Rock layers (6)
I believe the answer is:
'rock layers' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for strata that I've seen before include "Layers as of rock in the earth's crust" , "hard beds" , "Layers of rock on earth's surface" , "levels of society" , "Layers or thicknesses of earth" .)