Take note: making observations in the field

Survey geologist Sarah Bremmer stands on the side of the road with a blaze-orange vest and a notebook. She is conducting fieldwork for a new geologic map and is in the process of thoroughly describing the rocks in an exposed roadcut to understand how different rock layers are distributed in the area.

Geologist Sarah Bremmer posing at rocky outcrop wearing a hi-viz vest and a hat.
WGNHS geologist Sarah Bremmer poses at an outcrop while out in the field, summer 2023.

Sarah is in southwestern WI—Grant County—to map flaky beds of shale within the Glenwood Formation. She hypothesizes that this shale is an aquitard separating two major aquifers in this area. An aquitard impedes or prevents groundwater from moving vertically underground. Previous studies indicate that the shallower aquifer in Grant County is more susceptible to contamination, so Sarah’s study will provide more insight into how well the deeper aquifer is protected from contamination by the shale aquitard. She is out in the field to map the distribution of the shale layer by examining rocks exposed at the surface.

Good fieldwork requires good field notes

Fieldwork is an integral part of geologic mapping, and the most critical aspect of fieldwork is taking good notes. There is no worse feeling for a scientist returning from the field, than being unable to disentangle jumbled or sparse fieldnotes. Detailed field observations are a principal reference when making interpretations for a new geologic map. That notebook Sarah is using on the side of the highway is invaluable, even with modern technology. In fact, the Survey has a digital archive of field notebooks from Wisconsin geologists dating back to 1882 that can still be referenced for current projects!

Scanned images from notebooks with written text and geological sketches that compares notes from 1883 and 2020.
The archived field notes from Wisconsin geologists in the 19th century look very similar to today’s! The top two images are notes taken by W.W. Daniels in August 1883 in northcentral Wisconsin (Lake Superior Legacy Collection notebook #10). The bottom two images are notes taken by Sarah Bremmer in August 2020 in Grant County. Click the image to zoom in.

Most geologists learn to take field notes in their college geology courses. This process begins with detailed descriptions of the rocks: mainly grain size and sorting, and the presence of different minerals, textures, and patterns. Next, fieldnotes will focus on where two rock layers meet, called a contact. Is the contact sharp or gradual, flat-lying or tilted? Are there fractures present, and do they cross-cut the contact? How thick are the rock layers? Often a sketch and photos (annotated in the notebook!) are used to provide context to these notes.

Geologists will use these observations to make an initial identification of a rock unit and to predict what they may find at nearby outcrops. However, it is important to note observations rather than interpretations when in the field, and to focus on collecting as many observations as possible. Interpretations may change later, but observations shouldn’t!

Fieldwork in Grant County requires some digging

Before stepping foot in the field, Sarah always creates a general overview of the project in her field notebook. This often includes a site map and background material. Sarah emphasizes that this is a critical exercise to help keep her project’s main goal in mind. Out in the field, it is easy to get lost for hours in the small details, especially when you are face-to-face with an outcrop!

Rocky outcrop with white lines that trace the contacts between the different rock units.
This outcrop in Grant County reveals the different rock units relevant to Sarah’s project: (bottom to top) the thick sandstones of the St. Peter Formation, the thin shaly Glenwood Formation, and the blocky Platteville Formation. Note the different colors, layers, and patterns of erosion.

For the Grant County project, her goal is to map the details of the shaly Glenwood Formation, which sits between two other rock units: the sandstone of the St. Peter Formation above and the carbonate rocks of the Platteville Formation below. Sarah faces a couple of hurdles in the field. First, because there is substantial forest cover in much of Wisconsin, describing and interpreting these rock units can be a challenge. Second, shale is a softer rock type made up of clay and silt that is rarely exposed, and she will need to search for it and dig around to confirm its presence. However, there are other clues that can help Sarah to identify rocks in the field. For example, geologists commonly associate St. Peter Formation sandstone with pine trees, and finding groundwater springs can indicate that underlying bedrock is acting as an aquitard.

Sarah also notes observations of the landscape that can help her identify the contacts between each of these layers. For instance, when different rock types erode in different ways, they can create recognizable features in the land surface. Sarah can look for an escarpment (a rock cliff) that often indicates the St. Peter Formation sandstone and/or an abundance of loose carbonate rocks as evidence of the Platteville Formation carbonates. The Glenwood Formation, in contrast, hides very well and often the only evidence at the surface is a few chips of green shale among the rubble and plants. But when Sarah can find those loose chips, she knows where to dig! Sarah goes through this process at several locations over a field season, and pieces together the field notes from each outcrop back at the office as she begins to draw her map.

Back at the office, Sarah digitizes and compiles her notes so she can quickly access and analyze them. Here, she can flag areas of interest and add detail to rock unit descriptions based on her fieldnotes. Her primary observations from the field can quite literally “ground truth” her interpretations and identify areas of research that may need more attention.

Close up photo of the contact between two rock units. A yellow fieldnotes book rests against the rock for scale.
This rock outcrop shows the contact (white line) between the green, thinly bedded shale of the Glenwood Formation (bottom) and the tan, chunky carbonate of the Platteville Formation (top) near Lancaster, WI. Field notebook for scale.

We’re going to need a bigger notebook…

All this sleuthing and digging is a lot of work and time consuming, but Sarah has had good luck finding the hidden Glenwood Formation – and has one full notebook so far! The next part of the project will focus on mapping a carbonate rock unit above the Glenwood Formation, the Pecatonica Member of the Platteville Formation, which she thinks is also acting as an aquitard in the area. This means more field work and detailed notes.

These notes will contain rock descriptions, contact locations, landscape sketches, sunscreen smears, some loose silt grains, and a lot of hard work. While every project can look a little different, getting out to observe the rocks and taking detailed notes is a critical part of mapping Wisconsin geology.

Further reading and resources

Learn more about this project in Grant County here

Learn more about the Wisconsin aquifers here

Peruse the Lake Superior Legacy Collection, including the archived field notebooks here

This blog post was written by UW-Madison undergraduate Erica Kallas, who works at WGNHS as a Social Media and Geology Assistant and edited by WGNHS Communications Specialist Mel Reusche