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Alluvium
A
Abrasion
The wearing away of rock by moving water that carries small rock particles. The large pieces are slowly worn away, and fine powder is produced.
The effect is much the same
as using sandpaper. Rivers carry fine rock particles (sediment) that abrade the banks and bed of the river channel. At the same time, the rock particles bump together, making them finer and more rounded. This is called attrition.
Alluvial fan
A broad cone of material that builds up in mountainous areas where a fast-flowing river enters a flatter valley. Because the
speed of the water slows as it reaches the valley, the coarser and heavier material is dropped, and
it gradually builds up into a fan- shaped cone. (See also: Alluvium.)
Alluvium
Any material laid down by a river. The smaller pieces are also often called silt or mud. The larger pieces are called pebbles, cobbles, and boulders.
Alluvium is laid down on the bed of a river and also across the surrounding land when a river floods over its banks. The material it is carrying then settles out
and creates a layer. This layer is often silty or muddy, and that is why people use the words “silt”
or “mud” instead of alluvium. However, the alluvium that spreads over a flood plain may also contain sand and even pebbles.
You can see alluvium by looking at the banks of a river. They are made not only of rock, but of alluvium. (See also: Sediment.)
 Alluvium—Running water can carry a wide variety of materials from boulders and pebbles through grit, sand, and silt, to clay. These pictures show silt and clay in the Colorado River. The silt settles out, but the clay remains in suspension.
 Alluvium—Pebbles, grit, and sand.
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