The mingled sand and water flowing down the bank, the water inclines ever to separate from the sand, and while the latter is detained by its weight and by friction beneath and on the sides, the water flows in a semicylindrical channel which it makes for itself, still carrying much sand with it . When the flowing drop of sand and water in front meets with new resistance, or the impetus of the water is diminished, perhaps by being absorbed, the drop of sand suddenly swells out laterally and dries, while the water, accumulating, pushes out a new sandy drop on one side and forms a new leafy lobe, and by other streams one is piled upon another . . .