Grand Canyon-Illustrator of Stratigraphic Principles

Photo by Kevin Hefferan

The Grand Canyon illustrates many of the fundamental stratigraphic principles used in geology. Stratigraphy is defined as the study of rock layers with respect to time. Stratigraphic principles are "laws" that provide the keys to:

a) understand the original orientation and distribution of rock layers, and

b) determine the relative ages of rock bodies.

Stratigraphic Laws include:

Law of Original Horizontality-infers that sedimentary rock layers were originally deposited as flat-lying (horizontal) layers.

Law of Lateral Continuity-states that sedimentary rock layers are deposited over large areas. A useful analogy in Wisconsin is the distribution of snowfall over a region: snow does not simply fall along Division  Avenue, but accumulates in a region encompassing Stevens Point, Wausau, Wisconsin Rapids, etc.

Law of Superposition-states that, in a cross-section view, rock layers are oldest at the bottom and become progressively younger upwards.

Law of Cross-Cutting Relations-infers that a rock body (e.g. igneous dike) cutting through another rock body (sandstone beds) is younger than the layers it intrudes; that is, the igneous dike would be younger than the sandstone beds.

Question:

Which of the above listed stratigraphic laws is not illustrated in the photograph above?

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