How did the work of geologists support Darwin's theories about evolution?

Before Darwin proposed his Theory of Evolution, it was widely believed that the Earth was only 6,000 years old. Geologists noticed that the Earth changed gradually over time (tectonic plates shifting to create volcanoes, mountains, etc) in order for such changes to create the Earth known at that time, it would have to be building up such changes over millions of years, thus creating the idea of Earth's immense age which allowed for evolution to create the diverse species known today. This theory of gradualism (as it was called) was adapted into Darwin's theory to apply to the species and their gradual changes.