1 68 PHYSICAL GEOLOGY 



glaciers. A typical example of such a glacier is the Malaspina in 

 Alaska (Fig. 151). It is formed by the union of several glaciers which 

 move down the valleys of the St. Elias range upon a nearly flat plain. 

 The area of the united glacier is nearly 1500 square miles, about the 

 size of Rhode Island. The lateral margin where the ice is probably 

 1 coo feet thick is covered with a belt of morainic matter a few feet 

 thick and several miles wide, on which grows a luxuriant vegetation. 

 Extensive areas of bushes are found and, near the outer edge, trees 

 some of which reach a diameter of three feet. On the surface of 

 the nearly stagnant glacier are numerous ponds in which stratified 

 deposits are laid down. The central portion of the glacier is com- 

 paratively free from debris and is much broken by crevasses into which 

 streams from the melting ice flow. Piedmont glaciers are rare at the 

 present time, but were much more numerous during glacial times, 

 when they existed at the foot of the Alps, the foot of the mountains 

 of western North America, the southern Andes, and elsewhere. 



Continental Ice Sheets 



Up to this point mountain glaciers have been discussed because, on 

 account of their small size and accessibility, they are more easily 

 studied and their phenomena are better known than are those of the 

 great continental glaciers such as now cover Greenland and the Ant- 

 arctic Continent. At one time ice sheets covered the northern 

 portions of North America and Europe. These were of great extent, 

 those of North America covering an area estimated at 4,000,000 

 square miles ; of long duration, and probably of great thickness. 

 The stratified and unstratified drift so conspicuous in many of these 

 once glaciated regions was formerly believed to have been transported 

 to its present position and the underlying rock scratched and polished, 

 by a great flood (Mosaic flood) which swept down from the north, 

 carrying with it pebbles and bowlders which striated and grooved the 

 rocks over which they were borne. The term drift is a relic of this 

 ancient theory. One cannot obtain a clear conception of the con- 

 ditions which existed in North America alid Europe during the 

 Glacial Period (p. 645) without a study of the existing continental 

 glaciers of the polar regions. 



Greenland. — Greenland is a continent 1400 miles long and 900 

 miles wide. Of this area, fully three quarters are covered at all times 

 with ice, the only inhabited portion of the continent being a narrow 



