1909] BURNS— BOTANICAL SURVEY OF HURON VALLEY 449 



of Ann Arbor. Its longest diameter is nearly directly east and west. 

 Red contours on the map (fig. 4) show that the bottom of the post- 

 glacial lake was divided into four basins; a rather shallow ridge 

 running north and south divided it into an eastern and western half, 

 and each half is in turn divided into a northern and southern basin. 

 The central ridge was twelve feet (3 . 65 m ) deep at the lowest point, 

 and in the center it came above the surface, forming an island. 



In the eastern half, the northern basin reached a depth of seventy 

 feet (21.33™) and the southern basin thirty-five (io.66 m ) feet. 

 However, a large part of this half of the lake was comparatively shal- 

 low. Over a large area the depth was about ten feet (3™). The 

 other half was also divided into a deep northern basin which reached 

 the depth of seventy-two feet (21 .94™) and a shallower southern basin 

 thirty feet (9. i4 m ). The greater part of this half of the post-glacial 

 lake was deep. At some time in the past a ditch was dug through 

 the bank on the north and the level of the water was lowered several 

 feet. The conflicting stories told by the older inhabitants make it 

 impossible to determine when this occurred. The vegetation, how- 

 ever, shows that it has been a number of years. 



The map of the present distribution of plants at this lake (black 

 parts of fig. 3) shows that the peat deposit is largely confined to the 

 south and east. The principal vegetation of the lake is the bog 

 sedge. Only on the south have the tamaracks and the bog shrubs 

 gotten a foothold, though " islands" of these are rapidly spreading over 

 the bog sedge in many places. The absence of the bog flora around 

 the shore of the island in the center of the lake (the vegetation shown 

 on the map is Scirpus lacitstris), along the northern side of the west 

 half, and on the southeastern corner, is no doubt to be explained in 

 part by the action of the wind, wave, and shoreward push of the ice. 

 However, it must be seen from the map that these places do not stand 

 in a definite relation to the points of the compass. Such vacant 

 places are found where there is shallow water. In just such places the 

 actions referred to above would be most intense. It seems, therefore, 

 that the contour of the bottom rather than the direction of the wind 

 is the controlling factor. The small lake in the northeastern corner 

 is over a very deep basin; the deep western half is for the most part 

 open water. The narrow channels of open water on the eastern 



