The Rheinisches Schiefergebirge is composed of Devonian and Lower Carbon sedimentary rocks, similar to Harz and Frankenwald. The rocks are folded intensely and speckled with ignious (volcanic) rocks like diabases and keratophyres.
This area is part of the socalled Variszian geosyncline, which is a large area of downlift which happend during the Variszian time. During this downlift phase, the depression was continually filled by sediments, among them limestones. Later the area was both lifted and folded, which is generally called orogeny, but there was never a alpine mountain range. The uplift rate depends on the area, the southern parts like Hunsrück and Taunus, were lifted the most. The uplift started during the Pliocene, at the end of the Tertiary. The Rothaargebirge is a much younger anticline which was lifted for a rather high amount, but has now stopped.
The Rheinisches Schiefergebirge geography is separated into the areas left of the Rhine (linksrheinisch) and right of the Rhine (rechtsrheinisch). Left of the Rhine are Hunsrück, Eifel and Hohes Venn composed of lower Devonian and older sediments. Right of the Rhine are Taunus, Westerwald, Siegerland, and Sauerland. The Taunus and the Siegerland are also composed of lower Devonian sediments. The Westerwald, the Vogelsberg and part of the Eifel consists of tertiary and quarternary basalt. The Sauerland consists mostly of Middle and Uppder Devonian layers, mostly carbonatic rocks (limestone).
Several parts of this area are karstified: