Thought Puzzle:  You find a gneiss at the top of a large mountain.  What story does it tell us of the past?

The highly metamorphosed nature of the gneiss tells the story that it was once buried deeply and thus subjected to high temperature and pressure.  It must have been in the deep roots of a high mountain range.  The stuff that used to be on top of it is clearly gone now (it being at the top of a mountain and all).  Thus, a second chapter of the story is that an immense amount of rock, that used to be on top of the gneiss, has eroded away and what was once the deep mountain roots has become the top of the mountain.

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