A vertical section through different layers of the soil is called the soil profile. Each layer differs in feel (texture), colour, depth and chemical composition. These layers are referred to as horizons. The uppermost horizon is rich in humus and is called the topsoil or the A-horizon. The next layer has a lesser amount of humus but more of minerals called the B-horizon. The third layer is the C-horizon, which is made up of small lumps of rocks with cracks and crevices.