In a way this field, called geochronology, is some of the purest detective work earth scientists do.
There are two basic approaches: relative age dating, and absolute age dating.
Look for “absolute” ages such as cornerstones, dates carved into fresh concrete, or dates stamped on manhole covers.
Absolute age dating is like saying you are 15 years old and your grandfather is 77 years old.But the most accurate forms of absolute age dating are radiometric methods. Sedimentary rocks in particular are notoriously radioactive-free zones.This method works because some unstable (radioactive) isotopes of some elements decay at a known rate into daughter products. Half-life simply means the amount of time it takes for half of a remaining particular isotope to decay to a daughter product. Good discussion from the US Geological Survey: geochronolgists just measure the ratio of the remaining parent atom to the amount of daughter and voila, they know how long the molecule has been hanging out decaying. So to date those, geologists look for layers like volcanic ash that might be sandwiched between the sedimentary layers, and that tend to have radioactive elements.The Geologic Time Scale (courtesy of the Geological Society of America) UC (Berkeley) Museum of Paleontology Exhibit on Geologic Time For an informative description of the historical development of the Geologic Time Scale and the principles of geology that have been used to develop it, please visit the Berkeley Museum of Paleontology web page cited above.I also encourage you to visit the links within the Berkeley page to learn more about the divisions (Eras, Periods, etc.) of the time scale, their stratigraphy, life forms and other useful bits of information. Students will learn about the principles of Stratigraphy and application of various techniques.