In the not too distant past, people believed that the Earth was only a few thousand years old. Unaware of long-term geological processes, they thought the Earth had always been similar to what it appeared to them.
In 1788 a geologist named James Hutton proposed that the Earth might be more that a few thousand years old. He believed that rocks, mountains, and valleys had been slowly changed by by natural forces including rain, heat, cold, volcanoes, and other natural forces yet to be discovered.
In 1830, just before Darwin's voyage on the H.M.S. Beagle, a geologist Charles Lyell furthered Hutton's ideas by insisting 
that Earth's past events must always be explained using the scientific method. He believed that forces from beneath were causing land to move, shift, push up, twist, bend, bury, and even shove ocean floors up to become mountains.
Today geologist have verified Hutton and Lyell's ideas and believe that the Earth is over 4 billion years in age. Darwin had hoped for an age of at least several hundred million years so that natural selection would have time to operate to explain diversity of species. He would be thrilled to see today's evidences.
Geologists worldwide have gathered data on Earth's tectonic movements (see animation of last 750 million years of movements), fossils, and have conclusive evidence through radiometric dating that the Earth is billions of years old.
Pictures of Hutton & Lyell from http://www.ucmp.berkeley.edu/
Picture of Grand Canyon from http://www.indospectrum.com/travels/gc.html
