Uniformitarianism - "The Present is the Key to the Past"
Uniformitarianism is a geological doctrine. It states that current geologic processes, occurring at the same rates observed today, in the same manner, account for all of Earth's geological features. Thus, it assumes that geological processes are essentially unchanged today from those of the unobservable past, and that there have been no cataclysmic events in earth's history. As present processes are thought to explain all past events, the Uniformitarian slogan is, "the present is the key to the past."
Uniformitarianism - Glossary of Geology
Uniformitarianism is defined in the authoritative Glossary of Geology as "the fundamental principle or doctrine that geologic processes and natural laws now operating to modify the Earth's crust have acted in the same regular manner and with essentially the same intensity throughout geologic time, and that past geologic events can be explained by phenomena and forces observable today; the classical concept that 'the present is the key to the past'." (Robert Bates and Julia Jackson, Glossary of Geology, 2nd edition, American Geological Institute, 1980, pg. 677).
Uniformitarianism - James Hutton and Sir Charles Lyell
The doctrine of Uniformitarianism was significantly advanced by James Hutton (1726-1797) in his publication, Theory of the Earth (1785). Hutton influenced Sir Charles Lyell (1797-1875), who is acclaimed as the father of modern geology with his work, Principles of Geology (1830-1833, a three volume work). Lyell, in turn, influenced Charles Darwin, who later wrote (1859). Lyell is responsible for the general acceptance of Uniformitarianism among geologists for the past 150 years.
Uniformitarianism - Post Gradualism
In regards to Uniformitarianism, Warren D. Allmon writes, "As is now increasingly acknowledged, however, Lyell also sold geology some snake oil. He convinced geologists that because physical laws are constant in time and space and current processes should be consulted before resorting to unseen processes, it necessarily follows that all past processes acted at essentially their current rates (that is, those observed in historical time). This extreme gradualism has led to numerous unfortunate consequences, including the rejection of sudden or catastrophic events in the face of positive evidence for them, for no reason other than that they were not gradual." ("Post Gradualism", Science, vol. 262, October 1, 1993, pg. 122).
Uniformitarianism - Catastrophism
Uniformitarianism, together with the Geologic Column presupposed by Lyell based on uniformity, have been disproved by geologic features such as poly-strata fossils, misplaced fossils, missing layers and misplaced layers (including layers in reverse order or "ancient" layers found above "modern" layers). Furthermore, observed cataclysmic events such as the eruption of Mt. St. Helens in 1980 have validated Catastrophism, which is contrary to Uniformitarianism. We now know that catastrophe has had a significant role in forming Earth's currently observable features.