Cobra Effect · Earth

The rocks that revealed deep time

How rocks on a Scottish shore showed that the Earth is immensely old.

7 cards, read aloud in 3:28, with a test and sources.

An hourglass with most of its sand still in the upper bulb

In the eighteenth century, many people in Europe believed the Earth was only about six thousand years old.

Scholars had added up the generations in the Bible to date the Creation. On that timescale, mountains and valleys must have formed quickly, perhaps in great floods. James Hutton, a Scottish farmer, doctor and naturalist, came to doubt it. He had spent years watching how slowly the land around him changed.

Hutton saw that rivers carry the land into the sea, grain by grain.

Rain washes soil and sand off hills and fields, and rivers carry it downstream. In lakes and seas, it settles to the bottom in layers. Buried deep enough for long enough, those layers harden into rock such as sandstone. The process is so slow that a lifetime barely shows it.

He argued that forces deep inside the Earth push old sea beds up into new land.

Hutton found layers of rock on land that must once have formed underwater. Many had been tilted, folded, or even stood on end. He reasoned that heat from deep inside the Earth could lift and bend rock. Then the new land would wear away in its turn, and the cycle would begin again.

In June 1788 Hutton went by boat to a rocky headland called Siccar Point.

He sailed along the coast of Berwickshire, in Scotland, with two friends, the mathematician John Playfair and the young scientist James Hall. They were searching for a place where two very different kinds of rock met. At Siccar Point they found it, clearly exposed on the shore. It looked just as Hutton’s theory said it should.

Layers of grey rock stood almost upright, with red sandstone lying across their tops.

The grey layers had formed flat, on the sea bed, and then been tilted until they stood almost on end. Next they had been worn down, and then new layers of red sandstone were laid across them. At the pace Hutton had watched, each of those steps would take an enormous stretch of time. Geologists now know that the two rocks differ in age by tens of millions of years.

Playfair later wrote that looking so far back into time made his mind feel giddy.

Hutton’s theory was published that same year. He concluded that he could find no sign of a beginning, and no prospect of an end. His writing was hard to read, and his ideas spread slowly. After Hutton died in 1797, Playfair published a clearer account of them in 1802. Later geologists, including Charles Lyell, built on the idea that the Earth is immensely old.

So when change looks too slow to matter, ask what it adds up to over time.

Hutton looked at grains of sand and saw how mountains could be made and worn away. Slow processes, given enough time, can build and destroy whole landscapes. Measurements of radioactive elements in rocks and meteorites now put the age of the Earth at about four and a half billion years. Siccar Point is still there, and geologists still travel to see it.

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