How did rocks and rivers shape the Great Lakes?

The rocks in the Great Lakes Basin are of two main types: metamorphic/igneous and sedimentary. The metamorphic/igneous rocks formed long ago, when molten rock hardened and the heat changed other rocks nearby without melting them. This happened over one billion years ago. The sedimentary rocks in the Great Lakes Basin formed in an ocean that covered most of the basin beginning about 600 million years ago. Use maps skills to relate the hardness of rocks to topography and lake depth, describe the topography of the Great Lakes area, and relate the pre-glacial drainage system to the present size and
position of the Great Lakes.

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