Principle of Cross-Cutting Relationships

The principle of cross-cutting relationships holds that any geological feature, such as an igneous intrusion, a fault, or a fracture, that cuts across an existing rock formation must be younger than the rock formation it cuts through, since the cutting feature could only have formed after the rock it disturbs already existed. Danish scientist Nicolas Steno first articulated a version of the underlying reasoning in his foundational 1669 work on stratigraphy, and the principle was later more explicitly formalized and applied by geologists including James Hutton in the late eighteenth century as the young science of geology developed methods for establishing relative chronology in rock sequences. The principle provides one of the most direct and widely applicable tools available for establishing the relative order in which geological events occurred at a given location, since a geologist can read the sequence of cutting and cut features directly from the rock exposure itself without needing to determine any absolute numerical age. Cross-cutting relationships remain a fundamental working principle in field geology today, routinely used together with the principle of superposition and other stratigraphic principles to reconstruct the geological history of a rock outcrop.