Long before geologists tried to quantify the age of the Earth they developed techniques to determine which geologic events preceded another, what are termed "relative age” relationships.
These techniques were first articulated by Nicolas Steno, a Dane living in the Medici court of Italy in the 17th C.
First is the Law of Superposition - which states that in a section of horizontal strata, the top layer is the youngest and the underlying layers are increasingly older with depth.
Here is an example: The Davin Shale appears oldest, followed by the Hutt Ashstone, the Cabin Limestone, Lower Coal Seam, Raven Sandstone, Blackman Shale, then Fossil Bed Limestone.
Then the rocks were folded into shallow anticlines and synclines.
This was followed by an erosional event and the deposition of the Basin Conglomerate, followed by the Log Creek Mudstone, This was followed by the intrusion of the basalt dike (although the timing of the intrusion is not entirely clear).
This was followed by deposition of Loren Shale and finally the Jackson Sandstone.
Another period of structural deformation occurred, tilting the beds and finally the formation of the current erosional surface.
The laws of physics and chemistry that governed geologic processes in the past are the same as those that govern processes now and in the future.
The geologic timescale is a chronology (calendar) of events on Earth based on obtaining ages of past events.
These ages have been derived from relative dating and absolute dating (radiometric dating) of rock layers and fossils.
(a) Relative Dating This technique uses principles of stratigraphy (rock strata) and the study of fossils (palaeontology) to determine the relative ages of rocks and sediments. Field geologists' rely on a number of simple techniques for dating rocks and constructing geological successions. The Law of Strata Identified by Fossils is a little bit more complex.
(relative geologic timescale) (b) Absolute Dating Following the discovery of radioactivity in 1895, radiometric dating techniques were developed to determine the absolute ages, i.e. In the succession of strata, each layer represents the geographical conditions that occurred over that area at the time the layer was deposited.
In each period of earth's history different fauna and flora evolved with similar faunas being characteristic of similar beds.