GROUND-ICE, ice formed at the bottom of streams while the temperature of the water is above freezing-point. Every thing points to radiation as the prime cause of the formation of ground-ice. It is formed only under a clear sky, never in cloudy weather ; it is most readily formed on dark rocks, and never under any covering such as a bridge, and rarely under surface ice. On a cold clear night the radiation from the bottom is exces sive, and loosely-grown spongy masses of anchor-ice form on the bottom. On the next bright sunny day heat from the sun may detach them and they will rise to the surface with considerable force. Probably a thin film of stationary water rests upon boul ders and sand over which a stream flows, and this, becoming frozen owing to radiation, forms the foundation for the anchor ice and produces a surface upon which the descending frazil-ice (see below) can lodge. The ice which rises to the surface has a spongy texture and frequently carries gravel with it. This "anchor-ice," as it was called by Canadian trappers, frequently forms darns across narrow portions of rivers where the floating masses are caught.
"Frazil-ice" is a Canadian term from the French for "f orge cinders." It is surface ice formed in spicules and carried down wards in water agitated by winds or rapids. The frazil-ice may render swiftly moving water turbid with ice crystals. (See H. T. Barnes, Ice Formation with special Reference to Anchor-ice and Frazil-ice, 1906.)