THE WORK OF SNOW AND ICE 129 



water, and most of them not more than 100 feet, but they are some- 

 times a mile or more across. In the South Polar regions the bergs 

 are still. larger. 



As icebergs sail away from land, they carry some of the debris 

 which was in the bottom of the glacier. As the floating ice melts, 

 the debris which it carried falls to the bottom. The northern ice- 

 bergs do not appear to carry much debris far. The common notion 

 that the banks of Newfoundland were made by berg deposits prob- 

 ably has no foundation in fact. 



The courses of icebergs are determined partly by winds, and 

 partly by currents in the ocean. Those of the North Atlantic occa- 

 sionally reach the track of ocean commerce. Since they are some- 

 times surrounded by fog, they may be a menace to navigation. 



Ancient Glaciers and Ice-sheets 



There have been times in the earth's history when glaciers were 

 much more extensive than now. The latest of these periods is 

 known as the glacial period, when mountain glaciers were more 

 numerous and larger than now. In our own country, glaciers 

 existed even in the mountains of New Mexico, Arizona, and Nevada. 

 The amount of ice in the glaciers of Utah or Colorado was then far 

 greater than all that now exists in the United States south of Alaska. 

 At the same time, a great area east of the Cordilleran mountain 

 system, some 4,000,000 square miles in extent (Fig. 136), and lying 

 partly in Canada and partly in the United States, was covered with 

 an ice-sheet, or continental glacier. 



The ice-sheet of North America seems to have originated in two 

 principal centers, one on either side of Hudson Bay. The beginning 

 of each was doubtless a great snow-field. At first these snow-and- 

 ice fields grew by the addition of snow, and later by the spread of 

 the ice to which the snow gave rise. The two ice-sheets finally 

 became one by growing together. This great continental glacier 

 did not originate in mountains, but on high plains. 



When it was largest, this ice-sheet covered all of New England, 

 the northern parts of New Jersey and Pennsylvania, and much of 

 Ohio and Indiana. Its edge crossed the Ohio River where Cin- 

 cinnati now stands, and advanced a few miles into Kentucky. 



