Geology 101: Reading the story in the rocks

by Carolyn Gramling
Thursday, January 5, 2012

David Harwood’s field geology course gives future teachers an introduction to several of geology’s most fundamental principles, including the stratigraphic basics described by Nicholas Steno in 1669. Go to the head of the class with this quick primer.

Steno’s Principles of Stratigraphy:

Principle of Superposition:

In any sequence of strata (rock layers), the oldest layer is on the bottom and the youngest is on the top.

Principle of Initial Horizontality:

Because sedimentary rock layers form when particles in a fluid, such as water, slowly settle, the layers are originally deposited horizontally (“parallel to the horizon”). Deformation or tilting happens later.

Principle of Horizontal Continuity:

Strata can be assumed to have been continuous laterally, “unless some other solid bodies stood in the way.”

Principle of Cross-Cutting Relationships:

Rocks that intrude or cut across other layers of rock are younger than the rocks they disrupt.

Other Concepts:


The concept that “the present is the key to the past,” as geologist Charles Lyell wrote in 1830 in “Principles of Geology.” In other words, the same natural processes and laws that operated in the past are operating today.

The Precept of Multiple Working Hypotheses:

Geologist Thomas Chrowder Chamberlin described this concept in 1890: Rather than searching for confirmation of the prevailing wisdom, or “ruling theory,” scientists should consider and collect information on many possible explanations at once — and should not expect any single explanation to account for everything.

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