Differences in unconformities suggested by earlier classifications are more fancied than real. The criteria of scale, either areal or temporal, and concordance-discordance do not serve as practical bases for classification. Moreover, the aim of a nomenclature should be to answer the question which is, in the case of unconformities, "what is the evidence?", not "what is the kind?"

Examples demonstrate that evidences for unconformities are: (1) truncation of allochems and stylolites; (2) occurrence of encrusting organisms; (3) cement-supported detritus; (4) cement discontinuities; (5) megascopic erosional relief and bed truncation; and (6) truncation of faults and lithostratigraphic and biostratigraphic units on maps and cross sections.

These evidences fall into the following operational categories: I--petrographic; II--macrographic; and IIIa--lithocartographic and IIIb--biocartographic unconformities.

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