Geological layers (6)
I believe the answer is:
strata
'geological layers' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for strata that I've seen before include "levels of society" , "Layers of sedimentary rock" , "Geological layers" , "Layers or thicknesses of earth" , "Layers of rock on earth's surface" .)