the landscape we see today. The weathering-resistant diabase remained as witnesses of volcanic activity between the clayey-sandy rock layers and form the peaks of many mountains as hard rocks.
age of the rocks : Diabase: Upper Givetian main greenstone train; Givet stage, transition from Middle to Upper Devonian (around 380 million years before today)
Access: The cliffs can be reached via a short path from Hillebachsee.