A recent geological discovery helps UW-Madison geologists refine the story of Wisconsin's last ice age.
'Up until now, there have been no dates on when the last glaciation began in Wisconsin,' said Dave Mickelson, UW-Madison professor emeritus of the department of geology and geophysics. Mickelson was one of three geologists who discovered and dated glacial lake sediments buried on UW-Madison's campus.
The sediments, discovered at the excavation site for the new Microbial Sciences building on Babcock Drive, allowed the scientists to date Wisconsin's last glaciation at approximately 25,000 years ago.
Mickelson and Tom Hooyer of the Wisconsin Geological and Natural History Survey discovered lake sediments buried between two layers of glacial till'sand and gravel deposited directly by a glacier.
'It's like a lake sediment sandwich,' Hooyer explained. 'What that tells us is that the ice was there once, and then it receded just a little bit, allowing a lake to form in front of the ice margin. The ice sheet then re-advanced into that lake, subsequently burying any lake sediment and covering it up with more till. The lake sediment was actually preserved.'
Previously, scientists could only estimate the last time parts of Wisconsin were covered with ice based on radiocarbon dates from central Illinois. In Wisconsin, there was no organic matter, such as trees or leaves, buried and preserved in the glacial sediment.
'Up here, it was so cold. It was tundra, no trees, probably very little vegetation. So there was nothing to get preserved,' Mickelson said.
Without organic matter, scientists had no material for radiocarbon dating. But a relatively new technology, called optically stimulated luminescence, has allowed Mickelson and his colleagues to date tiny quartz crystals inside the lake sediment.
Steve Forman, an expert in the dating technology from the University of Illinois at Chicago, worked on the lake sediments. Measuring the amount of radiation that soil particles gave off to the crystals when the lake sediment was buried under glacial till, Forman determined how long that sediment had been underground.
'It's telling me how long it was buried for,' Forman said. 'Once those quartz crystals are basically buried and shielded from sunlight, they build up a signal. And basically it's a signal from exposure to small amounts of environmental radiation.'
When the giant ice sheet flowed down from Canada almost 30,000 years ago, a section called the Green Bay Lobe reached as far as present-day Janesville. Mickelson said that at its furthest reach, the margin of the glacier passed through areas where Sauk City, Cross Plains, Verona and Brooklyn, Wis. now sit.
After the ice retreated, it formed the lake which left its sediment here on campus.
Between a few hundred to a thousand years later, the glacier re-advanced almost as far as present-day West Towne Mall, and buried the glacial lake in the process. Incidentally, when the glacier later receded for the last time, it left one large lake that encompassed what are now present-day lakes Mendota, Monona, Wingra and Kegonsa.
Hooyer said the new dates derived from the 'lake sediment sandwich' are slightly older than what many local geologists had suspected.
'It doesn't really change the original story,' Hooyer said. 'It helps refine the story.'
