267 



might almost be defined as that portion of the beach which is devoid 

 of vegetation." The lower beach of the Beach area physiographic- 

 ally speaking, exists in two modifications, one consisting of a very 

 gradual slope, which may be concave, and the other of a relatively 

 steep slope. Beaches of the first type are but very little elevated 

 above the average level of Lake Michigan. The sancl is damp, either 

 to the very surface or, at least, to within one or two millimeters of 

 it. Just at the edge of the lake is a little ridge which permits water 

 to be retained beyond it. This water forms what is termed a beach 

 pool. Being almost at the level of the lake, drainage back into the 

 lake is very slow. In rainy seasons or at times of frequent north to 

 southeast winds the beach pools may remain for a long time. During 

 the ordinary growing season the sand is never sufiiciently dry to be 

 blown about in the wind. In beaches of the second type, the slope 

 is much greater and the water from each wave drains away very 

 rapidly. As a result, two to three centimeters of dry sand form the 

 surface. This sand is, of course, easily blown about in the wind. 



Neither of these two types of the lower beach bears vegetation 

 of a pennanent nature. In beaches of the first type, the one-celled, 

 motil alga, Chlainydomonas, together with Oscillatoria, may occur in 

 such numbers as to cause the wet sand to appear green. This con- 

 stitutes the Chlamyd ornonas association. These algae occur also in the 

 waters of the lake, but their optimum habitat seems to be the beach 

 pools which occur near the outlets of sewers or near the mouths of 

 creeks bearing sewage. (See Plate XLIV, Fig. 2.) The sand 

 around the pool is mushy and rather greenish in color. The ridgelet 

 between the beach pool and the lake is very low (10 cm. at most) 

 and very narrow. Every north to southeast wind will cause the 

 waves to run over the ridge and flood the pool with sewage-laden 

 water from the nearby sewer. This constant flooding, together with 

 the rather frequent rains, resulted in a permanent pool during the 

 season of 1909. Small snails appeared, and upon them as well as 

 upon other living forms the sanderlings shown in the figure are 

 feeding. 



Aside from the algae, vegetation upon the lower beach is purely 

 accidental. One such case is that of a large willow log which was 

 broken in three pieces and washed up to the edge of the lower beach 

 by the tidal wave of April 29, 1909. The original source of this log 

 is not known, for nowhere in the Beach area are there willows of 

 such size. The logs lie just within the reach of every ordinary wave. 

 Succeeding storms have partially covered the logs with sand, which 

 is constantly kept moist by the waves. From the logs themselves, 



