Piggyback Thrust

 

A Piggyback Thrust is a type of reverse fault created by extreme compression of the crust during a plate collision.  The "piggyback" is where several thrust faults occur in succession.  this is one of the key aspects of an orogeny, where mountains are built.

The far northwest of Scotland shows a superb example of this called the Moine Thrust (shown in the cross section above).  This area was key for geologists to understand the process of mountian building, before plate tectonics was even discovered.