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Relative Dating (Steno's Laws):

    Long before geologists tried to quantify the age of the Earth they developed techniques to determine which geologic events preceded another, what are termed "relative age” relationships.  These techniques were first articulated by Nicolas Steno, a Dane living in the Medici court of Italy in the 17th C.  His four observations on relative age relationships have been coined “Steno’s Laws” and are fundamental to the study of rock strata, or stratigraphy. 

1.    Law of Superposition

In a sequence of rock strata, the oldest layer will lie below or underneath the youngest.
Law one
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2.    Law of Original Horizontality

Layers of sediment, such as you would have in the bottom of a lake, or the ocean, are deposited by gravity into flat layers.
Law two
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3.    Law of Cross-Cutting

If a rock layer is cut by a fault or igneous intrusion, the rock that is cut must be older than the layer that cuts it.
Law three
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4.    Law of Lateral Continuity

Rock layers will extend outwards until the environment that produced them changes.

Law four
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For classroom activities based on using Steno's Laws to examine relative geologic time click here (
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Last modified October 02 2008 03:06 PM

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