A photographic collage depicting the diversity and evolution of life on Earth through the last 600 million years.
The oldest fossils are at the bottom and youngest at the top.
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 time can not determine the actual year a material was deposited or how long deposition lasted; it simply tell us which events came first.
Techniques of accurately dating rocks is crucial in establishing the time-gap that exists.
The classic type is the angular unconformity of which Thornton Force in Ingleton and Combs Quarry, Foredale are perhaps the best known local examples.
This means that a quartz sandstone deposited 500 million years ago will look very similar to a quartz sandstone deposited 50 years ago.
Making this processes even more difficult is the fact that due to plate tectonics some rock layers have been uplifted into mountains and eroded while others have subsided to form basins and be buried by younger sediments.
With out individual time stamps the process of dating these structures could become extremely difficult.