Contact between rocks over long periods of time causes rocks to smooth.
Primary rock types are sandstone, and shale.
Contact between shale over long periods of time causes them to smooth.
what smooths over long periods of time due to contact?

[a] oxygen
[b] calcite
[c] h2o
[d] clouds
[e] erosion
[f] shale
[g] bamboo
[h] rivers