Glacial Lake Wisconsin
During the last part of the Wisconsin Glaciation, central Wisconsin was occupied by a proglacial lake called Lake Wisconsin. Lake Wisconsin formed when the Green Bay Lobe of the Laurentide Ice Sheet reached the Baraboo Hills in south-central Wisconsin, damming the lowland to the north, which is now occupied by the south-flowing Wiscon-sin River. At its greatest extent the lake was 115 km long, and its greatest depth was 50 m.
Lake Wisconsin occupied a number of separate but interconnected topographic basins, including a main basin and seven smaller basins to the south of the main basin. During most of its history, Lake Wisconsin drained to the northwest through one of several outlets, all of which emptied into the Black River, a tributary of the Mississippi River. Sometime before the maximum expansion of the Green Bay Lobe during the Wisconsin Glaciation, Lake Wisconsin emptied southward through the Devils Lake outlet to the Wisconsin River, a tributary of the Mississippi River. At the end of Lake Wisconsin’s existence, it drained to the southeast out the Alloa outlet, to the Wisconsin River. The southern basins drained, at various times, through various combinations of a dozen separate outlets.
Wind, slope wash, soil creep, cryoturbation, and solifluction have destroyed most of the beach ridges and terraces of Lake Wisconsin. Based on elevations of the few remaining beaches plus the elevations of the lake outlets, ice-rafted erratics, offshore deposits, the break in slope at the west edge of the outwash plain on the east side of the main basin, and shore-ice collapse trenches, we have been able to reconstruct three shorelines; from oldest to youngest, these are: the Wyeville shoreline, which has been tilted S45°W at 0.1 m/km; the Johnstown shoreline, tilted S60°W at 0.6 m/km; and the Elderon shoreline, tilted S55°W at 0.3 m/km. They intersect each other along a northwest-southeast line through the northwestern outlet. The shorelines are tilted because the Earth’s crust had been depressed under the weight of the glacier but later rebounded with the removal of the weight.
The Wyeville shoreline probably formed about 19,000 years ago when Lake Wis-consin first came into existence as the glacier expanded into central Wisconsin during the last part of the Wisconsin Glaciation. The Johnstown shoreline formed about 15,000 years ago when the glacier was at its maximum extent and the Earth’s crust was at its maximum depression. The Elderon shoreline formed about 14,000 years ago, just before the lake finally drained, after the crust had begun to rebound as the glacier wasted back near the end of the Wisconsin Glaciation.
Tens of meters of offshore sand was deposited in the deep basins of glacial Lake Wisconsin. Much of this sand is included in the Big Flats Formation. The sand was deposited in part by density currents derived from glacial meltwater rivers. In most areas, one or more beds of offshore silt occur within the sand sequence; the uppermost one (the New Rome Member) was probably deposited somewhat before the Johnstown Phase of glaciation.
The final drainage of Lake Wisconsin was probably catastrophic. As the Green Bay Lobe wasted from the east end of the Baraboo Hills, the narrow ice dam was breached by water from the lake, causing the water level in the Lewiston basin to drop tens of meters, probably within a few days. This caused water from the main basin and other southern basins to flood into the Lewiston basin through a breach in the Johnstown moraine; this flood at least partly cut the sandstone gorges at the Wisconsin Dells.X