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The inland seas had long since disappeared and new mountains had risen on the continent when the great ice sheets of the ice ages advanced from the north to cover northeastern Minnesota, eventually turning this mineral-rich region into the world’s best canoe country. During four major periods of glaciation, which began almost 2 million years ago, the glaciers altered the landscape considerably. Evidence of the last glacial advance and recession (the Wisconsin Glaciation, which occurred from about 100,000 to 10,000 years ago) is everywhere in the Boundary Waters today. Parallel grooves, called striations, are visible on many rock ledges that were scoured by the ice. Glacial debris, from small pebbles to huge boulders, is widespread. Here and there, you will see erratics, large boulders that were carried by the glaciers and left off in new locations when the glaciers melted.

Perhaps the greatest distinction of the border lakes area is the presence of exposed bedrock. This region is unlike the rest of Minnesota, which is almost completely covered by glacial deposits. This domination of exposed bedrock in the Boundary Waters resulted in distinctive patterns of lakes and ridges, which reflect the underlying rock structures. In the eastern third of the region, the lakes form a distinctive linear pattern. Long, narrow lakes give the terrain a notable east-west “grain.” These lakes appear in two major types of rock formations. The lakes on the Duluth Gabbro formation, which is exposed over an area from Duluth north and east to the Canadian border, developed their particular pattern because alternating bands of less resistant rock and more resistant rock are oriented east-west. Erosion removed more of the less resistant rock, creating lake basins. In the area where the Rove Lake formation is exposed—along the international border from Gunflint Lake to Pigeon Point (the very tip of the Arrowhead)—the east-west linear pattern has a different cause. In this area intervening ridges separate the lakes. These ridges are the exposed edges of south-sloping layers of dark igneous rock that was intruded into sedimentary rocks after they were deposited. The north-facing slopes of the ridges are very steep and form escarpments 200–500 feet high. Huge piles of talus blocks cover the lower parts of many escarpments, the result of erosion by the advancing glaciers as they passed transversely over the ridges.