Relative dating principles of rocks
Our editors will review what you’ve submitted and determine whether to revise the article.Join Britannica's Publishing Partner Program and our community of experts to gain a global audience for your work!Presumably if all the world’s outcrops were integrated, sediments representing all of geologic time would be available for examination.This optimistic hope, however, must be tempered by the realization that much of the Precambrian record—older than 541 million years—is missing.By this technique it is possible to detect depth variations in electrical resistivity, self-potential, and gamma-ray emission rate and to interpret such data in terms of continuity of the layering between holes.
Such an array of attributes represents an ideal, and much stratigraphic geology is rendered difficult because of departure of the natural fossil assemblage from this ideal.
Correlation based on the physical features of the rock record also has been used with some success, but it is restricted to small areas that generally extend no more than several hundred kilometres.
The first step is determining whether similar beds in separated outcrops can actually be traced laterally until they are seen to be part of the same original layer.
Correlating two separated outcrops means establishing that they share certain characteristics indicative of contemporary formation.
The most useful indication of time equivalence is similar fossil content, provided of course that such remains are present.