The area encompasses much of the geologic feature called the Penokee Range (Martin 1916: p. 355). The rugged topography was formed millions of years ago and has since weathered and eroded to its present configuration, featuring a 320-foot rise in elevation. The underlying rock is very resistant, with four glacial events failing to pulverize it. Two rivers, the Potato and the Montreal, break through the range in this section and form classic water gaps (Martin 1916: p. 357). To the south, Alder Creek slowly flows parallel to the range over resistant shale and harbors alder and swamp conifer vegetation. Depressions dug by glacial action are now scattered lakes and bogs. The bedrock-influenced landscape is mostly vegetated with forests of aspen, sugar maple, yellow birch, and basswood. Small patches of hemlock, white cedar, and white pine occur in the uplands, and black spruce, cedar, tamarack, and black ash are found in the bogs. Habitat types adjacent to the hills include alder thicket, mixed hardwood/conifer forest, mature conifer forest, cool season grasslands, scattered planted conifers, and bog. Very rare plant populations thrive in the unique geological features. Much of this land is owned and managed for forest production and recreation by Iron County Forest.
This IBA harbors a large, intact block of mature upland deciduous forest that provides excellent habitat for many high priority birds. It is one of the top sites in the state for Black-throated Blue Warbler and is considered a core habitat for this species, with up to 12,000 acres of suitable habitat available. Other forest interior species with high populations here include Least Flycatcher, Black-throated Green Warbler, and Ovenbird. At least one Northern Goshawk territory has been confirmed in this area. A small population of Golden-winged Warblers breeds in the alder thickets lining Alder Creek. Bogs harbor small numbers of Yellow-bellied Flycatcher, Winter Wren, Nashville Warbler, Canada Warbler, Northern Waterthrush, White-throated Sparrow, and Lincoln?s Sparrow. The breeding avifauna is very diverse due to the size and varied habitats of this IBA and contains numerous neotropical migrants.
In the past, areas of steep topography throughout the Penokees received less intensive logging compared to more accessible northern forests. The entire range was subject to deep-hole mining which ended in the 1960?s and left the area relatively undisturbed. This left the extensive forested areas and mature, structurally complex stands so favorable to Black-throated Blue Warblers and other priority species within this IBA. Development, different types of forest harvesting, over-browsing by deer, and invasive species can degrade habitat for forest interior species by fragmenting the forest and increasing edge, reducing tree species and age diversity, and simplifying forest structure. Iron County Forestry has recognized the value of the Penokee Range for birds has set aside a 1,700 acre area of the County Forest, known as the Penokee Biological Reserve, as a ?no cut area?. The Iron County Forest 15-Year Plan integrates habitat needs of priority species into forest management activities. Active management already has enhanced habitat for Black-throated Blue Warbler in some places. Modification of certain harvest regimes could develop this area into one of the largest management areas for this species in the state.