The principle of lateral continuity refers to the fact that sedimentary beds originate as continuous layers that extend in all directions until they eventually into a different type of sediment or until they thin out at the edge of the basin of deposition (Figure 1A). For example, when a river creates a canyon, we can assume that identical or similar strata on opposite sides once spanned the canyon (Figure 1B). Although rock outcrops may be separated by a considerable distance, the principle of lateral continuity tells us those outcrops once formed a continuous layer (Figure 1C). This principle allows geologists to relate rocks in isolated outcrops to one another. Combining the principles of lateral continuity and superposition lets us extend relative age relationships over broad areas.
E J. Tarbuck, F. K. Lutgens, Illustrated by D. Tasa