I came across the word alluvial whilst reading Tess of the D’Urbervilles.
Immediately he began to descend from the upland to the fat alluvial soil below, the atmosphere grew heavier; the languid perfume of the summer fruits, the mists, the hay, the flowers, formed therein a vast pool of odour which at this hour seemed to make the animals, the very bees and butterflies drowsy.
So, a quick google search leads me to explore this geological landform.
Firstly, if you were to come across this landform, you would say you have found some Alluvium which is a word that stems from Latin alluere which means to “wash against”.
Basically, alluvial soils are sediment that has been deposited by running water, whether that be a river, stream etc. This soil is often very fertile and is used as grazing pastures for cows and other animals.
How is it formed?
First of all, the topsoil of the earth is slowly weathered and eroded away by the rain and other weather elements.
The rain carries the soil (with its nutrients) to the nearest water source.
The soil is now carried along by the river or stream at high speed.
Deposition occurs when a river or other moving water source runs out of energy and drops its load of material.
The characteristics and type of deposit are reflective of the power of the water source.
The faster the water (i.e the more energy it has) the larger the material deposited. As the velocity of water decreases, very fine particles of silt and sand are deposited. Finer silt and clay particles are only dropped by the river when it is a very slow moving water source, for example, a delta.