By Bee Candelaria and Ella Johnson
In an effort to answer our core questions of chronology and use of the site we reached out to Dr. Mary Savina, a retired Geology Professor at Carleton. She, along with the Arb Director Nancy Braker, visited the Tuesday lab during Week 6 and consulted with us about the project. We hoped to gain a better understanding of the kinds of rock quarried at the site and possible uses for the products of the quarry.
After a visit to our site, Dr. Savina wrote “I am confident that the rock unit exposed at this site is part of the Prairie du Chien group, most likely the Shakopee Formation which is the upper part of the Prairie du Chien.”(Savina 2021) The Prairie du Chien group includes Oneota Dolomite (aka Kasota limestone) and Shakopee limestone. Both formations developed in the Early Ordovician period 485 to 470 million years ago (Tweet 1935). This group covers parts of Minnesota, Iowa, and Illinois at varying thicknesses (Tweet 1935). On-campus the thickness is at minimum 170 feet deep on the bald spot and up to 265 feet near the Rec Center (Savina 2021). Oneota stone is less sandy than Shakopee and is used in construction more often, including the construction of Scoville Hall on campus (“Asset Detail” 1982). Shakopee is referenced more often for its use as an aggregate stone used as gravel or filler more than as a construction medium.
After identifying the formation found at the quarry, Dr. Savina addressed the possible use of the space. Dr. Mary Savina informed us that while the formation was not man-made, she did believe that workers exploited the loose rock. Human intervention would be, according to Dr. Savina, shown through sharp edges and fresh rock faces caused by shovels or hammers in the quarrying process, as well as long, flat benches in the rock.
“The overall pattern of the escarpment is natural. That is, the valley walls of the Cannon River have exposed the Shakopee Formation through erosion by the river. However, I think people have quarried in the particular area you are studying. In other words, they have taken advantage of a natural exposure and exploited it as a source of building material or aggregate (loose stone in piles). One piece of evidence for human exploitation is that flat bench cut across the rock just below the cliffy exposures. In most natural exposures I’ve seen, the rock forms cliffs with few or no benches. I didn’t look very closely at either the cliff or the loose rock to look for evidence of hammering, shovels, etc. Stones that have been hammered out of an exposure will have sharper corners and fresher faces than those that erode naturally from a cliff.”
– Dr. Mary Savina
These signs are seen in our quarry sight (shown below).
Both Shakopee and Oneota limestone were quarried in and around Northfield. Shakopee is referenced for its use in foundations and basements, but no specific locations are named. A report from 1935 describes mining in Northfield as ever gaining importance in the region but having abandoned quarries (Thiel 1935). It’s possible that one of these abandoned quarries could be our site, but it is not directly referenced.
Dr. Savina’s consultation was indispensable in our research into the use and formation of our quarry site. With her help, we were able to identify the type of rock quarried and its possible uses. She also gave us insight into how the work at the site was completed, giving a more complete picture of site use during the quarrying period.
“Asset Detail.” National Parks Service, U.S. Department of the Interior, npgallery.nps.gov/NRHP/AssetDetail?assetID=8c21d42e-e1e4-4b46-a631-954fdc62db21, 6 April 1982.
Savina, Mary. Received by Bee Candelaria, Re: Quarry Site, 24 May 2021.
Thiel, George A, and Carl E Dutton. The University of Minnesota Press, 1935, pp. 1–201, The Architectural, Structural, and Monumental Stones of Minnesota.Tweet, Justin. Practical Guide to MNRRA/Metro-Area Bedrock Geology, 1 Jan. 1970, equatorialminnesota.blogspot.com/2014/02/practical-guide-to-mnrrametro-area.html.