Regional Landscape Ecosystems of Michigan, Minnesota, and Wisconsin
SUB-SUBSECTION IX.6.1. Gogebic-Penokee Iron Range
DISCUSSION: Sub-subsection IX.6.1 is noted for steep ridges of Keweenawan (late Precambrian) basaltic lavas and conglomerates, which rise several hundred feet above the adjacent lake and till plains. The ridges extend from northern Wisconsin to the northern end of the sub-subsection and on to the tip of the Keweenaw Peninsula of Michigan; they are part of the Lake Superior syncline.
ELEVATION: 850 to 1,890 feet (259 to 576 m).
AREA: 1,361 square miles (3,526 sq km).
STATES: Michigan and Wisconsin.
CLIMATE: Growing season ranges from 100 to 114 days in Wisconsin (Wisconsin Agricultural Statistics Service 1987) to 120 to 130 days in Michigan (Eichenlaub et al. 1990). Extreme minimum temperatures ranges from -40½F near Lake Superior to -50½F farther inland. Average annual precipitation ranges from 31 to 36 inches. Annual snowfall ranges 60 to 200 inches; highest amounts are in Michigan along Lake Superior, and lowest amounts are in Wisconsin, further inland.
BEDROCK GEOLOGY: The sub-subsection is underlain by Precambrian bedrock, primarily Keweenawan basalts and conglomerates, but also including iron-rich marine sandstones and dolomites of Huronian age, as well as Archean bedrock. A part of the narrow band of Huronian-age (middle Precambrian) bedrock forms the iron-rich Gogebic-Penokee Range near the Michigan towns of Ironwood and Bessemer and the Wisconsin towns of Hurley, Pence, and Upson. Erosion-resistant conglomerates form the steep ridges, between which veins of highly eroded lavas form lakes and wetlands. Copper-rich lava flows were extensively mined.
LANDFORMS: Beginning south of the Keweenaw Peninsula, the volcanic bedrock ridges form a narrow, 1- to 2-mile-wide, 200- to 400-feet-high linear rock ridge, from Houghton in the northeast to Bergland in the southwest. This ridge is broken in several places by streams that have eroded through the bedrock. The Porcupine Mountains are a band 3 to 4 miles wide and 10 miles long, of Keweenawan basalt and conglomerate ridges.
South of Bergland and into Wisconsin, the Keweenawan bedrock ridge is partially or completely covered with either fine- or coarse-textured till.
LAKES AND STREAMS: The only large lake is Lake Gogebic, near the southern edge of the sub-subsection. A few small lakes are on the till plain; two lakes are among the bedrock ridges of the Porcupine Mountains: Lake of the Clouds and Mirror Lake. Rivers include the Presque Isle, Montreal, and Black, rapid streams on bedrock.
SOILS: Soils are red loams and sandy loams, derived from the iron formations eroded and abraided by the glaciers (Hole 1976). These are classified as Orthods and Inceptisols (Hole 1976). Locally there are areas of boulders and broken rock.
PRESETTLEMENT VEGETATION: On the bedrock ridges, red pine, white pine, red oak, and paper birch grew on the thin soils. On till, northern hardwood forests were dominated by sugar maple and hemlock, with basswood and yellow birch. Wetland forests were dominated by conifers.
NATURAL DISTURBANCE: Windthrow was locally common on exposed bedrock ridges and on large ground-moraine ridges. In Michigan, these windthrows were occasionally quite large, baring entire ridgetops of trees (Albert 1990). Canham and Loucks (1984) also show areas of windthrow on similar landscapes in Wisconsin.
PRESENT VEGETATION AND LAND USE: Tourism and forestry remain important to the economy. Copper and iron mining were major industries, but almost all mines are now closed.
RARE PLANT COMMUNITIES: None identified to date.
RARE PLANTS: Wisconsin: Dryopteris fragrans (fragrant fern), Polystichum braunii (Brown's holly fern).
RARE ANIMALS: Falco peregrinus (peregrine falcon).
NATURAL AREAS: Michigan: State Wilderness Areas: Porcupine Mountains; State Scenic Sites: Presque Isle River, Union Springs.
PUBLIC LAND MANAGERS: Michigan: State Parks: Porcupine Mountains, Lake Gogebic; State Forests: Copper Country; National Forests: Ottawa. Wisconsin: National Forests: Chequamegon.
CONSERVATION CONCERNS:
BOUNDARIES: Parts of the sub-subsection have been mapped in greater detail by the ECS mappers on the Ottawa and Chequamegon National Forests. My treatment is similar to that of the Ottawa National Forest. Detailed studies of the bedrock exposures and glacial drift depths along the southern boundary might justify expansion of this sub-subsection unit to the south; the GLO surveyors mapped several exposures of volcanic bedrock farther south, but these exposures were very localized.
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