The Taunus is a part of the Rheinischen Schiefergebirges.
The underground is a saddle of lower Devonian sediments. There are several spots of limestone, former reefs, which are karstified.
The Lahn-Dill-area with the socalled Hörre-Zug form a complex synclinale structure with three distinct parts. The rocks are from the lower, middle, and upper Devon, and lower Carbon. The Hörre-Zug is continued in the Dillmulde, an area called Kellerwald.