Hard beds (6)
I believe the answer is:
strata
'hard beds' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for strata that I've seen before include "Layers as of rock in the earth's crust" , "Layers of rock on earth's surface" , "Geological layers" , "Beds of sedimentary rock" , "Layers as of rocks in earth's crust" .)