Zone
3: How do we read the rocks?
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.
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.
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