GLACIERS. 667 



3. FREEZING AND FROZEN WATER. 



Water performs part of its geological work in the act of freezing, 

 and another part when frozen, in the condition of snow and ice. 



1. WATER FREEZING. 



Rending and disintegration from expansion. — As water in freezing ex- 

 pands on reaching 39j° F., the freezing-process in the seams of 

 rocks opens those seams, tears rocks asunder, and tumbles fragments 

 and masses down precipices ; or in porous strata it crumbles off 

 the surface, and causes disintegration. Consequently, bluffs in a 

 cold climate, like the trap hills of Connecticut and the highlands 

 of the Kudson, have a long talus of broken stone made mainly by 

 this means, — while in a tropical climate the precipices are generally 

 free from fragments. This cause of degradation goes on incessantly 

 in all icy regions where there are melting and freezing, and may have 

 originated much of the soil and drift of the globe. 



2. ICE OF RIVERS AND LAKES. 



Ice forming along streams in which there are stones envelops 

 the stones in shallow water, even to a depth of two or three feet, or 

 more in the colder climates. Other stones and earth fall on the ice 

 from the banks. When the floods of spring raise the stream and 

 break up the ice, both ice, and stones, often float down stream with 

 the current, or are drifted up the banks high above their former 

 level, or are spread over the river-flats. 



Ice sometimes forms about stones in the bottom of rivers when 

 the rest of the water is not frozen, and is then called anchor-ice. In 

 this condition, it may serve as a float to raise the stones and to 

 transport them with the aid of the current. 



The same modes of transportation are exemplified in lakes as in 

 rivers, except that there is less current, and the stones are mostly 

 set back up the shore. Large accumulations of stray stones far 

 above the ordinary level of the lake are in some places thus made. 



3. GLACIERS. 



1. General features, formation, and movement of Glaciers. 



1. Nature of Glaciers. — Glaciers are accumulations of ice descend- 

 ing by gravity along valleys from snow-covered elevations. They 

 are ice-streams, 200 to 5000 feet deep or more, fed by the snows and 



