Why a Pebble Feels Smooth When Its Parent Rock Was Rough: How We Learned to Read Rounded Stones

My niece once held up two stones from a family walk and asked me a fair question. One was a jagged lump from the track near the top of the hill, and the other was a grey, egg-shaped pebble from the stream at the bottom. "Did someone sand this one?" she said.

I gave her a muddled answer about water and time. It was true as far as it went, but I couldn't have said why the corners vanish first, or why the stream's stones are the roundest ones in the valley. So I went and looked into it. What I found was a good story as well as a good explanation.

Here is the short answer. A pebble feels smooth because it has been knocked and rubbed against other stones, thousands upon thousands of times, and every knock takes off the sharpest, most exposed parts first. Water does the carrying, but it is mostly the stones that do the wearing. The rest of this piece is about why that works, how people worked it out, and a jar-of-gravel experiment you can try at the kitchen table.

How people first made sense of rounded stones

For a long time, rounded stones didn't seem to need much explaining, or the explanations were dramatic. Rounded cobbles lying far from any obvious source were often put down to sudden, violent floods. The idea that a gentle process, repeated for an enormous span of time, could do the job was a harder one to accept.

James Hutton, the Scottish naturalist whose Theory of the Earth appeared in the late eighteenth century, argued along these lines. As I read him, his reasoning was that the gravel and sand in rivers and on beaches come from the wearing down of older rocks. Rounded pebbles buried inside solid rock, in what we now call conglomerate, were evidence that older land had been broken up and worn long before. Wearing is slow, so the history it implies is very long. Hutton is famous for finding "no vestige of a beginning" in the rock record, and rounded stones were one of the quiet clues behind that conclusion.

Charles Lyell, in the next generation, made the same habit of thought a working principle in his Principles of Geology. If you want to know how the ancient past shaped the ground, look first at what rivers, waves and weather are doing today. A stone in a stream is an ongoing experiment, and you can watch it happen.

Later in the nineteenth century, some people stopped watching and started experimenting. The French geologist Gabriel Auguste Daubrée tumbled rock fragments in rotating drums, sometimes with water, to see what happened to them. As I understand it, the fragments lost their corners and edges, got smaller, and left behind a trail of sand and fine silt. That is the same result you will see in the jar experiment later. I'm describing his work only in broad strokes, because I haven't read the original reports closely. I'd rather say that plainly than dress up details I can't vouch for.

Why the corners go first

Take a lump of rock freshly broken from a cliff. It has flat-ish faces, a few sharp edges and some pointed corners. Now start knocking it against other stones, the way a river in flood does.

Corners meet other stones first. When two stones collide, they touch at their most protruding points. Imagine an impact that pushes with 10 newtons of force (roughly the weight of a one-kilogram bag of sugar). If that force lands on a corner with a contact area of one square millimetre, the pressure is about 10 newtons per square millimetre. Spread the same force over a flat face of 100 square millimetres and the pressure is a hundred times smaller. These numbers are only an illustration, but the principle behind them is real. The same push is far more damaging when it is concentrated on a tiny point.

Corners also have little behind them. A corner is a thin wedge of rock, with no bulk of stone backing it up. Rock is much better at resisting squeezing than at resisting pulling apart, and a wedge hit near its tip tends to snap off small flakes and grains. The same blow on the middle of a face has a great deal of rock around it to share the load.

Wear slows as the stone rounds. Once a corner has been blunted, it meets its neighbours over a larger area. The pressure drops, and each knock does less harm. So rounding is self-limiting: fast at first, then slower and slower as the stone approaches a smooth shape. A pebble never becomes a perfect sphere. Its internal structure matters too. Rocks that split along layers tend to wear into flat discs, which is why good skimming stones are so easy to find in some places and so hard to find in others.

It isn't only knocking: weathering rounds corners too

There is a second process that can round a rock without it going anywhere, and it's a good example of how the same geometry works in a different setting.

Rock in the ground is attacked by slow chemical weathering, where water and dissolved gases gradually alter the minerals. Think about how exposed each part of a block is. A flat face is open on one side, an edge on two sides, and a corner on three. A corner therefore gets attacked from more directions than anything else and wears back fastest. Geologists call the result spheroidal weathering. Blocks of jointed rock sitting in soil can end up as rounded boulders, sometimes with a rind that flakes off in layers, without having travelled at all.

This leads to a misconception I held for years: that a round stone must have been rolled along by water. Roundness is a clue, but not proof. A rounded boulder sitting where it formed is a different story from a rounded pebble that has come down a river. To tell them apart you look at the setting: whether the stones lie in sorted layers, whether they're mixed with sand, and whether they match rocks found upstream.

Smooth to the touch is not quite the same as round

This is another place where I had been careless with words. Geologists separate roundness, the sharpness of the corners, from the surface texture, meaning how rough or polished the skin of the stone feels.

A fresh break through a rock exposes a surface that is rough at a very small scale. Mineral grains stick up, tiny steps and cracks cross it, and your fingertips feel every one of them. Rubbing against sand grains, and against other stones, grinds those high points down until the surface is level at that fine scale. That is what we feel as smoothness.

It doesn't always work evenly. A rock made of a hard mineral set in a softer one may keep a slightly gritty feel, because the soft part wears away faster. I notice this in some granite pebbles, which often feel a little rougher than a pebble of fine, uniform rock.

