i88 



PHYSICAL GEOLOGY 



Icebergs 



Formation of Icebergs. — On account of the muddiness of the water in front of 

 a glacier which enters the sea (Fig. 177) and also because of the danger from the frag- 

 ments of ice which continually break off from the glacier without warning, it has 

 been impossible to determine definitely how great icebergs are formed. Ice breaking 

 from that portion of the front of a glacier which is above the water produces small 

 bergs, but large ones do not usually have this origin. The two figures (Figs. 178, 



177. — Nunatak Glacier, Alaska, entering the water of a fiord and 

 discharging icebergs. (U. S. Geol. Surv.) 



179) show two theories of the origin of icebergs. The first theory (Fig. 178) holds 

 that near sea level a glacier is cut back by wave action and melting, leaving a pro- 

 jecting ice foot some distance beneath the surface of the water, which gradually 

 thins toward the end. Great icebergs which suddenly appear from the water some 

 distance from a glacier are believed by the adherents of this theory to have come 

 from the ice foot, from which they had been broken by the buoyancy of the water. 

 The second theory (Fig. 179) holds that the upper part of a glacier which enters 

 the sea will project beyond the lower part, both because of the more rapid movement 

 of the top than the bottom and because of the melting of the ice by the water. In 

 proof of this ir is stated that large masses, extending to the very top of the ice 

 front, shear off and sink vertically into the water, disappear for a few seconds, and 

 then reappear, almost to their original height, before they turn over. If the glacier 

 projected under the water to within 300 feet of the surface, it would cause the 

 m;iss to turn over at once. According to this theory most of the small bergs consist 

 of masses broken from the ice precipices; larger ones are formed when a piece shears 

 off .Hid smks into the water; and ice detached under the water may also form bergs. 

 A thud theory (Fig. 180) holds that the front of the glacier is broken off by the 

 l)n'»\ .in, y of the water. 



