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The Geological Time Scale 

Featured QuisScientists believe the earth to be about 4.5 billion years old - an almost incomprehensible amount of time. Geologists have separated this time into eons, eras, and periods, each with its own name. Periods are the most basic unit of geologic time. Many of their names are derived from Western Europe where much of the initial time scale development was done.

Rocks representing most of these time periods are found in Alberta. The best way for geologists to see subsurface rocks in their proper sequence is to look at oil well drill cores. Normally, the well starts in younger rocks and cuts through successively older rocks, and if one continued drilling, the Precambrian basement of Alberta would eventually be reached.

During the drilling process, rock fragments are brought to the surface and identified by a geologist. These rocks are used to produce a diagram, called a rock column, that shows rock types, thicknesses, and fossils. Related rock layers are grouped into formations and given a name. Formations are often combined into larger entities called groups. Formations are the basic rock unit that geologists use when they make geological maps.

An idealized rock column for Alberta

Reprinted from A Traveller's Guide to Geological Wonders in Alberta by Ron Mussieux and Marilyn Nelson with the kind permission of the authors and the Royal Alberta Museum.

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