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Deep Time Traveling With a Riverbed Pebble

Most of us do not think twice about that pebble we find on the beach by the river. Nor do we think about what is behind its smooth, flattened form that makes a stone ideal to skid upon the water’s surface.

pebbles by the beach

This rounded pebble was once a sharp, rugged piece of rock, thrust out of a larger geological body, such as a cliff upon the hills. When we hold the pebble, we do not think of the knocks and gashes that broke the rock into smaller pieces or the movement of the water as it transported and sculpted each stone.

The transportation of sediments in rivers occurs in three main ways; solution when chemicals from rocks are dissolved in the water, suspension when light grains are carried along by the current, saltation where small pebbles and stones bounce along the riverbed; and traction, when larger rocks and even boulders are rolled along the riverbed.

The continual laying down of sediment these ways over hundreds of years shapes and forms the landscape around. The study of geography tells us that the deposition of a pebble beach along a river occurs during a state of low energy, as the sediment is deposited via the route of least resistance. Eventually, the river breathes out a sigh of relief, letting go of the sediment, leaving it behind on a river beach, sometimes as sand or a pebble for us to pick up and throw back into the water… or time travel with.

We don’t need a spaceship to time travel. We can simply peer into the texture and colors of a pebble in our hands, like a detective. Geologists would use a hand lens to do so, and they know that when holding this pebble, we are accessing not thousands but millions or even billions of years of the Earth’s history.

Mineral jewels and tiny patterns etched into each pebble by the hand of nature tell geologists which rock formations they belong to. These formations are simply groups geologists use to characterize rock beds according to their age and characteristics.

A stone within a sandstone

Formations usually describe geological ages in terms of millions of years. In a sandstone pebble, we can see how grains are packed linearly, with individual grains in a rough texture. Each layer marks a moment in time when layers were compacted, with each sandwiched layer representing millions of years.

We may even find a small stone solidified within that sandstone pebble that is even older, perhaps billions of years old from another rock bed, that fell the base of a cliff to become buried under more and more sandwiching horizons. Holding the stone, these compacted sandy layers over hundreds to thousands of years would eventually become solidified rock, facing more and more pressure, with chemical changes over millions of years. Over that time, the rock is buried beneath other rocks, pushed around plate tectonics which thrusts layers on top of one another, like an infinitely complex Jenga game.

Discovering deep time

Not every layer is preserved through the millennia for us to find today; some have been cut into by wind, water, and stone, through the natural process of erosion. When enough horizons are missing, or there is clearly one horizontal bed cutting into inclined layers below, geoscientists see a missing piece of history.

It was a sandstone’s gap in time, called an unconformity, which ignited the study of geology as we know it. Humans have been harnessing the rocky treasures of the deep for millennia through mining, but the academic study of geology is what brought us the awareness of deep time – that the planet is not only several thousands of years old, as was previously believed for centuries.

It was geologist James Hutton in the Highlands of Scotland in 1788 who made this discovery, looking up an angled bed of rock upon another horizontal bed and imagining that it must have taken much more than hundreds or even thousands of years for this transition to occur naturally. This discovery contradicted the belief at the time that the planet was only 6000 years old. On this one bed of titled rock on a small Scottish island, the Isle of Arran was the profound discovery in time.

Millions to billions of years

The fascination of deep time is what keeps many academic geologists devoted to their craft. I remember how one of my lecturers took our class out to a remote outcrop in the Scottish Highlands, mentioning the sheer age of the rocks and the concept of deep time before glancing off into the distance in wonder.

In these dramatic, sinuous hills, I studied some of the oldest rocks on Earth – named the Lewisian Gneiss. These are not typical sandstones, but once upon a time, they would have been before the planet’s tectonics crushed them at depth and melted them into a new form, leaving no trace of a grainy sandstone layer to see. Instead, new forms would emerge – splashes of colored minerals such as red jasper and lime green serpentine, upon a backdrop of woven fabric called ‘foliation’ – where minerals had melted and recrystallized.

These rocks are complex in history and beauty, being 3 billion years old. Unlocking the history of these rocks is like constantly undoing a string knotted for millions of years. That is what makes the study of geology such a challenge, but when you find answers that can make sense, considering all evidence, you gain insights into the secrets of nature.

For some, deep time could put our place on the planet in perspective, that we are but a speck in a vast desert on Earth’s geological timeline. When we hold that pebble at the river, which may be a sandstone enclosing the stone of one of the oldest rocks on Earth, we can ‘time travel,’ as we glance at the deep time, seeing something much more significant than ourselves that has been at play on the planet for billions of years.

Author: Clarissa Wright
[Geology; University of Birmingham; Nature]

Illustration: Clarissa Wright


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