It depends on what kind of soil and sand model you’re doing. Whether it’s just soil and sand, or if you’re also incorporating other rocks and elements. But for most models, the answer would be sand.
Sand will remain at the bottom as its made up of rocks.
Explanation:
As sand is apart of the soil and soil is made up f liquids gases and solids and sand are natural that comprises of the 85% of the particles. And soil has decomposed parts of plants and animals and has clay, mud and thus is darker in color and sand is light in color.
