1922] Peattie,—Coastal Plain Element in Flora of Great Lakes 67 
In this manner the Great Lakes began to form, and they rose 
until a low point was found in the barriers to the south or until the 
receding ice uncovered outlets to the north. First one outlet and 
then another functioned, as the alternate advances and retreats of the 
ice altered the height of the water, and as the land tipped with the 
release of static pressufe exerted by the glacier. As a consequence the 
lakes many times changed their level, shape, and direction of outflow, 
but for us only the last stages are important. Whatever vegetation 
may have followed the first retreats of the glacier, it must have been 
wiped out again at each readvance. But when the ice had receded 
for the last time, it left ia undisturbed possession of the soil some sort 
of vegetation of which we may expect to find traces to the present day. 
Figure 3 is a map of glacial Lakes Chicago and Warren and the 
connectives, consisting of the Grand River across Michigan and the 
Finger Lakes and Mohawk-Hudson system of connection with the 
ocean. The continuous character of what are now Lakes Michigan 
and Erie is plainly shown and some suggestion of their connection 
eastward is indicated. 
In Figure 4 we have another and subsequent stage in the history 
of the Great Lakes. This is the Lakes Duluth, Chicago, and Lundy 
stage. The Grand River connective is considered as probably not 
functioning at this time, but an exceedingly large Mohawk-Hudson 
connective with the ocean is affirmed'. In Figure 5? we have the 
Algonquin stage. The Great Lakes are now at a maximum, all joined 
by wide connectives and opening very distinctly into the Champlain 
Sea. The Mohawk-Hudson connective is functioning, and the ice 
has retreated well away from the lakes. 
MIGRATION OF THE COASTAL PLAIN FLORA. 
The glacier never returned after this. The evidence indicates 
that during the Algonquin stage there undoubtedly took place a 
shore-wise migration of the coastal plain flora. It might be well 
questioned whether the migration did not take place in one of the 
periods preceding. It is hard to fix the exact time and this might 
appear dogmatic, but during the last of the Algonquin stage, at 
least, there was certainly ample opportunity for such a migration, 
and there is evidence that it did then occur. 
1J am aware that some geologists challenge the existence of this connective, but 
the evidence of the plants is conclusively for it, as I shall show. 
2 Figs. 3, 4, 5, are reproduced from Taylor, The Glacial and Post Glacial History 
of the Great Lakes Region, Rep. Smiths. Inst. 1912, 291-327, by the kind permis- 
sion of Mr. Taylor and of the Smithsonian Institution. 
