1922; Peattie,—Coastal Plain Element in Flora of Great Lakes 63 
THE COASTAL PLAIN EXTENSIONS CONNECTED ONLY 
WITH THE GREAT LAKES. 
We have seen that the coastal plain types are closely associated with 
the Great Lakes and that away from these waters and from the 
lakes or rivers connected with them, there are no stations where any 
considerable number of coastal plain plants occur. They form, in 
various areas along the Great Lakes, asthey do not elsewhere off 
the true coastal strip, a conspicuous part of the vegetation. Of 
course, the Mississippi Valley is an exception. It abounds in coastal 
plain types butitis not a discontinuous sort of extension but rather 
forms an arm of the flora of the basin of the Gulf of Mexico, and as 
such it is noticeable that not many of the coastal plain types which 
characterize the Mississippi Valley are the same as those which are 
found extended along the Great Lakes. The Mohawk Valley and the 
Finger Lakes of New York State might also be taken as exceptions, 
in that they are not outlets or tributaries of the Great Lakes. But 
it will be possible to show that in times not far distant they were pre- 
cisely these. 
GLACIAL History OF THE GREAT LAKES. 
The Wisconsin Ice Sheet destroyed all plant life in the glaciated 
area, so that all the vegetation around the Great Lakes can have come 
there only in post-pleistocene times. The interest for the botanist 
begins during the important period when the glacier was receding 
for the last time and the Great Lakes were in process of formation. 
This period has been thoroughly investigated and described by Lever- 
ett and Taylor in The Pleistocene in Indiana and Michigan and the 
History of the Great Lakes'. This we may summarize as follows: 
The glacier had piled up and left in the course of its various ad- 
vances and recessions a border of moraines fringing the lakes which 
lay to the south. The moraines exist more or less along the whole 
vast extent of the margin, but nowhere are they more marked than 
over the rolling country of Ohio, Michigan, Indiana, Illinois, and 
Wisconsin. As the ice receded the great melting floods poured out 
from its edge, and, meeting with the moraines, were dammed back 
into the pre-glacial river systems to the north which had been deep- 
ened by glacial gouging. 
1 Monographs of the U. S. Geol. Surv. iii. 
