Page:Popular Science Monthly Volume 49.djvu/184

170 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

 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.