It is called a soil deposit.
loess
Trees affect the soil in a good way. They are an important part of erosion control. Tree roots bind the soil, while their leaves break the force of wind and rain on soil. They reduce water runoff and sediment deposit after storms, and conserve rainwater.
nebraska
Silt
In geology, overburden soil is the soil and other material which lies over a rock formation or mineral deposit.
The red comes from high iron deposit in the soil.
Delta
delta
It can flood and deposit silt (fine soil).
It was largely placed there by glacial deposit in the ice age.
A delta.