Hedberg (1937) published the first paper that distinguishes several kinds of Stratigraphic classification, each with its own distinct units. It proposed separate Stratigraphic units to show variations in lithology, paleontology, mineralogy, and age. Lithostratigraphic (rock-stratigraphic) had been distinguished from chronostratigraphic (time-stratigraphic) units in an earlier paper by Schenck and others (1936), which was an outgrowth of an unpublished manuscript written by Hedberg in 1932 in which the terms “rock-stratigraphic” and “time-stratigraphic” were coined. The distinction between Stratigraphic units based on paleontology and those based on age was pointed out by Hedberg (1937) and was at about the same time emphatically noted by Pia (1937). Although Schenck and Muller (1941) emphasized the need for separate rock-stratigraphic and time-stratigraphic units, they did not distinguish between time-stratigraphic and biostratigraphic units.

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