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

Fact 2: Sandstone, shale, and limestone are sedimentary rocks.

Given the two facts above, answer the question "what is smoothed by contact over long periods of time?" with the following options: 
- shale
 - earth
 - eras
 - Roads
 - soils
 - water
 - clouds
 - ground


 

 shale        