Between the years of 17, James Hutton and William Smith advanced the concept of relative dating.Hutton, a Scottish geologist, first proposed formally the fundamental principle used to classify rocks according to their relative ages.Geologists use several principles to find the relative age of geologic layers. Remember, contact metamorphism is changes in a rock due to contact with magma or lava.
Geologist are often trying to learn more about Earth and its history.
It is widely accepted that the Earth is about 4.5 billion years old. For example, the Colorado river has eroded away parts of the Columbia plateau.
He concluded, after studying rocks at many outcrops, that each layer represented a specific interval of geologic time.
Further, he proposed that wherever un-contorted layers were exposed, the bottom layer was deposited first and was, therefore, the oldest layer exposed; each succeeding layer, up to the topmost one, was progressively younger.
The example used here contrasts sharply with the way conventional scientific dating methods are characterized by some critics (for example, refer to discussion in "Common Creationist Criticisms of Mainstream Dating Methods" in the Age of the Earth FAQ and Isochron Dating FAQ).