“Sequence dating” (S.D.), is simply the observation that, like a layer cake, the top levels or strata, with their artifacts, are the most recent periods while the lower strata contain the earlier periods. But it must be remembered that not all strata are to be found at the same depth, so each stata must be carefully examined and dated. Number 1 in the photo represents a well or cistern dug in the Hellenistic period into the previous Persian and Iron Age periods. When features are cut into previous periods, like digging modern pits, graves or foundations, this can make reading the pottery difficult, as there can be later pottery down in earlier strata. Sometimes a probe is used to get a sense of what periods lie below the surface.