The plural strata has occasionally been used as a singular since the 18th century and is sometimes given the plural stratas<there was a strata of Paris which mere criticism of books fails to get hold of — Ezra Pound><a Roman burial ground suggests stratas of corruption and decay — Connie Fletcher, Booklist>. Current evidence shows senses 2, 3b, and 4 so used, with 3b the most common. Singular strata is persistent but not frequent. Strata may someday establish itself as a singular like agenda, but that use is still not established.
Examples of STRATUM
<the lower strata of society have been hit especially hard by this economic downturn>
<the level of writing in that pop novel is several strata beneath that of serious fiction>