Hutton Uncomformity

Geologists' understanding of the Earth is in part based on the information gathered from exposed parts of the planet's crust, such as cliff faces, mountains, or canyons. These rocks reveal the history of the Earth, and enable scientists to piece it together. A classic example is the Hutton Unconformity where the rock outcrop is exposed at Siccar Point, Scotland. It was here in 1788 that James Hutton realized many geological truths that remain valid today.

Shale and Sandstone

Credit: Craig Chesek

There, in a sea cliff, Hutton observed flat-lying layers of red sandstones resting on top of nearly vertical layers of gray shales. He realized that the gray shales, which had been deposited in water, must have been uplifted, tilted, eroded, and then once again submerged by an ocean from which the red sandstones were deposited. This so-called "unconformity" represents an extensive period during which no sediments were deposited — 20 million years in the case of the Hutton Unconformity. This discovery overturned previous beliefs that the Earth was only 4,000 years old, and spawned the field of modern geoscience.