The Green Lake basin exists within a river-carved valley that originated prior to the Wisconsin glaciation. It is likely that the movement of the glacier also deepened the valley, as well as smoothed its sides. The valley finally flooded when a recessional moraine about high blocked the exit of water at the west end of the basin during the Cary stage of glacial reduction. The basin is composed mainly of Potsdam sandstone. Beginning just below the water surface is a layer of Madison Sandstone ( thick), followed by dolomite, St. Peter Sandstone ( thick), and Trenton limestone. The shoreline is diverse, ranging from sandy beaches to swamps to steep cliffs where the bedrock is exposed, such as in the image on the left.
