The current land mass of Isle Royale is the result of a geologic syncline (a fold in rock layers) that formed the Lake Superior basin. One end of the fold forms Isle Royale, while the other forms the Keweenaw Peninsula. There are two major bedrock formations on the island: the Portage Lake Lava series, covering about 85% of the island, and the Copper Harbor Conglomerates, covering about 15% of the island. These geologic formations were modified by four glacial periods, with the last ice-advance occurring some 12,000 years ago. This ice-advance altered the ridges and valleys, leaving a relatively thin mantle of glacial deposits, which are only a few inches to about 4 feet in depth. The thinnest deposits are usually located near the present day shorelines of Lake Superior. As the glacier retreated from the Lake Superior basin, its subsequent melt waters began to create many distinct lakes. Geology and Soils - Isle Royale National Park (U.S. National Park Service)