Principles of relative dating in geology
Ask yourself how the things that are happening in the world today might end up being recorded in the sediments that are now or soon will be deposited.
How would today's sediments appear to a geologist millions of years in the future examining outcrops of sedimentary rock that originated in our time?
By the end of the 19th century, geologists had used these principles to put together an outline of the geological history of the world, and had defined and named the eons, eras, periods, and epochs of the geologic time scale.
They did not know how many thousands, millions, or billions of years ago the Cambrian period began, but they knew that it came after the Proterozoic Eon and before the Ordovician Period, and that the fossils unique to Cambrian rocks were younger than Proterozoic fossils and older than Ordovician ones.
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.
The secondary rocks were thought to include interlayered basalts, which Werner thought formed by combustion of buried coal layers.
The Scottish geologist James Hutton (1726-1797) argued that granite and basalt by solidification within the earth (as opposed to precipitating in from oceanwater).
This matching process is called correlation, which has been an important process in constructing geological timescales.(light brown) Next, fossil-rich sedimentary rocks were precipitated.These rocks are tilted due to deposition on the non-horizontal surfaces of primitive rocks.To determine the relative age of different rocks, geologists start with the assumption that unless something has happened, in a sequence of sedimentary rock layers, the newer rock layers will be on top of older ones. This rule is common sense, but it serves as a powerful reference point.Geologists draw on it and other basic principles ( to determine the relative ages of rocks or features such as faults.
In a way this field, called geochronology, is some of the purest detective work earth scientists do.