80 ° Rhodora [May 
THE ATLANTIC COASTAL PLAIN ELEMENT 
IN THE FLORA OF THE GREAT LAKES. 
DoNnALD Curnoss PEATTIE. 
(Continued from page 70.) 
TOPOGRAPHY OF THE GREAT LAKES SHORES AT THE CLOSE 
/ OF THE GLACIATION. 
But were there really any warm shallow lagoons and bays? "There 
seem to have been, for a countless number of shallow bays and old 
sand spits enclosing lagoons have been traced by many geologists 
and shown to have existed extensively during the Algonquin stage. 
It is significant that the stations for coastal plain isolations along 
the Great Lakes occur precisely where some of the spits and lagoons 
still exist, as at Presque Isle and Cedar Point on Lake Erie, and around 
Pine and Dune Park at the head of Lake Michigan where a long era of 
spit-forming and lagoon-forming is just drawing to a close. 
TOPOGRAPHY AND FLORA OF THE Post-TOLLESTON BEACHES. 
The Post-Tolleston is that period of transition between the glacial 
Lake Chicago and modern Lake Michigan and is synchronous with 
the Algonquin stage over the Great Lakes as a whole, particularly 
with the latter phases of the Algonquin. During this period the strand 
line of Lake Chicago receded in places about six miles and this it did 
slowly, building up barrier beaches and forming lagoons behind them, 
and then receding a little more, to build up another bar and lagoon. 
The lagoons are, many of them, of recent date and their beaches are 
still intact. 
The result is a topography which is scenically monotonous, being a 
series of ridges and hollows which are about one hundred meters wide, 
the hollows never more than three or four meters above the level of 
the present lake, and the ridges scarce two meters above the hollows. 
There are by actual count 32 such ridges in the four miles between the 
village of Gibson and the lake shore’. 
The hollows nearest to the present shore are still what might prop- 
erly be called lagoons; they are occupied by swamp and pond types 
among which are coastal plain plants, while on the sandy ridges near 
the shore are many strand plants of the Atlantic coast. But as we 
1 Leverett and Taylor, loc. cit. 357. 
