The story begins about three billion years ago. This lesson condenses and describes millions of years of geologic history to help you appreciate how long it took to form the Great Lakes and the origin of its many treasures, including rich mineral deposits.

During the Precambrian Era, which started about 3 billion years ago and ended 600 million years ago, a great deal of volcanic activity occurred. This actions and sedimentary deposits were the source of the rich mineral deposits found in this region. During the Precambrian Era, the only types of life existing on the Earth were algae, fungi, and bacteria

After billions of years of volcanic activity, central North America was flooded several times during the Paleozoic Era., which lasted more than 350,000 million years. The flooding brought different soil materials, such as mud, clay, and sand, as well as various forms of sea life, to the Great Lakes Basin area. During the Paleozoic Era, the first fish, insects, reptiles, conifers, and tree ferns appeared on Earth. The Mesozoic Era followed on the heels of the Paleozoic Era. It lasted over 167 million years and brought dinosaurs, mastodons, birds, mammals, gymnosperms, and flowering plants to the Lakes area. The decomposition and accumulation of plants and animals during this Era added further to the mineral resources of the Great Lakes area.

The Era we are in now, the Cenozoic Era, started 70 million years ago. Eras are divided into epochs, and scientists now believe that the Great Lakes got their start prior to the Pleistocene Epoch, also know as the Ice Age, which occurred in North America about 10,000 years to 15,000 years ago. The region where the Lakes are now originally was carved out before the Ice Age by an ancient river system that emptied into the Hudson Bay or St. Lawrence River Valley.

During the Ice Age, glaciers covered central North America as far south as Kansas and Nebraska, as far east as New York and as far west as the Northern West Coast. In some places, the glaciers were over 6,500 feet thick, almost a mile-and-a-quarter high. Through the sheer weight of the ice, coupled with the varying hardness of the rocks beneath it, the glaciers tore up the river terrain, creating natural dams and dikes that obstructed the drainage of the ancient river system. As the glacier receded from North America, the Great Lakes began to form from the melting receding glacial water, which had enlarged the original river basin. During the Ice Age, modern humans, saber tooth tigers, mammoths, and numerous other animals began to roam the Earth. In addition, the first grasslands, herbaceous plants, and forests developed.

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