why is cross stratification not considered to be a sueful criterion in the identification of ancient sediments

Answer 1

Sueful?

Answer 2

Because they are time events (megaripples, foreset deposits, dunes, etc.) which have limited lateral continuity; the contained beds of sediment have limited extent for correlation purposes. They are cut off by the next cross strata in all directions. One use of cross stratification is if curved, up will face the concave side, a help in folded outcrops. Horizontal might be problematic in the case of megaripples and dunes, like finding one bone of a dinosaur without having the configuration of the entire skeleton.