22 AGRICULTURE. 



paratively recent geological times; indeed, they 

 are found to-day, though in 'much less size and 

 number than formerly. 



The causes of the climatic changes which led 

 to the formation and again to the disappearance 

 of the glaciers are unknown. At the time of 

 the great expansion these ice sheets covered 

 nearly all of North America down to 40 north 

 latitude. 



Wherever, in high latititudes or altitudes, 

 more snow falls in winter than melts in summer, 

 glaciers are formed (Fig. 6). These glaciers 

 carry with them (i) upon their surfaces, (2) 

 frozen in their interior, and (3) pushed along 

 in front or beneath them, great quantities 

 of rock of all degrees of coarseness, from the 

 gigantic boulders weighing tons to the finest 

 clay. Rocks over which they pass are striated 

 and polished (Fig. 7), and both these and the 

 materials carried may be ground into clay by 

 the enormous pressure of the slowly moving 

 mass. The drift, or this deposit, is distributed 

 over vast tracts, and is stratified or unstratified. 

 The stratified drift is deposited by the water of 

 glacial streams, while the unstratified is simply 

 dropped by the melting ice. 



At the present time there are great tracts of 

 glacial ice : (a) the Alpine, occupying narrow 

 mountain valleys, as those of the Alps; (6) the 

 Piedmont glacier, like the Malaspina, of Alaska 



