A fenster is best described as which feature?

• A. An eroded area of a thrust sheet that exposes younger rocks beneath the thrust
• B. An area exposing older rocks within a thrust sheet
• C. A window through which deeper rocks are visible
• D. A fault that juxtaposes two different rock units

Answer: (A)

Fenster is a structure in structural geology where erosion has cut away part of a thrust sheet (nappe), exposing rocks from beneath. The rocks seen through this “window” are the ones that lie below the thrust sheet and are typically older than the rocks that form the nappe itself. That direct, visual description—being a window through which deeper (and therefore older) rocks are visible—best captures what a fenster is.

The idea that it exposes younger rocks isn’t consistent with how nappes stack in thrust belts, where the overlying sheet is generally younger than the rocks beneath. A fenster isn’t just a generic exposure or a fault juxtaposition; the defining feature is the window-like view into the deeper, older rocks inside the thrust system.