i2 4 WORK OF SNOW AND ICE 



stream, and in some cases accumulate in vast "jams" behind 

 obstructions in the river. Where a jam forms above a bridge, the 

 bridge may be swept away. Some jams occasion disastrous floods 

 above their sites, and when they break, the waters accumulated 

 above may sweep down the valleys with destructive violence. 

 Poleward-flowing rivers are especially subject to such floods. The 

 snows of their upper basins melt while the lower parts of the streams 

 are still frozen over. The free discharge of the upper waters is 

 thus prevented for a time, and freshets follow. 



On the sea. In high latitudes, ice is formed along the sea- 

 shore. Unlike fresh water, sea-water condenses until it freezes, 

 at a temperature of 26 to 28 Fahr., the variation being due to 

 the amount of salt in the water. In polar regions the sea ice attains 

 a depth of eight or ten feet at least. Floating ice of much greater 

 thickness is sometimes seen, but it is doubtful if it represents ice 

 formed by the freezing of undisturbed sea-water. The geologic 

 importance of ice formed on the sea is slight. 



Snow-fields. Over the larger part of the land, the snow of winter 

 does not endure through the summer, and when it melts, the water 

 follows the same course as rain; but in cold regions where the fall 

 of snow is heavy, some of it remains unmelted from year to year, 

 and constitutes perennial snow-fields. High mountains and the 

 lands of high latitudes are the common habitats of snow-fields. In 

 North America there are numerous small snow-fields in the western 

 mountains, from Mexico to Alaska, their number and size increas- 

 ing to the north. In the United States there are few snow-fields 

 south of the parallel of 36 30', and most of the many hundreds 

 north of that latitude (excluding Alaska) are small. Snow-fields 

 comparable to those of the northwestern part of the United States 

 and British Columbia occur in the higher mountains of Europe and 

 Asia, while in South America there are snow-fields of small size even 

 in equatorial latitudes. Small snow-fields occur on the highest 

 peaks of tropical Africa, and in the mountains of New Zealand. 

 For reasons which will appear later, much of every large snow-field 

 is really ice. 



Besides these fields of snow in mountain regions, there are fields 

 of much greater extent in polar regions. The greater part of Green- 

 land is covered with a single field of ice and snow, the size of which 

 is estimated at 300,000 to 400,000 square miles (Fig. 128), an area 

 400 to 600 times as large as the snow-and-ice-covered area of Swit- 



