14 A NATURALIST IN THE GREAT LAKES REGION 



set at regular intervals to serve as fixed standards. Forests are 

 covered, killed, and uncovered. 



When on the old dunes, covered now with woods, a great 

 tree falls and so exposes the loose sand again to the wind, or when 

 the changing contour of the land sets the wind currents against 

 some new and poorly protected spot, a great hole is blown out 

 of the land and the sand is carried inland to be deposited in some 

 new spot as a moving dune. These blow-outs (Fig. 14) are 



FIG. 14. A blow-out in the dunes 



characteristic features, marked with the wreckage of former 

 forests. 



As has been suggested above, these agents that wear away 

 the land are also agencies of land formation. The ocean, whose 

 waves pound the shore debris into sand and still finer mud, 

 carries this material by its undertow and currents out into the 

 quieter portions and drops it offshore in sand bars and mud 

 banks. These deposits accumulate to great depths, hundreds 

 and even thousands of feet in thickness. The rivers bring down 

 their tons of material to add to the accumulation. Thus the 



