340 Annals of the Carnegte Museum. 



The Andropogon Dune- formation. 



As the cottonwoods of the Populus dune-formation rapidly grow in 

 height, they constantly act as an obstacle to the drifting sand, and the 

 ridge grows higher and higher, soon completely burying any accom- 

 panying Salix. The upward growth of the ridge continues usually as 

 long as the cottonwood has an abundance of lower bushy limbs which 

 serve to catch and retain the drifting sand. With increasing age, 

 however, the lower limbs of the trees begin to die away and the upper 

 growth of the dune ceases — the top of the ridge may even be blown 

 away. Generally with the dying off of the lower limbs of the trees and 

 the cessation of the upward growth of the dune, there appears another 

 dune-plant, Andropogon furcatus, which in a measure takes the place of 

 the lower limbs of the trees in protecting the top of the dune, or ridge, 

 from the action of the wind. Sparingly associated with the Andropogon 

 are usually a few species from the Panicum-Artemisia formation : 



Artemisia canadensis, Euphorbia polygoiiifolia, 



Artemisia can data, Lathyrus viaritimus. 



The Andropogo7i dune-formation, as above described, is typically 

 represented on the ridge commencing near the Key Post and running 

 along the lake-shore nearly to the Light House. About a half-mile 

 east of the Light House the lake is cutting into the shore and in places 

 has carried away part of the ridge, thus exposing the lower buried 

 portion of the tree-trunks to view. Where exposed the trees had been 

 buried from nine to fourteen feet in the dune (see Plate XXXVH), 

 and at the base of the exposure, about one and one-half feet above the 

 water of the lake, the trunks were about two inches in diameter, in- 

 creasing upwards to six to eight inches in diameter at the top of the 

 ridge. The buried portion of the trunks had numerous dead limbs in 

 various stages of decay and scattered among these limbs, extending 

 nearly to the top of the ridge, were many strong roots. As men- 

 tioned in the discussion of the historical development and probable 

 age of the peninsula, one of the undermined cottonwoods which had 

 been cut off to free a telephone wire showed twenty-six rings of annual 

 growth. In places this ridge is over twenty feet high and the cotton- 

 woods are still growing vigorously, the top of the ridge being mainly 

 held in place by the Andropogon. 



