Crossword Genius

Rock etc a source of chalk (10)

Ross

I believe the answer is:

cretaceous

'of chalk' is the definition.
(I've seen this before)

'rock etc a source' is the wordplay.
'rock' is an anagram indicator (the solver must 'rock' the letters around).
'etc'+'a'+'source'='etcasource'
'etcasource' with letters rearranged gives 'CRETACEOUS'.

(Other definitions for cretaceous that I've seen before include "Chalky" , "of chalk" .)

I've seen this clue in The Sunday Times and The Times.
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