26 



Nature- Study Agriculture 



The 



glaciers; 

 what they 

 did 



W. T. Stilling 

 FIG. 17. Water- worn pebbles. Brick and glass above ; stone and wood below. 



These ice sheets were hundreds of miles in extent, 

 covering a large part of what is now the United States 

 and Canada. Much of the soil of our country owes 

 its formation, or at least its distribution, to these glaciers. 

 They acted like mammoth plows, scooping out deep 

 channels in some places, and piling up great ridges of 

 earth and rock in other places. Their action may be 

 compared to that of a carpenter's gouge, for at the 

 bottom of these moving masses were fastened stones 

 which cut their way through the underlying earth, 

 leaving marks still plainly visible. The geologist Dana 

 estimates that the region south of Hudson Bay, where 

 the ice sheet began to form, must originally have been 

 from three to five thousand feet higher than it is at 



