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University of California
University of California Museum of Paleontology: Geology and Geologic Time
This site provides information about the geologic relationships of rock layers, as well as much about geologic time and the history of Earth.
Talk Origins Archive
Talk Origins Archive: Radiometric Dating and the Geological Time Scale
A lengthy article with lots of information about radiometric dating, including pictures and charts.
Other
Planetary Society: Relative and Absolute Ages in Histories of Earth and the Moon
A lengthy scholarly article that discusses the geologic time scale, its history of development, age-dating events that occurred in different eras, and how absolute and relative dating are used to assess the ages of the Earth and Moon....
Other
Utah Geological Survey: How Do Geologists Know How Old a Rock Is?
Geologists generally know the age of a rock by determining the age of the group of rocks, or formation, that it is found in. The age of formations is marked on a geologic calendar known as the geologic time scale. Development of the...
University of California
Ucmp: A Relative Dating Activity
An learning activity to help young scholars grasp the concept of relative dating and geologic time.
Open Curriculum
Open Curriculum: Studying the History of Life
This in-depth article illustrates the concept of geologic time, and helps the learner understand why fossils are rare.
US Geological Survey
Relative Time Scale
This is a good resource for anyone who wants to understand the historical development that led to our understanding of relative dating. This site also has links to index fossils and a table of geologic time.
US Geological Survey
The Numeric Time Scale
This is a good source for getting an in-depth description of using radioactive decay to measure the ages of rocks. In addition to describing the process of radiometric dating, this resource also includes a geologic time scale and four...