Arizona Museum of Natural History location (4)

I believe the answer is:

mesa

(Other definitions for mesa that I've seen before include "Flat-topped hill with steep sides, common in the southwestern US" , "Flat tableland with steep sides" , "Flat-topped mountain" , "US steep-sided tableland" , "In the US south-west, a steep-sided flat-topped hill" .)

I've seen this clue in The Wall Street Journal.

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