Vertical sorting is the tendency of sediment to separate according to size and weight as it deposits. A cut-section of a vertically sorted deposit shows the largest, heaviest stones on the bottom layer, medium-sized stones in the middle and the lightest sediment on top.
Rivers and streams pick up sediment as they flow downhill. As the stream slows, the energy needed to carry heavy sediment is lost, and the heaviest, most compact stones drop to the stream bed. Lighter particles take less energy to carry, so they remain in solution longer and are deposited later. This gradient sorts deposits in an identifiable way.