Replicate core samples
Three replicate cores from one location, adjusted to the same penetration depth. The obvious boundary between silt (dark) and clay (light) is aligned in all cores.  This is the expected situation when sediments are undisturbed.
Sometimes a core (upper) may lack a prominant zone  that is seen here in its two replicates (lower).  This is especially common in harbor sediments which have been dredged repeatedly, and natural zonation overturned.