Before the last glacier, a somewhat different Wisconsin River drained the north-central part of the state, running around the east end of the Baraboo Hills.

Around 18,000 years ago, the Green Bay lobe of the Laurentide ice sheet crept in from the east, abutting against the Baraboo Hills.

With that outlet closed, the water backed up, filling the basin to the north and west, forming Glacial Lake Wisconsin.

Eventually it found a new outlet, flowing west to the Mississippi via the east fork of the Black River near City Point.

Streams from the glacier to the north and east also carried in sand and silt which settled at the bottom of the lake, roughing in the flat sandy Central Plain that we see today when we follow I-90/94.