Bill Nye is obssessed with erosion and explains that rocks change over time. He explains what causes weathering (although he calls it erosion): water, ice, wind, sand, and chemicals. He illustrates the process of erosion in his lab.
Hoggish Greedly is clear cutting a forest, which loosens the soil as there are no roots to hold the soil in place. The Planeteers take advantage of this and show Hoggish Greedly the effects of water erosion on loose dirt.
The narrator explains how sedimentary rocks form: from sediment deposited in water through the process of weathering and erosion. He also explains the layers of sedimentary rock, which include sandstone, limestone, and conglomerates.