Jennings : A Botanical Survey of Presque Isle. 343 



The description of this formation, as given for the forest succeeding 

 the white pine, applies equally well for the formation as it occurs on 

 the ridges, although the ridges attain in places- to a height of nearly 

 thirty-five feet. 



The Ammophila Dune-formation. 



For a distance of about half of a mile west of the Key Post, and 

 again for about the same distance west of the Light House Jetty, there 

 is a more or less broken line o( Ammophi/a-dnwc?, fringing the beach. 

 West of the Key Post the Ammophila now characterizes a weak line 

 of small dunes between the first cottonwood ridge and the beach. 

 Ammophila arenaria is a stiff upright grass growing just back of drift- 

 beaches along much of both coasts of the North Atlantic. The grass 

 propagates itself readily in a horizontal direction by vegetative methods 

 and is able to grow vertically for a number of feet, when continuously 

 buried by accumulating sand. In this manner the grass and the drift- 

 sand reciprocally operate to build up dunes, the grass acting as an 

 obstacle around which the drifting sand accumulates. There is a 

 limit, however, to the ability of the Ammophila to grow vertically 

 with the increasing height of the dune, and at Presque Isle this limit 

 appears to be reached at about fourteen feet above Lake Erie. The 

 effectiveness of the grass as a dune-former is materially impaired, 

 however, before this limit is reached, and the small dunes west of the 

 Key Post are mainly not more than four or five feet above the sur- 

 rounding sand-plain. Unlike the cottonwood ridge the Ammophila 

 dune always has gentle slopes, because of the radial propagation and 

 the small stature of the Amviophila. 



The structure of this formation is essentially : 



Fades. — 



Ammophila arenaria. 

 Prifuipal Species. — 



Psilocybe ammophila. 

 Secondary Species. — 



Cakile edentula, Euphorbia polygonifolia, 



Artemisia canadensis, Lathyrus maritima, 



Artemisia caudata, Andropogon furcatus, 



Panicum virgatum. 



During the most vigorous growth of the Ammophila dune there are 

 very rarely any plants present, except the facies and the principal 



