SUMMARY 



389 



try, so that it becomes necessary if possible to find a way 

 to check their movement. This has been done in some 

 places by planting certain kinds of grasses capable of 

 growing in the sand and 

 thus protecting the sand 

 particles from the action 

 of the wind. 



Sand dunes are found 



Fig. 117. 



along almost all low sandy coasts, and they render difficult 

 the building and maintenance of roads and railroads to 

 many beach towns. 



Summary. Besides the sculpturing of waves and rivers, 

 two other agents of erosion are glaciers and winds. Alpine 



glaciers are formed by 

 huge masses of ice and 

 snow crowding into 

 mountain valleys where 

 the snow never melts 

 entirely. Glaciers are 

 intersected by great 

 cracks called crevasses 

 and they carry accumu- 

 lations of debris called 

 moraines. Icebergs are 

 the ends of glaciers 

 which have broken off. 



The northern part of America was once covered by a huge 

 glacier at a time which we call the Glacial Period. This 

 glaciation has had a great effect upon the region covered. 

 Glaciers smooth out irregularities in the surface, grind 

 rocks, transport soil and bowlders, dam lakes, force rivers 

 to seek new channels arid on account of this create water- 

 falls. Thus the glaciers of the Glacial Period have had a 

 great influence upon the conditions of life. 



QUARRYING A SAND DUNE TO AJAKE 

 BRICK. 



