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Look at how different the bottom of Lake Huron is on either side of the Alpena-Amberley Ridge.
Northeast of the ridge, the lake floor is deep and rugged. It includes the deepest part of Lake Huron, reaching about 750 feet below the surface. Southwest of the ridge, the lake is much shallower and smoother.
The difference began before the glaciers arrived. Rivers had already cut valleys into softer layers of sedimentary rock, while harder limestone and dolostone remained higher. The Alpena-Amberley Ridge is one of those resistant bedrock highs.
Glaciers followed those existing low areas and greatly enlarged them. Northeast of the ridge, thick ice became concentrated in the main Lake Huron trough. Pressure and meltwater beneath the ice helped quarry fractured rock, while the moving ice carried it away.
The deeper the trough became, the thicker the ice it could hold, allowing the glaciers to excavate it even farther.
Southwest of the ridge, the Saginaw lobe spread out as a thinner sheet of ice and retreated earlier. It did not excavate its basin nearly as deeply.
Large glacial lakes later covered the Saginaw side and deposited sediment across parts of the shallower landscape, helping produce the broad, smoother lake floor seen today.
Map: NOAA
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