Heavy rock in Colorado city (7)

I believe the answer is:

boulder

I'm a little stuck... Click here to teach me more about this clue!

This might be a double definition.

'heavy rock' is the first definition.
'boulder' can be an answer for 'rock' (I've seen this before). I am unsure of the 'heavy' bit.

'colorado city' is the second definition.
(I've seen this before)

'in' acts as a link.

Can you help me to learn more?

(Other definitions for boulder that I've seen before include "Large smooth mass of rock" , "Large, loose eroded rock" , "Large stone" , "Colorado city" , "US city" .)

I've seen this clue in The Sun.

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