The Ironwood Formation of the Penokee Range
by Tom Fitz
The Ironwood Iron Formation is a bedrock layer that extends 75 miles from Lake Gogebic in Michigan to near Jackson Lake, south of Grand View, Wisconsin. The formation is 480 feet thick and is sandwiched between a quartz layer (the Palms Formation), to the south and a slate layer (the Tyler Slate) to the north. The originally horizontal sedimentary rocks were tipped to the north at an angle of 70 degrees so just the edge of the formation is near the surface today.
The Ironwood Formation is composed thin layers of the minerals magnetite (Fe3O4, an iron oxide) and quartz (SiO2). Magnetite is the mineral of interest to mining companies because it is mostly iron, and it is attracted to a magnet—a property that makes it simple to extract from crushed ore. The Ironwood Formation is a relatively rich mineral deposit, meaning the proportion of economic mineral is high and the volume of the deposit is large.
The economic and resource importance of the formation has been known for a long time—in fact, about 40 mines worked the Ironwood Formation in the 19th and 20th centuries. Between 1877 and 1967 more than 325 million tons of ore were extracted from mines located between Upson, Wis., and Wakefield, Mich., mostly from underground workings. It has been estimated that the Ironwood contains 2.2 billion tons of economic ore, making it one of the largest iron reserves in North America.
Tom Fitz is an associate professor of geoscience at Northland College in Ashland, WI. He can be reached at This e-mail address is being protected from spambots. You need JavaScript enabled to view it .
Block diagram showing the Ironwood Formation and adjacent bedrock layers.
The view is looking toward the west. (From USGS Professional Paper 1730)
