Rocky beds (6)

I believe the answer is:

strata

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'beds' is the definition.
(I have seen 'hard beds' mean 'strata' so perhaps 'beds' could also mean 'strata')

I cannot understand how the rest of the clue works.

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(Other definitions for strata that I've seen before include "Layers of rock on earth's surface" , "Layers as of rock in the earth's crust" , "Levels, grades or social classes" , "Layers into which rocks are divided" , "Layers of sedimentary rock" .)

I've seen this clue in the Evening Standard.

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