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.
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.
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.
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...
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.