Wide rings are produced during wet years and narrow rings during dry seasons.
The principles of relative dating for continuous stratigraphic sequences: (as put forth by scientists such as Nicolas Steno): Ice cores are obtained by drilling core samples of ice in glaciated regions, such as near the poles.
Visible light and dark rings can be found in such cores that are then analyzed to determine the age of the ice.
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.
Seriation uses the assumption that once a tool was developed, its use would become more widespread.
Stratigraphy uses the assumption that higher layers or strata were laid down after lower layers.
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.
By using dendrochronology scientists have dated certain living trees to having ages of around 4600 years.