Fact 1: Contact between rocks over long periods of time causes rocks to smooth.

Fact 2: Smoothed, grooved and polished rocks are produced by glacial erosion.

Given the two facts above, answer the question "What is erosion?" with the following options: 
- moonlight
 - looseness of dirt or compost
 - permineralization
 - water vapor
 - fog
 - The area was once underwater
 - fissures of hot, acidic water
 - contact between rocks


 

        contact between rocks 