1 9 03] BOG PLANT SOCIETIES 4*7 



in adjacent areas. As we know from the numerous physio- 

 graphic studies that have been made of glacial basins, many of 

 the lakes were formerly much larger than at present. Some of 

 them in early postglacial times had steep banks, which were 

 unfavorable to the development of shore vegetation. But by 

 the lowering of the water level consequent upon the cutting 

 down of the outlet, the shore line at present is a gradually slop- 

 ing one, and supports a "drained swamp" flora. In other cases 

 irregular arms, extending away from the main body of the lake 

 and protected from wind and wave action, doubtless supported a 

 bog vegetation during the tundra dominance. Since then they 

 have been separated from the main lake by a lowering of the 

 water level. Today we find in many such cases the bog vegeta- 

 tion still persisting in the depressions which were formerly arms 

 of the lake, while on the shore of the main body, which came to 

 be swampy at a much later period, the so-called " drained 

 swamp" flora occurs. One of the best examples of this is Turkey 

 Lake, Indiana. Here is an irregular lake several square miles 

 in extent, nearly surrounded by high moraines. At its south- 

 eastern end, through a less elevated portion of the moraine, it 

 formerly connected with several shallow depressions, 22 all of 

 which contain bog plants with varying proportions of swamp 

 plants. But on the now shallow margins of the present lake only 

 the swamp plants are found. At Eagle Lake 23 the same obser- 

 vations hold for a former extension of this lake toward the north- 

 west. But without multiplying examples, the relation of these 

 two groups of swamp societies seems to depend largely upon the 

 time when the swamps came into existence as swamp habitats. 

 If they have existed since the days of tundra conditions they 

 may show a bog flora today. If they are of recent origin, the 

 plants will correspond to the normal swamp plants of the present 

 climatic conditions. If of intermediate age, we may have vari- 

 ous mixtures of the two. Dr. Cowles informs me that the only 

 bog in the sand-dune region near Chicago which contains all 

 these typical bog plants is the one that occurs on the Valpa- 



22 For map and description of lake see Proc. Ind. Acad. Sci. 1895. 

 8 3Map opposite p. 118, Proc. Ind. Acad. Sci. 1901. 



