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.
In any sequence of strata (rock layers), the oldest layer is on the bottom and the youngest is on the top.
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.
Strata can be assumed to have been continuous laterally, “unless some other solid bodies stood in the way.”
Rocks that intrude or cut across other layers of rock are younger than the rocks they disrupt.
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.
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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