The present Great Lakes began to form with the retreat of the last glaciation –
the Wisconsonian. The final substage of the Wisconsonian glaciation began
retreating some 14,000 years ago and left the basin about 9,500 years ago. As the
glaciers retreated, large lakes were formed at their edge. These “proglacial
lakes” often deposited clays and other fine material, leaving patches of wet,
poorly drained soils in the basin. These lakeplains are now home to unique
biological communities in the basin.