115 



The destruction of the rehc dunes takes place thru the 

 same agencies that were instrumental in their formation. The 

 wave action during violent storms is one of the most potent 

 agencies of destruction, both because of its mechanical force 

 and the ready movement of sand grains when submerged. The 

 wind in general acts as a desiccating agent, but obtains direct 

 action where the sides of the dunes are unprotected. As the 

 surface dries, the outside grains no longer stick to the moister 

 ones within, but either fall to the base of the dune because of 

 gravity, or are blown away by the wind. This method of des- 

 truction is very slow because the dunes are abundantly sup- 

 plied with water by capillary attraction from the water table, 

 by spray blown in fromi the lake, by the quite frequent rains 

 of this region, and by dew which is often deposited upon them 

 during the nights when the sand cools down much faster than 

 the surrounding air. 



A more violent, though rather infrequent agency, is the dis- 

 ruptive power of freezing water. In instances of the action of 

 this agency in 1909, the dunes were thoroughly soaked by a heavy 

 rain, which was followed immediately by a drop in tempera- 

 ture from 0.5° to 12°C, the result of which was the cracking 

 of the dunes. The broken pieces were like rocks on ac- 

 count of the ice, but as soon as the ice at the surface evaporat- 

 ed, the wind scattered the loosened sand grains over the surface 

 of the beach. Several of the smaller relic dunes were thus 

 disintegrated during November 1909. The larger dunes merely 

 suffered the removal of 10-20 cm. of sand from around the 

 edge of the crown. The vegetation of the rim slipped down 

 and now serves to protect the dune during the winter. 



In the part of the area which has been under consideration, 

 the lake is advancing upon the shore rather rapidly, so rapidly 

 that the Juucus, even with its relatively rapid means of vege- 

 tative propagation, has been unable to retreat. It merely holds 

 the ground upon which it had formed a zone parallel to the shore 

 line. Consequently, as soon as the Juncus relic dunes are des- 

 troyed, the Juncus association will become non-existent in this 

 particular area. The Potentilla anserina association, on the 



