Geologic Principles—Cross-cutting Relationships

^{Pegmatite dikes exposed in Painted Wall cliff face. The light colored dikes are younger than the dark rock. Black Canyon of the Gunnison National Park, Colorado.}

Introduction

James Hutton’s observations related to uniformitarianism also serve as the basis for another important geologic principle called cross-cutting relationships, which is a technique used in relative age dating.

Principle of Cross-Cutting Relationships

The geologic axiom or general law that states that relative ages of rock units and geologic structures can be established: younger rocks or structures cut across older rocks or structures.

Cross-cutting Relationships—Intrusions

In short an intrusive rock body is younger than the rocks it intrudes. For example, Salisbury Crag, a prominent Edinburgh landmark known to Hutton, owes its relief to a thick sheet of resistant basalt. Hutton showed from the super-heated contacts below and above, and from places where the basalt actually invaded underlying and overlying beds, that the thick basalt body was not merely a flow that had formed in sequence. Rather it was intruded as hot magma into the surrounding sedimentary rocks long after they were deposited (Eicher 1976).

Cross-cutting Relationships—Faulting

Other similar relationships include faults being younger than the rock layers they cut and erosional surfaces being younger than the rocks they erode.

^{A splay of the Moab Fault in Arches National Park illustrates the principle of cross-cutting relationship. The fault is younger than the rock it displaces.
Photo by Allyson Mathis, first published in Moab Happenings.}

Clastic dike cutting across sedimentary rock strata. Badlands National Park, South Dakota.

A diabase dike at Hance Rapid on the Colorado River cross-cuts the Hakatai Shale. Grand Canyon National Park, Arizona.

Related Links

• NPS—Geologic Concepts

• NPS—Rocks and Minerals

• NPS—Geology

Geologic Time