A more or less snow-white mass of ice occupying an Alpine valley and moving slowly down its bed like a viscous substance, being fed by semi-melted snow at the top called nevé and forming streams at the bottom; it has been defined by Prof. J. D. Forbes as "a viscous body which is urged down slopes of a certain inclination by the mutual pressure of its parts"; in the Alps alone they number over 1000, have an utmost depth of 1500 ft., and an utmost length of 12 m.