Natural forces formed the Great Lakes; the lakes continue to shape the features of their watershed.
- Ancient igneous and metamorphic rocks form portions of the upper Great Lakes basin. Other rocks underlying the present day Great Lakes and surrounding watershed are sedimentary, originating during a time when shallow tropical seas covered the basin. Many of the rocks now exposed on land were deposited and shaped during the advance and retreat of glaciers.
- During the Ice Age, mile-thick sheets of ice covered the Great Lakes region multiple times depressing the crust with their weight. Ancient beach ridges mark previous lake shorelines. Since glaciers retreated (about 10,000 years ago), Earth’s crust has been adjusting upward in a process of isostatic rebound that continues today.
- Lake level changes influence the physical features of the Great Lakes coast. Lake water levels show changes and patterns that vary over periods of hours to millennia.
- Erosion – the wearing away of rock, soil and other earth materials – occurs in coastal areas as wind, waves, and currents in rivers and the Great Lakes move sediments.
- Sediments are a product of erosion and consist of fragments of animals, plants, rocks and minerals. Sediments are classified by grain sizes, from silt and clay to sand, cobbles and boulders. Sediments are seasonally redistributed by waves and coastal currents and help maintain beaches and coastal wetlands.