Summarize with students the rates of erosion and sedimentation, the amounts that have occurred, and how this leads scientists to conclude that the Earth is very old (many millions of years). Do the same with continental drift. Couple this with the differing fossils of flora and fauna found in successive layers of sedimentary rock. What is the overall picture that emerges?
Radio Active Dating
In the last century techniques of radioactive dating have been developed, improved, and made more accurate. The concept of radioactive dating is explained in the following video:
Note that this video is in reference to carbon dating, which is only accurate to 50-60 thousand years. However, the same concept applies to transitions of other elements with much longer half lives, and enable dating back to millions, hundreds of millions and even billions of years. Such dating methods have enabled the addition of the “millions of years ago” to charts of geological ages. Likewise, they lead to the conclusion that the Earth is 4.54 billion years old.
It is very difficult for us to conceptualize such big numbers. The following video will help.