Take Your Own Tour Through Geological Wonders Of Wisconsin

Bedrock Geology of Wisconsin
University of Wisconsin-Extension

Learn about the three distinct regions of Wisconsin geology with this helpful guide.

Region 1: Precambrian ‘core’ (northern WI)

Bedrock: Complex rock record of tectonic and volcanic activity from 2.8 to 1 billion yrs ago

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• Oldest rocks: 2.8 billion year old gneisses –part of ancient North American continent

Central Bayfield and southern Ashland Counties; Along Black River, and Wisconsin River

west of Whiting

• “Penokean” mountains (1.85 billion yrs old): Banded iron formation, slate, schist,

quartzite – a now-eroded range once comparable to the Appalachians in height

Mt Whittlesey, Mellen; Flambeau Flowage area

• Wisconsin Magmatic Terranes (1.9-1.8 billion yrs old): Volcanic and intrusive igneous

rocks – rhyolite, andesite, granite, diorite—formed in a subduction zone setting like the

Cascades today.

Marinette County Parks; Menominee River State Recreational Area; Jim Falls County Park,

(Eau Claire County); Eau Claire Dells County Park (Marathon County)

• Wolf River Batholith (1.5 billion years old): Volcanic and intrusive igneous rocks –

rhyolite, granite, syenite – formed in a hot spot setting like Yellowstone today

Rib Mountain State Park, Wausau; Waupaca City Parks (old quarries); Hayman Falls

County Park, (Shawano County)

• Mid-Continent Rift (1 billion years ago): Lots of basalt and some rhyolite extruded in a

setting comparable to the East African Rift today. When volcanism ceased, the rift

valley filled with sandy sediment that now forms the Apostle Islands

Copper Falls State Park; Pattison State Park; Amnicon Falls State Park; Big Bay State Park;

Interstate State Park; St Peter’s Dome on the North Country Scenic Trail

Topography reflects glacial scouring of hard rocks

‘Whalebacks’ or roche moutinée; striated and polished rock surfaces

Hint for finding bedrock outcrops: Much of northern Wisconsin is covered by dense forest

and wetlands, and it can be hard to find exposed bedrock. The best outcrops are often

found along streams, especially at waterfalls.

Region 2: Glaciated southeast

Bedrock: Mostly Paleozoic sedimentary rocks laid down in shallow seas

Ages: Late Cambrian through early Devonian (520-390 million years)

Ordovician and Silurian rocks are commonly fossiliferous

Parfrey’s Glen Natural Area; Mosquito Hill Nature Center, New London; Fonferek Glen

Park, (Brown County); Cato Falls County Park (Manitowoc County)

Exception: A few ‘windows’ into Precambrian ‘basement’ rock, such as at Red

Granite (1.75 billion year old igneous rock) and the Baraboo Hills (1.6 billion

year old folded quartzite – a single exposure of a buried mountain range)

Devils Lake State Park; Observatory Hill State Natural Area; Red Granite City Park

Topography generally subdued, reflecting glaciation; along Lake Michigan, ice age dunes

Terminal moraine of the Green Bay ice lobe creates a continental divide

between rivers that flow into Lake Michigan vs. those that flow into the

Mississippi system

Kettle Moraine State Park; Hartmann Creek State Park; Whitefish Dunes State Park,

Kohler-Andrae State Park

Exception: The Niagaran escarpment, the boundary between soft Ordovician

shale and overlying resistant Silurian dolostone

High Cliff State Park; Peninsula State Park; Calumet County Park (Hilbert)

Region 3: The Driftless Area – the unglaciated southwest

Bedrock: Paleozoic sedimentary rocks laid down in shallow seas

Ages: Late Cambrian through late Ordovician (520-440 million years)

Ordovician rocks are especially fossiliferous

Limestones and dolostones host lead-zinc deposits

Caves and other ‘karst’ features in limestones

Governor Dodge State Park; Cave of the Mounds; Crystal Cave (Spring Valley);

Elroy-Sparta Bike Trail

Pb-Zn related museums: Badger Mine Museum, Shullsburg; Rollo Jamison

Museum, Platteville

Topography shaped by rivers (a ‘fossil’ pre-glacial landscape)

Deep valleys, steep bluffs, ‘castellated mounds’

Wyalusing State Park; Natural Bridge State Park; Nelson Dewey State Park;;

Wildcat Mountain State Park