176 NEW VOKK STATE MUSEUM 



of grooves and polished surfaces and the material transported 

 by it. i' i' »>v! ! | ■■'{'■; 



The glacial phenomena are well marked. Ice worked blocks 

 of stone have a peculiar angular form, which does not occur on 

 water worn boulders. 



The theory of continental glaciation was first worked out in 

 Europe from studies of the glaciers of the Alps. These are the 

 result of a copious precipitation of moisture on the mountains 

 in the form of snow and the formation of snow ice. Large masses 

 of this consolidate and form ice rivers or glaciers, which slowly 

 move toward the valleys grooving and polishing the rocks over 

 which they pass and tearing off rock fragments, which in turn are 

 polished and scratched as they are dragged along in the base of 

 the ice. ; . 



Glaciers now exist in Iceland, Greenland and Alaska and in 

 other Arctic countries, also on some of the mountains of Wash- 

 ington, South America, Asia and Africa. They also abound 

 within the Antarctic Circle. 



Evidences of former continental glaciation occur in both hem- 

 ispheres. . 



In New York state the continental glacier extended as far south 

 as Long Island and Staten Island and formed at its front a great 

 ridge of transported rock debris, sand, gravel, boulders and clay, 

 at some points over 360 feet in height, which is called the ' ter- 

 minal moraine ' and is known locally as the back bone of Long 

 Island. ; 



After reaching its point of maximum extension and resting 

 there, perhaps for a long time, the ice sheet with a recurrence of a 

 warmer climate began to retreat. This retreat was not at an 

 even rate. There were periods of arrested motion and probably 

 of temporary advance as shown by the moraines of recession. 

 These are masses of earth, gravel and boulders which form small 

 hills and ridges. ; 



As the ice melted, great volumes of water were poured over 

 the land and the valleys were flooded. The streams thus formed 

 were loaded with sand and gravel which they carried for a dis- 



