Chronometric dating method
The main relative dating method is stratigraphy (pronounced stra-TI-gra-fee), which is the study of layers of rocks or the objects embedded within those layers.
This method is based on the assumption (which nearly always holds true) that deeper layers of rock were deposited earlier in Earth's history, and thus are older than more shallow layers.
The successive layers of rock represent successive intervals of time.
Since certain species of animals existed on Earth at specific times in history, the fossils or remains of such animals embedded within those successive layers of rock also help scientists determine the age of the layers.
These plants are eaten by animals who, in turn, are eaten by even larger animals.
Before the advent of absolute dating methods in the twentieth century, nearly all dating was relative.
The time it takes for one-half of the carbon-14 to decay (a period called a half-life) is 5,730 years.
By measuring the amount of carbon-14 remaining, scientists can pinpoint the exact date of the organism's death.
Since World War II, there has been tremendous success in the development of new methods for dating artifacts; the so-called `radiocarbon revolution' was only the first such discovery.
The increasing accuracy of the various new techniques has brought about major changes in archaeological research strategies.