Pebbles, sand and clay
Have you ever stood on the bank of a river and thrown stones in the water? You may have started by finding some small stones and watched them go "plip" as they hit the water surface. You may then have found some larger stones to make a bigger splash. Perhaps you searched for a flat stone and held it horizontally in your hand before flicking it towards the river. As the stone struck the water it bounced off and flew a couple of metres then bounced off the water again. It may have bounced and flown several times before it reached the opposite riverbank. The movement of the stone is called skimming and the stone itself is called a skimmer.

When you had finished skimming, you may have searched the gravelly bank for attractive looking pebbles and brought some home in your pocket.

The pebbles you played with by the river or made into a collection are millions of years old. Some may come from rocks that are hundreds of millions of years old. All of them originally come from rock that formed in the Earth's crust. At some time in the past, the rock was exposed at a cliff and large chunks were broken off it by the effect of weather. In time the chunks were washed into a stream and began their journey to the riverbank.

At first the chunks moved slowly. They were only washed along by the water when the streams were full. Through hot, dry summers they stayed in one place but tumbled on again when the streams filled again in winter. As the chunks struck the other rocks, bits chipped off and in time they became smaller with a smooth surface and a rounded shape. The bits that chipped off were ground up smaller to make the sand and grit you find among the pebbles today. If you looked around the riverbank you may have seen soft areas where your feet sink down. These are formed out of the smallest particles from the pebbles - mud and clay.

Why are pebbles different colours?
They are different colours because they come from different kinds of rock. Some pebbles made from granite are pink, black and white. Pebbles made from sandstone are yellow, brown or grey. You may find that some pebbles have lines of different colours in them. They may have formed from a rock called gneiss (say 'nice'). It has bands in it made from layers of different minerals.

Why are some pebbles flat?
Flat pebbles are made from rocks that split easily into sheets. For example, sandstone is a sedimentary rock that splits into sheets and slate is a metamorphic rock that also splits in this way. Granite, like other igneous rocks, does not split into sheets so there is less chance of skimming with a granite pebble than with a pebble of sandstone or slate.

Are all pebbles made out of rock?
Nearly all of them are but occasionally you may find one made out of a different material. This can happen because these materials are chipped away in a river just as rock is. Amber is a dark yellow substance, which was made by coniferous trees millions of years ago. It forms a hard material that can be worn smooth. A piece of brick can be worn into a pebble and so can pieces of glass.

What happens to all the small pieces that are chipped away from the pebbles?
They are carried along by the water. Sand hops along the bottom of the river while clay and mud are carried along in the water. They form a suspension in the water. At times, particles such as sand may settle in the river and break up even more as they are pushed together. Later the new particles are washed away again. Sometimes a fast flowing river can carry a huge number of particles. When this happens, the water looks brown. When the water slows down it dumps the particles it is carrying. They may be left in the middle of the river and form small islands or left on the sides and form a sandy riverbank.

What happens when a river goes round a bend?
The water on the outside of the bead flows faster than the water on the inside of the bend. The faster flowing water wears down the riverbank and riverbed on the outside of the bend. This makes a deep pool. Water moves through this pool carrying particles to the inside of the bend. Here the water is moving too slowly to carry the particles so they fall to the riverbed. In time a sandy riverbank builds up on the inside of the bend.

How are stones in jewellery made so smooth?
The small polished stones that you sometimes see in jewellery are not made in a river. They are made in a machine. Attractive pebbles or rocky fragments are collected and mixed with coarse grit in a drum. The drum is connected to a motor that turns it round and round. The drum is left rotating for about a week. As the drum rotates, the grit rubs on the stones and makes them smoother. The pebbles are then mixed with finer grit and left to rotate for another week. This process may be repeated a few more times. Each time finer grit is used. Eventually the surfaces of the stones become rounded, smooth and shiny.