THE QUATERNARY PERIOD 329 



before the British Scientific Association in 1840. For some years 

 the idea was opposed, especially by advocates of the so-called ice- 

 berg theory. Now, however, no important event of earth history 

 is more firmly established and no student of the subject ever 

 questions the fact of the Quaternary Ice age. 



Some of the proofs for the former presence of the great ice 

 sheets are as follows: (1) Polished and striated rock surfaces 

 which are precisely like those produced by existing glaciers, and 

 which could not possibly have been produced by any other 

 agency; (2) glacial boulders or " erratics," which are often some- 

 what rounded and scratched, and which have often been trans- 

 ported many miles from their parent rock ledges; (3) true 

 glacial moraines, especially terminal moraines, like that which 

 extends the full length of Long Island and marks the southern- 

 most limit of the ice sheet there; (4) the generally widespread 

 distribution, over most of the glaciated area, of heterogeneous 

 glacial debris (so-called "drift") both unstratified and stratified, 

 which is clearly transported material and typically rests upon 

 the bed-rock by sharp contact. In regions which have not been 

 glaciated, it is quite the rule to find that the underlying fresh rock 

 grades upward through rotten rock into soil. 



Ice Extent and Centers of Accumulation 



The best known existing ice sheets are those of Greenland and 

 Antarctica, particularly the former, which covers about 500,000 

 square miles. This glacier is so large and deep that only an 

 occasional high rocky mountain projects above its surface, and the 

 ice is known to be slowly moving outward in all directions from the 

 interior to the margins of Greenland. Along the margins, where 

 melting is more rapid, some land is exposed, but often the ice flows 

 out into the ocean, where it breaks off to form large icebergs. 



The accompanying map Fig. 209 shows the area of nearly 

 4,000,000 square miles of North America covered by ice at the time 

 of maximum glaciation, and also the three great centres of accumu- 

 lation and dispersal of the ice. The directions of flow of the ice 

 from these centres have been determined by the study of the direc- 

 tions of a very large number of glacial striae, as well as the direction 

 of transportation of the glacial debris. Greenland was also buried 

 under ice during the Quaternary period. 



