A process called relative-age dating helps scientists determine how old these rock layers are.
In order to evaluate the potential of the Schmidt hammer for relative age dating of Late Pleistocene moraines and rock glaciers, rebound (R) values were collected at 38 sites in two formerly glaciated valleys in the Western Tatras in northern Slovakia.
A large statistical population of measurements obtained from moraine and rock glacier surfaces was used to analyze the variability of mean R-values in the same lithology and overall southern valley exposition.
Four separate sets of glacial/periglacial morphosystems are present at different positions in the Western Tatras starting from valley outlets to the highest cirques, which represent successive phases of deglaciation.
The site also provides fact sheets on the age of the Earth and isochron dating.
Who's on First, University of California, Berkeley. This website is a book chapter about geologic time. This online version of their informative booklet contains short, content explanations about relative time, major geologic time divisions, index fossils for use in age dating, radiometric dating and the age of the earth.
This allows geologists to determine the age of a rock or strata relative to another rock or strata.
So, instead of saying when something happened, it puts events in the order they happened.
As the layers build up, they cement together to form sedimentary rocks.
This has also helped them to know how old the earth is. This simply means that sediments are usually deposited in horizontal layers.