Small rises that naturally precede bigger ones (9)

I believe the answer is:

foothills

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'small rises' is the definition.
The definition and answer can be both natural objects as well as being plural nouns.
Maybe they are linked in a way I don't understand?

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for foothills that I've seen before include "Low uplands by mountain range" , "What lie below a higher mountain" .)

I've seen this clue in The Globe and Mail.

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