Internal and external forces are geologic processes that shape the Earth’s surface. External forces include weathering and erosion. Erosion is the movement of weathered material by the actions of gravity, wind, water or ice. The Great Lakes are caused by glacial erosion. Slow, moving glaciers change the landforms by causing an erosion. The carved out basins (by the glaciers) and the subsiding crust, expanded the drainage basin, collecting water to form lakes.
