Geologic Principles—Cross-cutting Relationships

dikes visible in canyon wall
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.

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

Clastic dike
Clastic dike cutting across sedimentary rock strata. Badlands National Park, South Dakota.
hance rapids dike
A diabase dike at Hance Rapid on the Colorado River cross-cuts the Hakatai Shale. Grand Canyon National Park, Arizona.

Last updated: September 27, 2018