248 FIRST YEAR COURSE IN GENERAL SCIENCE 



of water, which is stored and used for city water supply, 

 for irrigation, or for hydraulic mining. 



267. Drift. The rock fragments, sand, and clay which 

 a moving ice sheet carries, are all left on the land beneath 

 when the ice melts. This deposit is called drift. It covers 

 the northern United States as far west as the Rocky Moun- 

 tains and as far south as the Ohio and Missouri rivers. 

 The British Isles, Scandinavia, and parts of Germany are 

 covered with a similar deposit. In river valleys, the drift 

 has been covered by deposits of flood material, but it may 

 still be seen on hillsides and even on high ridges where 

 are scattered loose stones, more or less rounded in shape, 

 varying in size from a pebble to a small house. The large 

 stones are called' boulders. They are often totally unlike any 

 other rock within many miles, but are similar to rocks 

 found farther north. The location of such boulders and 

 the direction of scratches upon rocks over which the 

 glacier passed, have helped to determine the direction in 

 which the ancient North American ice sheet moved 

 in general, toward the south. 



268. Glacial Lakes. If you examine a map of any of 

 the New England or other northern states, you find many 

 small lakes scattered over the country, while in the southern 

 and middle states there are very few. Some of the lakes 

 lie among hills; some are in the plains. The making of 

 lake basins is an important work of a glacier. 



In its progress, the ice with its rock tools scooped out hollows 

 in the softer rocks, and after the ice withdrew, the basin was a 

 receptacle for water from the hillsides around. Such lakes 

 are found in the Highlands of Scotland, in the Swiss and 

 Italian Alps, in Canada, and in some of the northern states. 



The terminal moraine sometimes makes a dam across a 

 valley and thus forms a basin to receive drainage. Lakes 

 made in this way are usually long and narrow. All of the 

 Great Lakes, except Lake Superior, were probably the re- 



