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In geology, uniformitarianism is the belief that Earth's physical structure is the result of currently existing forces that have operated uniformly (in the same way) since Earth formed roughly 4.5 billion years ago. Mountains rise, valleys deepen, and sand grains collect now the same way they uplifted, eroded, and deposited over these millions of years. The activities of the present are a key to those of the past.

Early theories of Earth's formation were based on a literal reading of the Biblical book of Genesis. In the mid-sixteenth century, Irish Catholic bishop James Ussher (1581–1656) counted the ages of Biblical characters and calculated Earth to be only 6,000 years old. Bound by tradition (and even by law) to work within this short time frame, scientists of the time had to explain the placement and composition of rocks with more acceptable theories like catastrophism—the belief that Earth changes suddenly during...