5° 



PHYSICAL GEOLOGY 



which were covered by sand dunes have been uncovered as the dunes 

 moved on. There are hundreds, perhaps thousands, of square miles 

 of buried towns and cities in Central Asia. 



One of the difficulties in connection with the maintenance of the 

 Suez Canal is the sand which is constantly being blown into it from 



Dunes held by mesquite bushes. (Photo. D. T. MacDougal.) 



the desert, necessitating frequent dredging. Many communities 

 in the past which depended for their existence upon irrigation have 

 been obliged to abandon their homes, because an unstable government 

 failed to keep the irrigation canals free from drifting sand. 



The drifting of sand has often affected the drainage of the land ; 



Fig. 29. — A sand dune covering a cabin. The origin of the sand is the seabeach. 



(Bermuda Islands.) 



the Grand Calumet River (Fig. 30) formerly emptied into Lake 

 Michigan in Indiana, but its mouth became so filled with drifting 

 sand that the course of the stream was reversed and it now empties 

 into the lake at Chicago, twenty-four miles distant. Large lakes 

 have been formed in consequence of the damming of rivers by dunes, 

 where they emptied into the sea. One such in France, Lake Cazaux, 

 has a width of nearly seven miles and a depth of 130 feet. 



I he ripples which mark the surfaces of sand dunes shift their posi- 

 tions gradually and, in general, are affected as are the dunes. 



1 he movement of sand dunes can sometimes be prevented by plant- 



