Certain types of stratigraphic units are based primarily on objective features of rocks which are physically discernible or physically measurable, such as lithologic character, fossil content, and electrical character. Other types of stratigraphic units are of a more subjective nature and are based on interpretation, in terms of geologic age, environment of deposition, etc., of the evidence supplied by the various objective features.

Rock-stratigraphic and bio-stratigraphic units are dominantly objective; time-stratigraphic and eco-stratigraphic units are dominantly subjective. Countless pages of geological literature have been wasted in fruitless controversy merely because of failure to differentiate these two types of stratigraphic units by clear and precise systems of classification and terminology. Likewise, effectiveness in the application of stratigraphic thinking to petroleum geology has been impeded by the existing confusion in stratigraphic classification and terminology. An attempt is made here to bring out the proper relations between rock-stratigraphic, bio-stratigraphic, time-stratigraphic, eco-stratigraphic, and other kinds of stratigraphic units.

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