Stratigraphic levels can be horizontal as well as vertical.
On beaches, where the configuration of the shoreline has changed through time, the earliest site may be inland, the later site closest to shore.
This is the concept of --or the Law of Superposition.
Stratigraphic sequences in the field, however, are sometimes unreliable.
You might ask students to picture a pile of newspapers that have been stacked every day for a week.