How were the Madison lakes formed?
Before the last glaciation, which started about 30,000 years ago, there was a deep river valley that ran under where Lake Mendota and the other Madison lakes are now. If you could imagine being up on one of the hills around Lake Mendota you would have seen a valley that was 400 feet or almost 500 feet deep.
The landscape during the time the glacier advanced had only a very thin soil covering and probably had not been covered by glaciers before. There was this big deep river valley, and the glacier extending out west of Madison to the Cross Plains and Verona area.
The material that the glacier was carrying slowly filled up that valley that was underneath the ice. The glacier only partly filled the valley, and the remaining parts are now occupied by lakes, or wetlands.
When the ice retreated about 18,000 years ago, it left behind the depressions that we now see as the lakes.
When the glacier retreated, a lot of meltwater was coming off the ice. At that time what’s now the Yahara River had a glacial moraine, an accumulation of debris, across it that dammed up the river. This resulted in one big single lake, called Glacial Lake Yahara, that was there for several thousand years.
Lake Yahara was about 10 feet higher than the highest Lake Mendota level.
As the river cut through that glacial debris over the years, the level of the water dropped. And it dropped enough so that it separated out into the lakes Madison has now.