Located on State Highway 6 in a wayside 0.4 miles north of Crosby (overlooking Portsmouth Mine Pit Lake)
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The Portsmouth was one of many iron and manganese mines that operated along the Cuyuna Range during the period 1911-1978. As much as 60 meters of glacial sediment cover the iron-bearing bedrock in this area; there are few natural rock exposures. Most ore deposits were discovered by magnetic surveys using a dip needle, which resembles a compass set on its side. During the 1890s, Cuyler Adams used this method to map much of the iron-formation. The name "Cuyuna" is derived from his first name, "Cuyler," and the name of his canine assistant, "Una."
At the Portsmouth Mine, about 13 million tons of iron and manganese ore were extracted from the slate, sandstone, and chert bedrock. These rocks were once clay, sand, and chemical precipitates that were deposited in an ocean about two billion years ago. They were subsequently buried deep in the earth where they were metamorphosed by heat and folded in a complex manner. The folding and associated fracturing provided pathways for fluids that precipitated the metal minerals and leached away silica. These processes concentrated the metals and created the high-grade ores, which have as much as 65 percent iron and 20 percent manganese.
The open-pit mining method used at the Portsmouth and other Cuyuna Range mines required the continual removal of water from the working pits, which could be more than 100 meters deep at some locations. When mining ceased, water slowly filled the pits, creating deep lakes with shorelines composed of glacial sediment. As a result, mine workings in the bedrock are no longer visible, and information about the rock is derived from the reports of mining geologists. The Cuyuna Range remains an important potential source of manganese, and mining may one day resume. The fact that one can barely see that a mine was here is a tribute to the effectiveness of mine-land reclamation practices and the natural growth of vegetation.
Erected by the Geological Society of Minnesota in partnership with the Minnesota Department of Transportation and the Minnesota Geological Survey. 1998
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