Well of course (das) "Fenster" is German for "window", but in geology and geomorphology, "fenster" refers to a peculiar type of landform, a window or hole eroded into a nappe (a thrust sheet exposed as surface rocks) so that the underlying (possibly older, possibly younger) rocks are at the surface. A fenster is the inverse of a klippe (see my diagram in that writeup).

Fenster are usually made possible by upwarp of the affected rocks, where the landscape erodes a high point to reveal the underlying layer. The amount of upwarp required does not need to be very great; just enough for the rivers draining the fenster to be able to flow out into a lower point of the nappe. Since tectonic thrusting is usually accompanied by folding, plenty of opportunities for the type of upwarp needed to create a fenster arise. It would be misleading to use the term anticline here instead of "high point", as the nappe and the rocks beneath are separated by an unconformity. The "folds" in the nappe may not match the "folds" in the underlying layer.

The Tauern region of southern Austria and northern Italy is actually the top of gneiss domes in a huge fenster in the Austroalpine nappes forming most of the Alps of the region. Inside the window, eroded into the underlying rocks, are the Hohe Tauern, one of the highest mountain groups in the Alps.


Longitudinal view:

                          fenster    /`-.______
                            |     _.'    nappe `-------.___
               _____________v__.-'___               _______\________
--------------'    ___.--\////\\\\\\/`-------------'////\\\\\\//////       
 nappe    ____.---'/\\\\\\//////\\\\\\//////\\\\\\//////\\\\\\//////

Aerial view:
                           __                             |
                          /  \           nappe            \
                 fenster>>    \                            \
     (                  | __  _|_.~                         |
      )                 |(  )( | `.                         \
     (            _.-.  | )(__)|                             \
      )          (    ) \(    `|~~~                           \
  .--'_    __   __)  (___\)    | `~                            |
 ((    )  (  ) (          \____|                              /
  ))  (____)(___)                                            |
--''                                                        / 

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