A stratum is a horizontal layer of any material; in geology, the term refers to a layer of rock having the same composition throughout. Strata is the plural of stratum."This is a stratum of volcanic rock." "This novel contains many strata of meaning." It is correct to say "One stratum is" and "All strata are." Neither Stratums nor stratas is generally acceptable as a plural, although both occasionally appear in print.
Dictionary of problem words and expressions. Harry Shaw. 1975.