AP Environmental Science
Main
The Scale
Sources
Student Websites
Multiple Choice Questions
Sources
http://www.earthsciweek.org/forteachers/geotime_cont.html
http://comp.uark.edu/~sboss/geotime.htm
http://www.britannica.com/EBchecked/topic/340003/life
http://pubs.usgs.gov/gip/geotime/contents.html