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Tauern Window (Tauernfenster), the most important geological structure in the Eastern Alps. Between the Brenner channel in the west and the Katschberg Pass in the east, and between the two lines running along Matrei/Brenner- Gerlos Pass- Salzachtal valley- Radstadt in the north and Brenner Pass- Matrei/Osttirol- Mölltal valley in the south (Matrei schuppen zone), the deeper-lying, Penninic nappe structures of the Alps appear arched and surrounded by several higher units (Eastern Alps). The deeper (inner) system is made up of central-gneiss cores and schist coating (Bündner Schist). They are concentrically enclosed by the lower east-alpine system (particularly visible in the Radstädter Tauern Mountains and the Tux Alps) and the higher east-alpine units, which were overthrust from a southern direction during the formation of the mountains in the Cretaceous.