Fact 1: Most canyons are formed by flowing rivers through erosion over long periods of time.

Fact 2: Erosion was monitored by means of erosion pins.

Given the two facts above, answer the question "How do we track canyon changes?" with the following options: 
- LEDs
 - wind
 - forces
 - river
 - slow
 - Pins
 - Raft
 - three


 

      Pins   