Four People Who Learned to Tell Time in Stone

Four People Who Learned to Tell Time in Stone

[1]In 1666 a Danish doctor named Nicolas Steno was working in Florence, Italy, when fishermen brought in the head of an enormous shark. Steno dissected it and noticed that its teeth looked exactly like the 'tongue stones' that people dug out of rock on the island of Malta. He argued that the stones were shark teeth that had been buried in sediment and turned to rock. That raised a bigger question. How does loose sediment become solid rock with objects trapped inside it, and how do the layers end up stacked the way they are?

[2]In 1669 Steno published a short book with his answer. He reasoned that sediment settles out of water one layer at a time, so each layer must be younger than the one beneath it. He reasoned that settling sediment spreads out flat, so a layer that is now steeply tilted must have been moved after it hardened. And he reasoned that a layer keeps spreading sideways until something stops it, so matching layers on two sides of a valley were once joined. Today these three ideas are called superposition, original horizontality and lateral continuity. Steno never saw the Grand Canyon, yet his rules work there and everywhere else, because gravity and water behave the same way in every place and in every century.

[3]More than a hundred years later, a Scottish farmer and doctor named James Hutton took Steno's rules to a sea cliff. In 1788 he and two friends sailed along the coast of Scotland to a place called Siccar Point. There they saw gray layers standing nearly on end, worn flat across their tops, with red layers lying gently across them. Hutton read the scene as a sequence. The gray layers formed flat, were tilted upright, were worn down, and then were buried by red sand. Each step takes an enormous amount of time. One of his friends later wrote that looking at the cliff made the mind 'grow giddy.' Hutton could not say how many years the cliff recorded, but he was sure the number was far larger than anyone had imagined.

[4]In the 1790s an English surveyor, William Smith, was planning canals through the countryside. He noticed that each rock layer held its own set of fossils, and that the same layers, with the same fossils, appeared in the same order wherever he dug. A fossil shell could tell him which layer he was standing on, even many miles away. Smith used this to produce, in 1815, the first geological map of an entire country, England and Wales. His idea that fossil species appear in a fixed order is what this lesson calls the law of fossil succession.

[5]Steno, Hutton and Smith could put events in order, but none of them could give a date in years. That had to wait for the discovery of radioactivity. In 1907 an American chemist, Bertram Boltwood, measured the amounts of uranium and lead in several minerals. Because uranium turns into lead at a steady rate, the ratio of the two worked like a clock. His samples came out between about 400 million and 2.2 billion years old, far older than most scientists then expected. In 1913 a young English geologist, Arthur Holmes, published a book arguing from such measurements that Earth was at least 1.6 billion years old.

[6]The clock kept being read more carefully. In 1956 the American chemist Clair Patterson measured lead in meteorites, which formed at the same time as Earth, and reported an age of 4.55 billion years. That number has barely changed since. Notice what each person added. Steno gave rules for order. Hutton showed that the order implied vast time. Smith showed that fossils let you match layers across a whole country. Boltwood, Holmes and Patterson turned 'older' and 'younger' into years. Geologists today use all of these tools together: relative dating to build the sequence of events, and absolute dating of igneous rock to pin that sequence to a calendar.