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Using these key or index fossils as markers, geologists began to identify a particular layer of rock wherever it was exposed.
Because fossils are believed to record the slow but progressive development of life, geologist use them to identify the relative age of rocks throughout the world.
Dendrochronology is a technique of dating past climatic changes through a study of tree ring growth.
Each year a tree adds a layer of wood to its trunk and branches thus creating the annual rings we see when viewing a cross section.
It only sequences the age of things or determines if something is older or younger than other things.
Some types of relative dating techniques include climate chronology, dendrochronology, ice core sampling, stratigraphy, and seriation.
Therefore, the actual length of geologic time represented by any given layer is usually unknown or, at best, a matter of opinion.
Visible light and dark rings can be found in such cores that are then analyzed to determine the age of the ice.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.He concluded, after studying rocks at many outcrops, that each layer represented a specific interval of geologic time.Further, he proposed that wherever un-contorted layers were exposed, the bottom layer was deposited first and was, therefore, the oldest layer exposed; each succeeding layer, up to the topmost one, was progressively younger.