MOVEMENTS OV THE LAKE WATERS 489 



dune from the land side. On the lake side some distance below the crest 

 the dry sand had all been blown off and the edges of moist layers pro- 

 jected. This dune is very old, as is evidenced by old tree trunks, now 

 much carved by blowing sand, which occur on its back slope. It is now 

 advancing very slowly inland on an adjacent piece of woodland. 



West of here, toward Port Stanley, other, but usually smaller, dunes 

 occur along the edges of the cliffs. 



Geologic Process on the Shores 

 movements of tbe lake waters 



The several forms of movement. — While the existence of periodic 

 movements of the waters of the Great lakes corresponding to the tides 

 are known, the amount of movement is insig-nificant. On all bodies of 

 water exposed to the action of winds, three distinct types of movement 

 are developed in addition to the tidal movements — the seiche, the cur- 

 rents, and the waves. The current movement may also be developed 

 directly or indirectly by gravity. All movement of the shore materials, 

 on a significantly large scale at least, must be through the action of one 

 or more of these movements, either acting separately or in conjunction. 



The seiche. — The amplitude of the seiche oscillation is directly de- 

 pendent on the wind velocity and persistence in a constant or nearly 

 constant direction. As an agent of transportation when acting directly, 

 it is practically powerless. Acting indirectly through the operation of 

 currents passing through narrow channels, it may possibly be able to 

 shift some very fine materials, such as clay held in suspension. Under 

 very favorable circumstances, in the Mun-ay canal, where a relatively 

 very powerful seiche current is generated, it was not found strong enough 

 to transport for more than a few feet even very fine silt when mechanically 

 stirred up from the bottom. IsTormally the only material moved by it is 

 floating debris. At the east and west ends of the two lakes the periodic 

 raising of the water during storms greatly broadens the portion of the 

 shore belt within the immediate zone of action of the storm waves. 



Currents. — In the report by Mr M. H. Harrington, issued by the 

 United States Weather Bureau,^ the currents of the Great lakes are 

 grouped under four heads : The body currents, a surface current due to 

 the prevailing winds, the return currents, and surf motion. The body 

 currents and the return currents may be regarded as constant. With 

 these should be associated the locally constant currents found at the points 

 of inflow and outflow of the main streams of the lake system — the Detroit, 



3 Currents of the Great lakes. Bulletin B, U. S. Weather Bureau, 1894. 



