Final answer:
Uniformitarianism, as suggested by Hutton and Lyell, is the principle that Earth's current geological processes have always operated in the same manner and at the same rate over long periods, shaping Earth's features gradually over time.
Step-by-step explanation:
The concept of uniformitarianism formulated by Hutton and Lyell suggests that the same natural laws and geological processes that we observe shaping the Earth today have operated in the same way throughout the Earth's history. This principle, often summarized as 'The present is the key to the past,' directly contrasted the prevailing views of their time, which attributed the geology of the planet to sudden, catastrophic events.
Instead, Hutton and Lyell proposed that features like valleys and mountains were the result of gradual processes such as erosion and sediment deposition occurring over vast spans of time. This concept reinforced the idea that Earth must be much older than the few thousand years previously thought, providing a framework that would later support Charles Darwin's theory of evolution, implying that given enough time, biological changes could occur through slow, continuous processes.