Westward expansion meant that the US acquired ever increasing amounts of land, and needed people, farmers, to settle there so it could maintained without a huge military presence
Superposition is the geologic principle which explains why the bottom stratum in a section of rock is the oldest. The word literally means placing one thing on top of another where the lower strata are the older.
Desert and Rainforest, I believe this could the right answer
Cutting down the trees! (Stop cutting them down)
And human presence disrupting the ecosystem (let the environment be)
