The Mammoth Lakes Basin is a large glacial cirque basin that was carved by the grinding movement of glacial ice and rocks entrained within the ice. The lakes in the lower basin, including Lake Mary, Lake George, and Horseshoe Lake, fill depressions in glacial moraine deposits. The lakes in the upper basin, including Duck Lake, Shelton Lakes, and Deer Lakes, fill depressions in the underlying bedrock (metamorphosed sedimentary rocks) that were scooped out by glaciers.