Up until the 18th century, it was common belief that the Earth’s age was about 6 thousand years old, basically the age that the Bible defines. This claim was about to be disrupted by a Scottish geologist named James Hutton, with the publication of "Theory of the Earth” in 1788, which introduced the concept of deep time, that the Earth had an age that was in millions of years, and that it was hard for humans to comprehend.
Hutton's concept of deep time emerged from his detailed studies of rock formations and soil layers in various parts of Scotland. One of Hutton’s most famous observations was made at Siccar Point, Scotland, where he found an angular unconformity showing layers of rock tilted vertically and overlaid by layers of horizontally bedded rocks. This geological formation visibly demonstrated that immense periods were necessary to allow for the deposition, tilting, erosion, and subsequent overlay of new material.
Hutton's ideas introduced the concept of geological time, which he believed to be effectively infinite, famously stating that in the rocks, there is "no vestige of a beginning, no prospect of an end." This radical perspective laid the foundation for the later development of the theory of plate tectonics and evolutionary biology, providing the vast timescales necessary for natural processes of change and adaptation.
A key for Hutton coming up with the idea was his principle of uniformitarianism, encapsulated by the phrase "the present is the key to the past,". His key observation was that the processes observed today have always operated in the same way, requiring long timescales to produce the geological features we see.
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