[from the text] At the commencement of the glacial period the topography and the drainage system of what is now Isle Royale were very similar to those of today, except that lakes and swamps were few or absent. This topography was but slightly modified by the invasion of the ice, and the most important change effected by glacial erosion was the excavation of rock basins in the preglacial valleys. Since the retreat of the ice the gradual emergence of the island from the waters of the lake has taken place. In some cases in closed basins appeared above the surface ready made; in others they were produced by wave-built bars thrown across the mouths of harbors or both ends of channels during pauses in the retreat of the waters. By continued emergence some of these rock basins and cut-off bays came to occupy positions far in the interior of the island. The tilting which followed the Lake Nipissing stage must have had some effect upon the island lakes thus formed. It may have brought about the partial draining of some, the enlargement and perhaps even origination of others, and occasional shifting of outlets; in all cases it must have produced a tendency toward migration to the southwest.