When Charles Darwin was a student in the 1820s, no science exerted a stronger hold on the popular imagination than geology. The public followed the latest theories and discoveries in amazement. Thousands bought books on the subject, and debates on the history of the Earth and its formation raged among scientists.

By this time, geologists had shown that Earth was not static or fixed--clearly it had undergone sweeping changes over time and was, in fact, still changing. This idea had a huge influence on Darwin's thought. Pondering the long, slow changes in Earth's history would later help shape Darwin's ideas about how plant and animal species, too, had changed over millions of years.

Shifting Layers of Roc

Hutton's Unconformity, Siccar Point, Scotland

(c) Marli Miller / Visuals Unlimited

The exposed layers of rock at Siccar Point, Scotland, tell a dramatic story of the ever-shifting movement within the Earth. Beneath the nearly horizontal layers of red sandstone, one can see vertical layers of gray slate. In the late 1700s, Scottish geologist James Hutton realized that these cliffs revealed a long, slow process. The vertical layers were originally laid down horizontally, then lifted and tilted by forces within the Earth. After being worn down by erosion, they sank underwater and were covered by sediment. This sediment also became rock, forming the red upper layers. Later the whole structure emerged above the water.

Such insights changed the way people understood Earth's history. Geology showed that Earth was much older than the Bible implied--processes like these would require millions of years. According to Hutton, Earth had been constantly changing for so long that there was "no vestige of a beginning, no prospect of an end."

A Story in Stone

The cliffs at Siccar Point, Scotland dramatically reveal how layers of rock have been raised, lowered, tilted and eroded over time. We now know that these processes took millions of years.