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Deep time is the concept of geologic time. The modern philosophical concept was developed in the 18th century by Scottish geologist James Hutton (1726–1797). Modern science has since established, after a long and complex history of developments, the age of the Earth at around 4.54 billion years.
An understanding of geologic history and the concomitant history of life requires a comprehension of time which initially may be disconcerting. As mathematician John Playfair, one of Hutton's friends and colleagues in the Scottish Enlightenment, later remarked upon seeing the strata of the angular unconformity at Siccar Point with Hutton and James Hall in June 1788, "the mind seemed to grow giddy by looking so far into the abyss of time."
Early geologists such as Nicolas Steno and Horace-Bénédict de Saussure developed ideas of geological strata forming from water through chemical processes, which Abraham Gottlob Werner developed into a theory known as Neptunism. Hutton's innovative 1785 theory based on Plutonism visualised an endless process of rocks forming under the sea, being uplifted and tilted, then eroded to form new strata under the sea. In 1788 the sight of Hutton's Unconformity at Siccar Point convinced Playfair and Hall of this extremely slow cycle, and in that same year Hutton memorably wrote "we find no vestige of a beginning, no prospect of an end". 
Other scientists such as Georges Cuvier put forward ideas of past ages, and Werner's ideas were incorporated into concepts of catastrophism by geologists such as Adam Sedgwick who inspired his university student Charles Darwin to exclaim "What a capital hand is Sedgewick for drawing large cheques upon the Bank of Time!". In a competing theory, Hutton's comprehension of endless deep time as a crucial scientific concept was developed into uniformitarianism by Charles Lyell in his Principles of Geology (1830–33). As a young naturalist and geological theorist, Darwin studied the successive volumes of Lyell's book exhaustively during the Beagle survey voyage in the 1830s, before beginning to theorise about evolution.
Physicist Gregory Benford addresses the concept, in Deep Time: How Humanity Communicates Across Millennia, as does paleontologist and Nature editor Henry Gee in In Search of Deep Time.  Stephen Jay Gould's Time's Arrow, Time's Cycle (1987) also deals in large part with the concept.
John McPhee discussed "deep time" at length in his 1981 book, Basin and Range, parts of which originally appeared in The New Yorker magazine. One of the metaphors McPhee used in explaining the concept of deep time was cited in Time's Arrow, Time's Cycle by Gould:
Consider the Earth's history as the old measure of the English yard, the distance from the King's nose to the tip of his outstretched hand. One stroke of a nail file on his middle finger erases human history.
Concepts similar to geologic time were recognized in the 11th century by the Persian geologist and polymath, Avicenna (Ibn Sina, 973–1037), and the Chinese naturalist and polymath Shen Kuo (1031–1095).
The spiritual implications of the concept of Deep Time have been explored by Roman Catholic theologian Thomas Berry. He proposed that a deep understanding of the history and functioning of the evolving universe is a necessary inspiration and guide for our own effective functioning as individuals and as a species. This view has greatly influenced the development of deep ecology and ecophilosophy. The experiential nature of the experience of deep time, has also greatly influenced the work of Joanna Macy and John Seed.
H.G. Wells and Julian Huxley, however, thought the difficulties of coping with the concept of deep time were exaggerated. "The use of different scales is simply a matter of practice," they said in The Science of Life. "We very soon get used to maps, though they are constructed on scales down to a hundred-millionth of natural size. . . to grasp geological time all that is needed is to stick tight to some magnitude which shall be the unit on the new and magnified scale—a million years is probably the most convenient—to grasp its meaning once and for all by an effort of imagination, and then to think of all passage of geological time in terms of this unit."
- Big History
- Deep history
- Clock of the Long Now
- The World Without Us, a non-fiction book by Alan Weisman.
- Orders of magnitude (time)
- General Relativity
- Palmer Zen.
- Kubicek 2008.
- Playfair 1805.
- Montgomery 2003.
- Rance 1999.
- Darwin 1831.
- Korthof 2000.
- Campbell 2001.
- McPhee 1998, p. 77.
- Toulmin & Goodfield 1965, p. 64.
- Sivin 1995, pp. iii,23-24.
- H.G. Wells, Julian S. Huxley, and G.P. Wells, The Science of Life (New York: The Literary Guild, 1934; orig. publ. 1929), p. 326.
- Campbell, Anthony (2001). "Book review: In Search of Deep Time".
- Darwin, C. R. (1831-07-09). "Darwin Correspondence Project - Letter 101 — Darwin, C. R. to Fox, W. D., (9 July 1831)".
- Korthof, Gert (2000). "A Revolution in Palaeontology: Review of Henry Gee's In Search of Deep Time".
- Montgomery, Keith (2003). "Siccar Point and Teaching the History of Geology" (pdf). University of Wisconsin. Retrieved 2008-03-26.
- Palmer, A. R.; Zen, E-an. "The Context of Humanity: Understanding Deep Time". In Critical Issues Committee. Geological Society of America.
- Rance, Hugh (1999). "Hutton's unconformities" (pdf). Historical Geology: The Present is the Key to the Past. QCC Press. Retrieved 2008-10-20.
- McPhee, John (1998) . "Basin and Range". Annals of the Former World. p. 77. ISBN 0-374-10520-0.
- Repcheck, Jack (2003). "Chapters 2 and 5". The Man Who Found Time: James Hutton and the Discovery of the Earth's Antiquity. Cambridge: Perseus Books. ISBN 0-7382-0692-X.
- Rossi, Paolo (1984). The Dark Abyss of Time: The History of the Earth and the History of Nations from Hooke to Vico, tr. by Lydia Cochrane, Chicago: University of Chicago Press, pp. 338, ISBN 0226728358.
- Sivin, Nathan (1995). Science in Ancient China: Researches and Reflections. Brookfield, Vermont: Ashgate Publishing Variorum series. pp. III, 23–24.
- Toulmin, Stephen; Goodfield, June (1965). The Ancestry of Science: The Discovery of Time. University of Chicago Press. p. 64.
- White, Andrew Dickson (1896). A History of the Warfare of Science with Theology in Christendom. New York: D. Appleton & Company.
- Winchester, Simon (2001). "Chapter 2". The Map That Changed the World: William Smith and the Birth of Modern Geology. New York: HarperCollins. ISBN 0-06-019361-1.
- Kubicek, Robert (2008-03-01). "Ages in Chaos: James Hutton and the Discovery of Deep Time". The Historian 70 (1): 142–143. ISBN 0-7653-1238-7.
- Playfair, John (1805). "Hutton's Unconformity". Transactions of the Royal Society of Edinburgh V (III).
- ChronoZoom is a timeline for Big History being developed for the International Big History Association by Microsoft Research and University of California, Berkeley
- Deep Time in Evolution (TV series). Note: this PBS/WGBH website advises “shockwave required.”
- Deep Time - A History of the Earth : Interactive Infographic