Geologic Time Scales

Eras of the Earth
Law of Superposition
Radiometric Dating



References:

Chronological Methods 3 - Superposition


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Geologic Time Scales


Law of Superposition

The geological time scale developed from a very simple observation. When you shoveled dirt, manure, or hay on a pile, the dirt at the bottom of the pile was in the pile first, and the dirt at the top of the stack was in the pile last. If you phrase this in terms of age, the oldest dirt will be at the bottom of a stack. That would be true if the pile began one hour ago, one million years ago, or one billion years ago.

Geologist used this simple observation to set up the most basic rule of determine geological time, the Law of Superposition, by the Danish geologist Nicolaus Steno. Steno simple stated that when examining a stratified rock, the oldest strata will be on the bottom, the youngest strata will be on the top of the stratification. Later in the 19th century, both the William Smith and Georges Cuvier extended Steno's law to include fossils by stating that in a series of fossil-bearing rocks, the oldest fossils will be at the bottom, with successively younger fossils above. Using the Law of Superposition extended to fossils allowed paleontologist to place relative dates upon their finds.

By also comparing fossils found in different locations, paleontologist could state with some confidence the relative ages of the fossils from those different locations. For example, comparing trilobites found in Scotland and Montana, gave paleontologist the confidence to state that both areas are the same relative age. But it was not until radiometric dating that paleontologist and geologist could put a reliable number on the Eons and Periods of Earth's past.