Some ingredients are organic, i.e., bits of once living material such as leaves, grass clippings etc., that are now decomposing. The other ingredients are inorganic, i.e., bits of stone that have been ground down into small particles by water and wind. Some water problems in the garden involve the organic materials.
Organic material holds water in the soil.
As long as it stays dry, it can and will be carried off the surface by wind. The inorganic materials sand, silt and clay remain behind. Soil particles vary in their ability to allow water into the soil, to a great extent due to particle size. Over long periods of time, years and years, these inorganic materials fractionate, i.e., separate into layers. The large particles, such as sand drop, the smaller ones, clay and organic material float to the top.