The Great LakesGlaciers often form lake basins by eroding out surface areas, leaving depressions in the bedrock and deepening existing valleys. The Great Lakes of North America are the result of a combination of erosion and deposition by continental ice sheets. Glacial erosion widened and deepened broad river valleys covered by the ice sheets. Moraines to the south blocked off the ends of these valleys. As the ice sheets melted, the melt water flowed into the valleys and was held there by the moraines to form lakes.
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