Geologic dating principles
An unconformity is a buried erosional surface or non-depositional surface, a contact between the rocks below and the layer of stratified rock above that is missing a significantly large interval of geologic time.
For example, deep in the Grand Canyon in Arizona, there are places where a layer of rock of Devonian age is right on top of a layer of rock of Cambrian age.
Geologic time covers the whole sweep of earth's history, from how and when the earth first formed, to everything that has happened on, in, and to the planet since then, right up to now.
Some are abrupt, such as an explosive volcanic eruption, an earthquake, or a landslide.Much of the most detailed and precise information that geologists have gleaned of earth's history comes from a branch of geology known as stratigraphy.Stratigraphy studies stratified rocks, - layered rocks, in other words, which are either sedimentary or volcanic - establishes their age sequence based on principles of relative geologic age, and reconstructs, from the evidence in the rocks and from their field relations as depicted on maps and cross-sections, the geologic history that they represent.Geologists still use Steno's principles, with some refinements and additions.
They are summarized as the principles of relative geologic age determination, sometimes referred to as the principles of relative dating.In the early 1800s, soon after James Hutton died, William Smith in England made the scientific case for what came to be called the principle of faunal succession.