 Comparing the record about the discordance in the record to the full rock column shows the non-occurrence of the missing species and that portion of the local "rock record", from the early part of the middle Eocene is missing there.This is one form of discordancy and the means geologists use to compensate for local variations in the rock record.With the two remaining marker species it is possible to correlate rock layers of the same age (early Eocene and latter part of the middle Eocene) in both South Carolina and Virginia, and thereby "calibrate" the local rock column into its proper place in the overall geologic record.