S. Oil formation starts with a bed of rock. Over time, this rock will have physical elements that erode away at it, for example if it has a river flowing over it, the water will erode the rock down (physical). During the winter, the rock will freeze and thaw, creating expansion and contraction and cracking, and the wind may also contribute to erosion too. As small particles are chipped away from the rock, small non-vascular plants like mosses may take advantage of the nooks and cracks and take hold there. As the moss starts to colonize the rock, they form a protective barrier to the weather erosion that is happening. As the plants age and decay, they slowly form an organic layer on top of the rock (biological). This organic layer is a food source for small animals and insects, and so it attracts them to the site. The animals live underneath the organic layer, creating the top soil layer. As the animals live and die here, they add to the organic matter, increasing it in size.
Rain is also slightly acidic and provides another means of breakdown of the rock via a chemical process.
As time goes by, and the topsoil level increases in size, other larger vascular plants can take hold here. Dicot plants with tap roots can take hold, penetrating deep into the soil and breaking up the rock as a biological process.
Over time the rock is broken down into the organic layer, the topsoil, the subsoil, the parent rock and the original bedrock under all the new layers.
The topsoil and the subsoil are different due to a number of reasons. The topsoil is where the most organisms live, where the most nutrients are, therefore where the most root growth takes places, and subsequently where the most aeration is in the soil. It is this level that we usually cultivate in gardening.
Below this layer there is the subsoil. It does not have many organisms living in it, and is therefore non of the above. The particles int he subsoil are very small and the air spaces are very small, suggesting that it would be clay based, and high in water content. Although it is lower in content in most things tham the topsoil, it holds a lot of water which can be useful in times of drought.