170
POPULAR SCIENCE MONTHLY.
tinent lowered all the waters of the lake region until their levels depended* upon the rims of the lake basins from which the waters overflowed. The upper lakes were the first to sink far within
![](http://upload.wikimedia.org/wikipedia/commons/thumb/b/be/PSM_V49_D184_Map_of_the_early_great_lakes.jpg/500px-PSM_V49_D184_Map_of_the_early_great_lakes.jpg)
Fig. 14.—Map of the Early Lakes. Broken shading represents extension of the early lake epoch; solid shading, a lower stage of Iroquois Gulf before the birth of Lake Ontario; modern lakes, by dotted lines.