Where wind, water or ice keeps working on the land for a very long time, a landform forms, bit by bit. To predict it, ask what keeps working there and what it does.

The rule: where wind, water or ice keeps working on the land for a very long time, a landform forms bit by bit; to predict it, ask what keeps working there and what it does.

So, back at the park stream, the stream keeps carrying dirt away and cutting down.

Give it thousands and thousands of years, and its shallow channel becomes a deep valley.