CK-12 Foundation
Ck 12: Earth Science: Determining Relative Ages
[Free Registration/Login may be required to access all resource tools.] Distinguishes how to tell which rock unit is older than the ones around it and so reveal the geologic history of the region.
Other
Radford University: Environmental History Timeline
Worrying over the environment is nothing new, just the term "environmental" is. Investigate the history of concerns dating back to ancient civilization and looking forward into the future.
Estrella Mountain Community College
Online Biology Book: Paleobiology: Fossils and Time
Find out how fossil evidence has been the major link to learning about Earth's geologic history. See several illustrated interpretations of a geologic time scale in this online biology textbook.