VEGETATION IN THE DUNES 



over them. Thickets and clumps of alders, as 

 well as of aspens and willows, are also com- 

 mon. 



The exceptional trees are a few scattered 

 white pines, small and frayed by the wind, 

 some thrifty red cedars, a couple of hunched 

 up hemlocks which bear no resemblance to the 

 stately forest trees, a few red maples, two 

 elms, dwarfed and stunted, that look large 

 only at a distance, and two small clumps of 

 red birches. The red or river birch is com- 

 mon in Texas, the lower Mississippi region 

 and Florida, and extends along the coastal 

 plain to Long Island. Prom there to Essex 

 County, Massachusetts, is a gap of one hun- 

 dred and fifteen miles where the tree is absent, 

 but it is common in the lower valley of the 

 Merrimac River and in southern New Hamp- 

 shire. Professor M. L. Pernald explains this 

 distribution by the former existence of a great 

 highway for plant migration, a sandy shelf 

 that extended out for miles all along the coast 

 from the southern parts of the United States 

 to Newfoimdland, a shelf which has largely 

 sunk beneath the waters since the close of the 

 glacial period. 



85 



