Correlation

The method of using similarities between geologic units to extend information about geologic sequences over large geographic areas is called correlation. In lithologic correlation, a unit is recognised by it's lithology (rock type) or a sequence of lithologies. The same distinctive sequence of sandstone (oldest), shale, dolomite, shale (youngest) is recognized over much of the north-central United States.

Using fossil assemblages to determine relative ages can result in very precise correlations. Because the A assemblage is missing in the second area, it can be inferred that there is an unconformity between the sequences with assemblages B and C in the second area.