The fundamental geological principle that the processes that are operating now to shape the world around us have been operating throughout the geologic past as well. This principle was first proposed by James Hutton in the mid-1700s, and implied that the Earth had to be billions of years old to create all of the rocks and structures we see today. Importantly, uniformitarianism does not require that all changes happen at the same rate; some processes are slow, like the deposition of sediment in deltas, others are fast, like volcanic eruptions. This principle is often paraphrased as "The present is the key to the past."

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