Water by itself, with nothing in it, is a poor sandpaper. The abrading is done by the load the water carries: sand, grit, and other pebbles. A clear, gentle stream carrying little sediment barely wears anything. A flooding river full of gravel is a different matter.

Do the roundest pebbles really come from furthest away?

The usual rule of thumb says yes, with some honest caveats. The reasoning runs like this. Each collision knocks off a little more. The longer a stone has been tumbled, the more collisions it has had, and the rounder it should be. Since transport along a river takes time and involves many knocks, stones that have gone a long way tend to be rounder than ones from near the source.

There is a line of evidence for this that doesn't depend on shape at all. A German engineer named Sternberg measured gravel along the Rhine and found that stones got smaller and smaller downstream, and that the decrease followed a regular pattern. This is now often called Sternberg's law, or downstream fining. It's well established that gravel gets finer going downstream. What scientists are still working out is how much of that comes from abrasion (stones wearing away) and how much from sorting (the river leaving the bigger stones behind while carrying the smaller ones on). In some rivers sorting seems to matter a lot, and I'm still learning how the balance varies from one river to the next.

Here are the caveats, because the rule of thumb needs them:

• Distance isn't the same as number of knocks. A steep, fast, flood-prone river can round a stone in a short distance. A lazy lowland stream might carry it a long way and do little. Waves on a gravel beach can round stones thoroughly without moving them far along the coast, because they just roll them back and forth.
• Hard rock and soft rock behave differently. Soft rocks round quickly but also crumble away, so they often don't survive long distances at all. Tough rocks last and travel. Many New Zealand river gravels are greywacke, a hard, dense sandstone, which is one reason gravel can survive a long journey to the sea.
• How you're carried matters. Ice is a good example. Rock carried in or under a glacier is gripped in place rather than rolled against its neighbours, so it often stays angular or only slightly rounded even after a very long journey. A sharp-edged stone in a glacial deposit doesn't mean it came from nearby.
• Small grains round slowly in water. Water cushions the impact between tiny grains, so sand-sized pieces often stay more angular than you'd expect.

So rounding is a useful clue about a stone's history, and not a ruler you can read distance from.

Try it yourself: a jar-and-gravel experiment

You can watch the principle at work with very little equipment. I'll say up front that my first attempt was a disappointment, and I'll explain why below.

What you need - A sturdy plastic jar with a tight screw lid (not glass, which can break) - A handful of angular gravel, such as crushed stone or driveway chippings. Rounded river gravel won't show much. - Water - A sheet of white paper - A pencil - Optionally, some soft pieces such as chunks of brick, sandstone or chalk

What to do 1. Pick five or six pieces of gravel and lay them on the paper. Trace the outline of two or three of them, or photograph them against the same background. Rank their corners by eye, from sharpest to roundest. 2. Put all the gravel in the jar, about a third full, and add water until the jar is about half full. Screw the lid on firmly. 3. Shake the jar steadily for five minutes. A rhythmic back-and-forth works better than wild shaking. If your arms get tired, take turns with someone, which is also more fun. 4. Look at the water. It should now look cloudy. That cloud is fine silt and rock flour, the material that has been worn off the stones. 5. Repeat the five-minute sessions several times, comparing your stones with the first outlines or photographs each time. 6. When you've finished, tip the muddy water onto the garden rather than down the sink.

What I found, and why it may differ for you. The water went cloudy within the first few minutes, so something was clearly being worn away. But with hard gravel, I could hardly see the corners change for a long while. I had expected too much. A river gets its results through thousands of hours of tumbling, and my jar gave it a few minutes. When I added chunks of brick and soft sandstone, the rounding showed up much sooner. Your results will depend on the stone you use and how long you shake, so please treat my experience as one example rather than a promise.

Variations to try - Run two jars side by side, one with water and one dry. Which one clouds, and which one makes more dust? - Use one jar of hard stone and another of soft, and compare which rounds faster and which produces more silt. - Add a handful of sand to one jar. Does it speed things up or slow them down?

If your results are messy or surprising, that's normal, and often it's the most interesting part. Write down what you did and what happened, and you've done real science.

What the jar can't show

A jar is a model, and models leave things out. The stones in your jar fall a short distance and hit a plastic wall, whereas a river carries stones along its bed, rolling, sliding and sometimes bouncing in floods. Real rivers also sort their stones by size and weight, so what you find on a bar of gravel isn't a random sample of what went in at the top. Real stones carry a history of weathering too, which the jar doesn't reproduce.

The rest I'm still learning. Researchers continue to work on how much of the downstream shrinking of gravel is abrasion and how much is sorting, and on how to read a pebble's past from its shape. These are open questions, and not ones I can answer for you.

Next time you're by a river

I think about my niece's question differently now. She asked whether somebody sanded the stone, and in a way the answer is yes: other stones did, slowly, one small knock at a time, working on the corners first because that is where they could do the most.

It strikes me how little the idea needed in the end. There was no special machine and no extraordinary force. There was only a very long time and a great many ordinary collisions, and it took people like Hutton, Lyell and the experimenters who followed to trust that ordinary things, repeated long enough, could shape a valley.

So next time you pick up a smooth pebble, try a small guess. Where on its journey did it lose its corners, and what else did it knock against along the way? There's a story in that stone, and some of its chapters are probably still unread.