Stratigraphic Classification and nomenclature
Of the many possible kinds of stratigraphic units, three have been particularly useful. They are
1. Lithographic (or Rock-stratigraphic) Units:
2. Biostratigraphic units
3. Chronostratigraphic (or Time-rock or Time-stratigraphic) units.
The above three types units are interrelated in many ways. However, a separate scheme of classification and terminology has developed for each type of unit. Considerable controversy exists about the relative importance of these three separate classification. However, each is important for particular purposes and relative importance varies with the circumstances. Lithostratigraphic studies constitute the first step in establishing the stratigraphy of any area/region and though lithostratigraphic and biostratigraphic studies should generally precede chronostratigraphic classification.