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.
[ While this may be true, a shrub in Tasmania could be 40,000 years old.
See Oldest Living Organism.] The Sheffield Laboratory now has a continuous master sequence for England going back to about 5000BC. This article should be a "must read" for any person interested in factualy accurate information on dating methods.
Cross-dating is a technique used to take advantage of consistencies in stratigraphy between parts of a site or different sites, and objects or strata with a known relative chronology.
A specialized form of cross-dating, using animal and plant fossils, is known as biostratigraphy.
See more information about "Strata" Smith and his original geologic map of England.
Information about Simon Winchester's delightful biography of Smith, The Map That Changed the World is available at Tree-Ring dating is based on the principle that the growth rings on certain species of trees reflect variations in seasonal and annual rainfall.
Chronological sequence is all that is really required.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.Earth's oldest living inhabitant "Methuselah" at 4,767 years, has lived more than a millennium longer than any other tree.Many of these links also appear where appropriate below.James Hutton and William Smith advanced the concept of geologic time and strengthened the belief in an ancient world.