CHICAGO AND VICINITT. 57 



present wave action, and is tenanted by biennials and perennials in 

 addition to the annuals. Fig. 33 shows a beach of this type, the lower 

 beach being smooth and even, the middle beach covered with debris, 

 while the upper beach has a scattered perennial vegetation. 



The beach at the base of cliffs shows similar subdivisions, though 

 the zones are much narrower as a rule. The vegetation, too, is much 

 the same, though some forms, as Strophostyles, have not been seen as 

 yet on the beaches of the dune district. At the foot of cliffs there often 

 occur alluvial fans of sand, which have been deposited by the torrents 

 during and following rain storms. These fans have a comparatively 

 rich vegetation, and species sometimes occur here that are not found 

 elsewhere on the beach. 



B. The embryonic or stationary beach dunes. — Wherever plants occur 

 on a beach that is swept by sand-laden winds, deposition of sand must 

 take place, since the plants offer obstacles to the progress of the wind. 

 If these plants are extreme xerophytes and are able to endure covering or 

 uncovering without injury, they may cause the formation of beach dunes. 

 Among the dune-forming plants of this type are Ammophila arundinacea 

 (Sand reed), Salix glaucophylla and 5. adenophylla (Glaucous and glan- 

 dular willows), Frunus pumila (Sand cherry), and Fopulus monilifera 

 (Cottonwood). The shapes of these beach dunes vary with the charac- 

 teristics of these dune-forming plants. Ammophila dunes are exten- 

 sive but low, because of strong horizontal rhizome propagation. Prunus 

 and Populus dunes are smaller but higher, because of the relative lack 

 of horizontal propagation and the presence of great vertical growth 

 capacity. Dunes are formed more slowly in protected places, and here 

 the dune-forming species may be plants that are ill adapted to the 

 severest beach conditions, such as the creeping juniper. A beach dune 

 of the type just described is shown w^ fig. 34. 



C. The active or wandering dunes. The dune complex. — The sta- 

 tionary embryonic dunes on the beach begin to wander as soon as the 

 conditions become too severe for the dune-forming plants. The first 

 result of this change is seen in the reshaping of the dune to correspond 

 with the contour of a purely wind-made form. The rapidity of this 

 process is largely determined by the success or failure of the dune- 

 formers as dune-holders. The best dune-holders are Calamagrostis, 

 Ammophila, and Prunus. 



There are all gradations between a simple moving dune and a mov- 

 ing landscape; the latter may be called a dune-complex. The complex 



