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Photo of the Glenwood Formation at Bridal Veil Falls, Minneapolis. Glenwood bed is indicated by the blue arrow. Image is original. March 2026.
The Glenwood Formation, composed of shale and sandstone, overlies the St. Peter Sandstone. It’s 3-5 feet wide. It was deposited during the Ordovician Period (485-444 million years ago). It is interpreted to be either a shallow marine shelf deposit or deposited in a coastal lagoon.
It often hides from view between the Platteville and the St. Peter. It is found only where the Platteville is found, since this unit is very erodible.
It acts as a confining layer to the aquifer layers below (water cannot pass through this layer nearly as easily/quickly as through the Platteville or St. Peter).
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Mossler, John H.. (1985). RI-33 Sedimentology of the Middle Ordovician Platteville Formation Southeastern Minnesota. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/60754🗗
https://equatorialminnesota.blogspot.com/2014/12/the-glenwood-formation.html🗗
https://www.pca.state.mn.us/sites/default/files/mm-overview.pdf 🗗