Minnesota

Eagle Mountain
       7 ½’ quadrangle: Eagle Mountain, MN
       Cook County
       Superior National Forest; Boundary Waters Wilderness.
       Brule Hills

Intrusive Igneous
2,301 ft (701 m)

Bedrock: Granophyre

Late Proterozoic ~ 1100 Ma

Red, porphyritic, quartz-orthoclase-plagioclase-altered hornblende-magnetite granophyre, commonly with quartz-lined miarolitic cavities. The granophyre on Eagle Mountain, part of the Middle Keweenawan Duluth Complex, is surrounded by Duluth gabbro at lower elevations. Layering dips gently south, typical of the whole complex, which is a huge sill-like mass.

Surficial Geology: Brown sandy glacial till left by Wisconsinan continental glaciers. Glacial striae indicate that the last ice sheet to cover Eagle Mountain moved almost due south, part of the Rainy Lobe. It was not affected by the younger Lake Superior Lobe of ice, which moved westward parallel to what is now the north shore of Lake Superior.

Soil Series: No soil survey has been published for Cook County. Presumably a pedalfer soil formed on glacial till under mixed forest.

Selected References:

  • Grout, Frank F., R.P. Sharp and G.M. Schwartz, 1959, The Geology of Cook County, Minnesota: Minnesota Geological Survey Bulletin No.39, 163 p., St. Paul, MN.
  • Taylor, Richard R., 1964, Geology of the Duluth Gabbro Complex near Duluth, Minnesota : Minnesota Geological Survey Bulletin, 63 p., St. Paul, MN.

Other suggested sources of information:


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