Drillers to search for rock patterns under Jefferson County

May 7, 2019

WATERLOO -- What lies beneath the earth’s surface can assist geologists with the knowledge of what was deposited by the glacier during the last ice age.

Understanding the layers below the surface, the space between the soft and hard rocks, allows geologists to construct maps of the thickness of the aquifer.

Taking samples of what lies below the surface is what the Wisconsin Geological and Natural History Survey is doing at the Wisconsin Department of Natural Resources parking lot on state Highway 19, west of Maunesha River crossing.

The crew is working on a mapping project for Dodge County, Pete Chase, a geo-technician at the Wisconsin Geological and Natural History Survey, said. Chase was at the site Wednesday, April 24 with a crew from the Illinois State Geological Survey. The crew had been there for about a week and was planning to wrap up its operations within a few days. The drilling took longer than anticipated due to the hard bedrock. The drill team used two diamond bits to cut through the rocks.

“We are drilling holes to get bedrock samples,” Chase said. “We are looking for a couple of things. Where the sandy soils begin and the hard surface takes over. We want to know the rock types between those surfaces,” he explained.

The Wisconsin Geological and Natural History Survey is mapping the bedrock geology of Dodge County at 1:100,000 scale based on well construction reports, drill core, geologic logs interpreted from drill cuttings, downhole geophysical logs, aeromagnetic anomaly data and sparse outcrops and quarry exposures.

The survey project will soon be expanded into Jefferson County to map the underground, Chase added. The bedrock maps are used as a resource for the aquifer level. The surface is deeper than originally thought, Chase said. The drill has gone down 395 feet to obtain samples.

The location was selected due to its proximity to Michel’s Quarry to the east. It was also selected because it is state land and easier to get approval for drilling, Chase pointed out.

Esther Stewart, a geologist at WGNHS, studies Wisconsin’s oldest rocks, the Precambrian, to understand the ancient history and to keep an eye toward how those rocks affect the most pressing geologic resource, groundwater. The resulting bedrock elevation surface will aid land use decisions and serve as the base for the Dodge County bedrock geologic map. Bedrock topography is low relief in the western and central part of Dodge County. Both bedrock elevation and topographic relief on the bedrock surface increases over the Niagara Escarpment, in eastern Dodge County. Two main pre-glacial drainage systems incised the bedrock surface. One broad, low-relief drainage system begins in northwest Dodge County and extends south into Jefferson and Dane counties.

Mapping of Jefferson County will begin in June.

“The objective is to understand the hard bedrock that is under the glacial deposit,” Stewart said. “The information is used to understand the natural resources in the county and understand ground water resources,” she added.

This is the fourth year of a four-year study, Stewart said. “We will publish a map through the Wisconsin Geological and Natural History Survey and submit maps to the United States Geological Society,” she said. The federal government is funding the project.

“They showed me the core and it is incredible valuable to understand how the rocks change,” Stewart said.

The geologist explained that many years ago, about 520 million years ago, the area was an island in a tropical sea. Bedrock was deposited around the island and the physical properties of the rocks changed as they got closer to the existing topography. There are similar rock types formed around the Baraboo Hills area, she said. That area was also an island surround by a tropical sea and storms caused the boulders to shed off the islands.

“It is interesting to take a closer look,” Stewart said.

Once the maps are reviewed, they will be published through the WGNHS.