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

Fact 2: Primary rock types are Sandstone, and Shale.

Given the two facts above, answer the question "what smooths over long periods of time due to contact?" with the following options: 
- oxygen
 - calcite
 - h2o
 - clouds
 - erosion
 - shale
 - bamboo
 - rivers


 

      shale   