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The Giessen nappe (German: Gießener Decke) is a tectonic nappe in the southeastern part of the Rhenish Massif in western Germany. The nappe is an "alien" (allochthonous) unit in the Rhenohercynian zone of the Hercynian orogeny, it was thrust over the usual slightly metamorphosed Devonian and Carboniferous sedimentary rocks in this zone.