Relative age and relative dating
Geologists have divided the Earth's history into Eras -- broad spans based on the general character of life that existed during these times -- and Periods -- shorter spans based partly on evidence of major disturbances of the Earth's crust.
The "relative" positions of layers and fossils to assign estimated dates to strata.
It only sequences the age of things or determines if something is older or younger than other things.
During the same interval of time in another part of the world, the land surface subsided and was covered by the seas.
From the results of studies on the origins of the various kinds of rocks (petrology), coupled with studies of rock layering (stratigraphy) and the fossils they contain (paleontology), geologists have associated layers of rocks with sequence of events thought to have occurred over hundreds of millions of years.
For example, it is believed that during a particular episode the land surface was raised in one part of the world to form high plateaus and mountain ranges.
Therefore, the actual length of geologic time represented by any given layer is usually unknown or, at best, a matter of opinion.
William Smith's collecting and cataloging fossil shells from rocks led to the discovery that certain layers contained fossils unlike those in other layers (see: fossil sorting).
Dendrochronology is a technique of dating past climatic changes through a study of tree ring growth.