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Mapping Wisconsin’s oldest rocks to better understand our groundwater today

Photo of Esther Stewart

Esther Stewart, WGNHS geologist

Esther Stewart, a geologist at the Wisconsin Geological and Natural History Survey (WGNHS), studies Wisconsin’s oldest rocks – the Precambrian – to understand our ancient history and with an eye toward how those rocks affect our most pressing geologic resource – groundwater.

Interestingly, her study of these very old rocks has been aided by finding some very old records in the basement of the Wisconsin Geological and Natural History Survey. For the past 70 years, historic drill hole records from the eastern Baraboo Hills were thought to have been lost; 47 of these were recently rediscovered in the records preserved by the Survey. The records were collected in the early 1900s for iron exploration of the rock layers called the Freedom Formation. The iron in that formation was once mined in the Baraboo region.

Stewart has prepared a new Precambrian bedrock map of the Baraboo Hills. She is also constructing a fresh understanding of the rocks beneath the surface by integrating lots of data – old and new.  She includes traditional surface mapping, a USGS aeromagnetic anomaly map, 34 drill cores, 226 historic drill hole records, 10 drill cuttings sets, and 89 well construction reports. Stewart explains that this Baraboo Hills mapping effort is important for understanding the geology that underlies much of south-central Wisconsin: “We can apply what we learn in the Baraboo area to areas with similar Precambrian geology that are more difficult to study because the Precambrian rocks are buried under many feet of overlying rocks and sediments.”

The top of the Precambrian rocks being mapped by Stewart is also the bottom of the Cambrian sandstone – the principal aquifer for municipal and industrial water supplies in much of Wisconsin. Understanding that surface allows Stewart to construct maps of the thickness of the aquifer. She is also finding structures in the bottom of the Cambrian sandstone with preferential mineralization that may affect the quality of the water in the aquifer.

For more information about Stewart’s research projects see her Story Map.

Published: June 14, 2017
Updated: February 12, 2019