Scienc-y question for ya'll. My mind blanked and I forgot what they're called :P
In case you are not joking, the answer is "sedimentary layers" ! ..... each layer of which can be called a "stratum", though it is more usual to refer to them collectively as "strata" [ plural of stratum - from the Latin, meaning "something thrown down" ]
Hmm...beds? Beds form by the deposition of layers of sediment on top of each other.
oh. My Science paper makes it sound like it has a really Scientific name :P Thanks x