sentences:  The Laurentide Ice Sheet decayed and receded as the Wisconsonian glaciation drew to a close, and Glacial Lake Agassiz formed from its meltwaters.Outlets to the north were blocked by the glacier, and the outlet to the south was dammed by the Big Stone Moraine which had been left by the retreat of the ice sheet.Lake Agassiz filled until it overtopped the moraine about 11,700 years ago.