US Geological Survey
U.s. Geological Survey: Geologic Time
A discussion of geologic time, how scientists use the concept, and how they have determined its span.
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.
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....
University of California
Ucmp: A Relative Dating Activity
An learning activity to help students 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...