We call those layers "soil horizons". The top horizon may be an inch (25mm) thick. We call that layer the "O" horizon or sometimes "topsoil". The next layer (horizon) is the "B" horizon. The next lower layer of soil is the "C" horizon. The bottom layer has many rocks and may be called the bedrock or "R" horizon for "rock". Deep down, there is always bedrock. But you may have to dig down a mile (a km) or more. When the soil dries, the soil may shrink and cracks will form in the soil. The soil in the top layer may fall down into the cracks. This causes the layers of soil to be changed because they are mixed. There may be many types soil where you live or only one type of soil. Different rocks make different soil textures. Different weather makes the different soil textures different. And so, there are different soils all over the world.