SAND DUNES AND SALT MAKSHES 



northern side of the dune. The exposed wood 

 is decayed and soft, but masses of hard 

 pitch can be found here and there on the 

 bark, so thoroughly infiltrated with sand 

 that they look like sandstone or pieces of 

 coral. 



The rate at which the dunes advance varies 

 greatly, but it depends chiefly on the season 

 of the year. One of the fastest dunes is un- 

 doubtedly the large one just mentioned that 

 is breaking over the birch grove, for here at 

 the southern end of the dunes the sand is 

 exposed to the full sweep of the north winds, 

 and the region is widely destitute of grass or 

 bushes. By means of marked trees I have 

 been able to obtain exact measurements of 

 the progress of the dune from time to time, 

 for the edge of the sand as it advances into 

 the grove is sharply defined. 



In the five winter months, from December 

 5, 1909, to May 15, 1910, the dune advanced 

 871/2 inches, or about 17 inches a month, while 

 in a little over five summer months, from May 

 15 to October 23, 1910, it advanced only 61 

 inches, or about 12 inches a month. The next 

 winter was a favorable one for dime move- 

 ment, for in the four and a third months, from 



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