prize9-5, Exercise 3: Older over younger

Model answer (check the details):

Normally younger layers lie on top of older ones (superposition). If older rock lies on top of younger rock along a surface, the likely explanation is that a sheet of rock was pushed over its neighbours along a low-angle thrust fault, or that a fold has been turned upside down.

In the north-west Highlands, Peach and Horne and their colleagues mapped the area in detail and showed that old Moine rocks had been moved many kilometres westward over younger rocks along the Moine Thrust. The pattern, together with crushed rock along the fault, explained why the layers looked in the wrong order.

Lesson: when the order looks impossible, test whether a fault or overturned fold explains it before assuming that the usual rule has failed.

Care: check the distances and dates in the Survey memoir or a modern summary.