Using these key or index fossils as markers, Smith could identify a particular layer of rock wherever it was exposed.
Because fossils actually record the slow but progressive development of life, scientists use them to identify rocks of the same age throughout the world.
Trees from the same species, growing in the same area or environment will be exposed to the same conditions, and hence their growth rings will match at the point where their lifecycles overlap.
Keyed to the relative time scale are examples of index fossils, the forms of life which existed during limited periods of geologic time and thus are used as guides to the age of the rocks in which they are preserved.
William "Strata" Smith, a civil engineer and surveyor, was well acquainted with areas in southern England where "limestone and shales are layered like slices of bread and butter." His hobby of collecting and cataloging fossil shells from these rocks led to the discovery that certain layers contained fossils unlike those in other layers.
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James Hutton and William Smith advanced the concept of geologic time and strengthened the belief in an ancient world.
This is made up of numerous regional tree-ring chronologies, particularly in the medieval and post-medieval periods, for which the laboratory now has more than 200 reference chronologies from many areas. By comparing the proportion of K-40 to Ar-40 in a sample of volcanic rock, and knowing the decay rate of K-40, the date that the rock formed can be determined.