Regional Landscape Ecosystems of Michigan, Minnesota, and Wisconsin
SUBSECTION V.4. Rock River Hill Country
DISCUSSION: The Rock River Hill Country subsection is an area of steep, dissected topography, which also contains broad outwash plains. Soils are often thin, developed from a recent silt-loam cap of loess over old Altonian- and Illinoian-age glacial drift. This subsection continues south into Illinois, where its western portion is called the Rock River Hill Country, and its eastern portion is called the Winnebago Section of the Northeastern Morainal Division (Schwegman 1973).
SUB-SUBSECTIONS: None.
ELEVATION: 740 to 1,157 feet (226 to 353 m).
AREA: 1,048 square miles (2,716 sq km).
STATES: Wisconsin (this subsection also continues south into Illinois).
CLIMATE: Growing season ranges from 150 to 160 days (Wisconsin Agricultural Statistics Service 1987). Average annual precipitation is 32 to 36 inches, and average annual snowfall ranges from 32 inches in the south to approximately 40 inches in the north (Wendland et al. 1992). Extreme minimum temperature ranges from approximately -30½F in the south to -35½F farther north (Reinke et al. 1993).
BEDROCK GEOLOGY: Subsection is underlain by Ordovician-age dolomite, with some limestone and shale (Ostrom 1981, Morey et al. 1982). Cambrian sandstone is exposed in the valleys of the Rock and Sugar Rivers. Bedrock is within 3 to 4 feet of the surface locally (Hole and Germain 1994).
LANDFORMS: The topography consists of dissected uplands of pre-Wisconsinan till and Wisconsinan-age outwash deposits, which form broad, flat to rolling plains (Hole and Germain 1994). Glaciation within the subsection is considered to date from at least 25,000 years ago (Zenda Formation at the east edge). Over much of the subsection, glaciation occurred as long as 130,000 years ago (Walworth Formation) (Clayton et al. 1991).
LAKES AND STREAMS: No lakes within the subsection; numerous small creeks within this highly dissected landscape. Among the larger streams are the Rock and Sugar Rivers.
SOILS: Pre-Wisconsinan till, and paleosols derived from this till, underlie a silt cap (loess) and are exposed on dissected uplands (Hole 1976). Wisconsinan-age outwash deposits form extensive rolling plains. Loess is about 2 feet deep over either leached sandy loam or loam, which is calcareous at a depth of about 6 feet. Under oak savannas, Typic Hapludalfs form; under prairie, Typic Argiudolls are representative soils (Hole and Germain 1994).
PRESETTLEMENT VEGETATION: The dominant vegetation over most of the landscape was upland prairie and oak forest. Tallgrass prairie was concentrated on a broad till plain in Green County and on both till and outwash plains on both sides of the Rock River in Rock County. Oak forest was quite extensive, especially along the more dissected bluffs and ravines of the numerous small streams of the subsection. Sugar maple-basswood was locally present along some streams, where the topography was hilly and often rocky. The largest areas of sugar maple-basswood forest occur along Sugar and Richland Creeks in Green County. Sedge meadow, wet prairie, and shrubs form linear corridors along several of the small creeks and also along the Sugar River. The Sugar River supports a significant corridor of flood-plain forest.
NATURAL DISTURBANCE: Fire maintained upland prairie and oak forests over much of the landscape. Native American land management with fire may be partially responsible for the persistence of tallgrass prairie. The Azatlan (Juntunen) culture had villages along the Rock River between circa 1400 and 1600, and the Winnebago tribe had villages along the Rock River between 1810 and 1830 (Tanner 1986).
PRESENT VEGETATION AND LAND USE: Heavy agriculture has caused major fragmentation and reduction of natural vegetation.
RARE PLANT COMMUNITIES: Tallgrass prairie, including mesic, wet-mesic, and wet prairie, along with a site for oak opening.
RARE PLANTS: Besseya bullii (kitten-tails), Chaerophyllum procumbens (wild chervil), Diarrhena americana (beak grass), Hypericum sphaerocarpum (round-fruited St. John's-wort), Napaea dioica (glade mallow).
RARE ANIMALS: Dendroica dominica (yellow-throated warbler), Hesperia ottoe (Ottoe skipper), Nyctanassa violacea (yellow-crowned night heron), Speyeria idalia (regal fritillary).
NATURAL AREAS: State Natural Areas: Abraham's Woods, Oliver Prairie, Browntown Oak Forest, Swenson Wet Prairie, Avon Bottoms, Newark Road Prairie, Lima Bog, Ward/Swartz Decatur Woods, Kessler Railroad Prairie, and Muralt Bluff Prairie.
PUBLIC LAND MANAGERS:
CONSERVATION CONCERNS: The pursuit of a joint "grassland" management project with Illinois, focused on the Sugar River corridor, has been suggested. Elsewhere the landscape is highly disturbed and fragmented. There is a great potential for managing this landscape, where management would target rare and declining grassland bird species, and, east of the Sugar River, the ornate box turtle.
BOUNDARIES: I chose to combine parts of two divisions recognized in Illinois, the Winnebago Section of the Northeastern Morainal Division and the Freeport Section of the Rock River Hill Country Division (Schwegman 1973). I separated the Winnebago Section, which has Altonian-age glacial drift that is better drained, from the adjacent Morainal Section of the Northeastern Morainal Division, which has many glacial lakes and poorer drainage. The Freeport Section to the west has both Illinoian and early Wisconsinan (Altonian) drift. Outwash is more extensive in the Winnebago Section, and eroded bluffs and dells are more common in the Freeport Section, but both of these features are shared by both sections. There may well be justification for dividing this subsection (V.4) into two sub-subsections.
I also chose to include this subsection as part of the savanna province, rather than as a part of the prairie province (cf. Bailey and Cushwa [1981], who treat Subsection V.4 as part of the Tall-grass Prairie Province). I had two reasons for this. First, the vegetation in Subsection V.4 is a mosaic of maple-basswood and oak forests, oak savannas, and prairie, in contrast to Section I and Section II, where prairie dominates broad expanses of land, and both savannas and forests are uncommon. Second, the annual precipitation in Subsection V.4 is heavier than in most of the tallgrass prairie, with a gradual increase in annual precipitation, and possibly more important, winter precipitation farther to the east. This climatic difference may be as important as the changes in topography for resulting in a mosaic of forest and prairie.
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