Geology Reference
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their matching halves preserved in rock inches away from each other could only be ex-
plained as the remains of once living creatures.
Steno's demonstration that tongue stones were petrified shark teeth convinced scholars
that fossils were indeed organic remains. His interest in the problem of solids enclosed
within solids—how fossils got into rocks—led Steno to deduce that the bottom layers in a
pile of sediment were deposited first. This is the foundational principle of modern geology,
Steno's Law of Superposition—the idea that the oldest sedimentary layers are on the bot-
tom and the youngest are on top. It's still valid centuries later; I used this same basic rule
to interpret the geologic story when I hiked out of the Grand Canyon.
Steno thought that some rocks were made of consolidated sediment washed off the land
and that other rocks precipitated from mineral laden waters. Fossils were the remains of sea
creatures buried by gradual deposition of sediment on the seabed. This was why fossils ten-
ded to be the most durable parts of marine creatures (teeth, bones, and shells). Soft tissue
decayed too rapidly to be preserved.
Steno's prescience is astounding given the time in which he lived and the countervailing
convictions of his peers. The impact of his shark head dissection and the short yet wildly
influential publication it spawned in the spring of 1667 show the serendipitous nature of
scientific progress. Steno subsequently began working on a longer masterpiece that laid
geology's foundation. In trying to explain how shark's teeth ended up in rocks, he devised
rules for how to read geologic history from the rocks themselves. Whereas Descartes and
Kircher developed their ideas from sweeping generalities based on classical ideas backed
up by little, if any, geologic evidence, Steno studied Earth's history by applying guiding
principles and logic. He didn't just make up a good story to explain how he thought things
worked; he went out and scoured the countryside for clues to build up ideas that were
grounded in field evidence.
As he grew increasingly enamored with geological problems, Steno began collecting
fossils on long hikes in the Tuscan mountains. Indulging Steno's curiosity, the grand duke
opened quarries and mines to expose what lay underground. The more Steno observed, the
more he became convinced that an ancient sea deposited fossil-bearing rocks. He also noted
how some rock layers lay at an angle to the horizon, meaning that they had been tipped up
on end after they were deposited.
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