Between the years of 17, James Hutton and William Smith advanced the concept of relative dating.

Relative dating is a dating method that used to determine determine the relative ages of geologic strata, artifacts, historical events, etc.

This technique does not give specific ages to items.

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.

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.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.The concept is considered by uniformitarian geologists to be a major breakthrough in scientific reasoning by establishing a rational basis for relative time measurements.However, unlike tree-ring dating -- in which each ring is a measure of 1 year's growth -- no precise rate of deposition can be determined for most of the rock layers.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).