Rock layers (6)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">rock layers</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)